Neurogenesis in the epithalamus, dorsal thalamus and ventral thalamus of the rat: an autoradiographic and cytological study

Spatial-temporal topography in neurogenesis of the macaque thalamus

Maternal injection of ³H-thymidine ([³H]dT) during gestation in non-human primates (NHPs) has been used to determine the time of neurogenesis for various brain areas, including the lateral geniculate (LGN) and the pulvinar (PUL) nuclei of the caudal thalamus. Here, we examine neurogenesis in the rostral thalamus, with focus on the mediodorsal (MD) and the anterior nuclei (ANT), to determine if neurogenesis of rostral and caudal thalamic nuclei is concurrent or instead temporally staggered. The MacBrainResource (MBR) search function identified archived cases (N = 10) of [³H]dT labeled specimens, with injection dates ranging from embryonic day 25 (E25)-E50 and postnatal sacrifice dates. Slides were scanned to create digital images for subsequent analysis using Stereo Investigator software. Labeled neurons were mapped within a contour that encompassed the entire rostral thalamus. These maps were superimposed onto closely corresponding sections from the online BrainMaps macaque atlas to facilitate analysis. Our novel approach uncovered a previously undetected spatial-temporal patterning of neurogenesis in the thalamus. At E30, labeled neurons were located in a compact medial band; at E38-E40, labeling was dense ventrolaterally, and at E43, labeling predominated laterally at rostral levels and was widely distributed at caudal levels. Peak neurogenesis occurs earlier in MD (E30-E43) and ANT (E31-E43) than in LGN (E36-E43) and PUL (E36-E45). Birth-dating of neurons in MD and ANT, two higher order relay nuclei implicated in the pathology of schizophrenia, provides further insight into the critical period of vulnerability during which early developmental perturbation may increase incidence of schizophrenia later in life.

Histogenetic Radial Models as Aids to Understanding Complex Brain Structures: The Amygdalar Radial Model as a Recent Example

The radial dimension expands during central nervous system development after the proliferative neuroepithelium is molecularly patterned. The process is associated with neurogenesis, radial glia scaffolding, and migration of immature neurons into the developing mantle stratum. Radial histogenetic units, defined as a delimited neural polyclone whose cells share the same molecular profile, are molded during these processes, and usually become roughly stratified into periventricular, intermediate, and superficial (subpial) strata wherein neuronal cell types may differ and be distributed in various patterns. Cell-cell adhesion or repulsion phenomena together with interaction with local intercellular matrix cues regulate the acquisition of nuclear, reticular, or layer histogenetic forms in such strata. Finally, the progressive addition of inputs and outputs soon follows the purely neurogenetic and radial migratory phase. Frequently there is heterochrony in the radial development of adjacent histogenetic units, apart of peculiarities in differentiation due to non-shared aspects of the respective molecular profiles. Tangential migrations may add complexity to radial unit cytoarchitecture and function. The study of the contributions of such genetically controlled radial histogenetic units to the emerging complex neural structure is a key instrument to understand central nervous system morphology and function. One recent example in this scenario is the recently proposed radial model of the mouse pallial amygdala. This is theoretically valid generally in mammals (Garcia-Calero et al., 2020), and subdivides the nuclear complex of the pallial amygdala into five main radial units. The approach applies a novel ad hoc amygdalar section plane, given the observed obliquity of the amygdalar radial glial framework. The general relevance of radial unit studies for clarifying structural analysis of all complex brain regions such as the pallial amygdala is discussed, with additional examples.

Amphibian thalamic nuclear organization during larval development and in the adult frog Xenopus laevis: Genoarchitecture and hodological analysis

The early patterning of the thalamus during embryonic development defines rostral and caudal progenitor domains, which are conserved from fishes to mammals. However, the subsequent developmental mechanisms that lead to the adult thalamic configuration have only been investigated for mammals and other amniotes. In this study, we have analyzed in the anuran amphibian Xenopus laevis (an anamniote vertebrate), through larval and postmetamorphic development, the progressive regional expression of specific markers for the rostral (GABA, GAD67, Lhx1, and Nkx2.2) and caudal (Gbx2, VGlut2, Lhx2, Lhx9, and Sox2) domains. In addition, the regional distributions at different developmental stages of other markers such as calcium binding proteins and neuropeptides, helped the identification of thalamic nuclei. It was observed that the two embryonic domains were progressively specified and compartmentalized during premetamorphosis, and cell subpopulations characterized by particular gene expression combinations were located in periventricular, intermediate and superficial strata. During prometamorphosis, three dorsoventral tiers formed from the caudal domain and most pronuclei were defined, which were modified into the definitive nuclear configuration through the metamorphic climax. Mixed cell populations originated from the rostral and caudal domains constitute most of the final nuclei and allowed us to propose additional subdivisions in the adult thalamus, whose main afferent and efferent connections were assessed by tracing techniques under in vitro conditions. This study corroborates shared features of early gene expression patterns in the thalamus between Xenopus and mouse, however, the dynamic changes in gene expression observed at later stages in the amphibian support mechanisms different from those of mammals.

Molecular anatomy of the thalamic complex and the underlying transcription factors

Thalamocortical loops have been implicated in the control of higher-order cognitive functions, but advances in our understanding of the molecular underpinnings of neocortical organization have not been accompanied by similar analyses in the thalamus. Using expression-based correlation maps and the manual mapping of mouse and human datasets available in the Allen Brain Atlas, we identified a few individual regions and several sets of molecularly related nuclei that partially overlap with the classic grouping that is based on topographical localization and thalamocortical connections. These new molecular divisions of the adult thalamic complex are defined by the combinatorial expression of Tcf7l2, Lef1, Gbx2, Prox1, Pou4f1, Esrrg, and Six3 transcription factor genes. Further in silico and experimental analyses provided the evidence that TCF7L2 might be a pan-thalamic specifier. These results provide substantial insights into the "molecular logic" that underlies organization of the thalamic complex.

Distinction in the immunoreactivities of two calcium-binding proteins and neuronal birthdates in the first and higher-order somatosensory thalamic nuclei of mice: Evolutionary implications

Comparative embryonic studies are the most effective way to discern phylogenetic changes. To gain insight into the constitution and evolution of mammalian somatosensory thalamic nuclei, we first studied how calbindin (CB) and parvalbumin (PV) immunoreactivities appear during embryonic development in the first-order relaying somatosensory nuclei, i.e., the ventral posteromedial (VPM) and posterolateral (VPL) nuclei, and their neighboring higher-order modulatory regions, including the ventromedial or ventrolateral nucleus, posterior, and the reticular nucleus. The results indicated that cell bodies that were immunoreactive for CB were found earlier (embryonic day 12 [E12]) in the dorsal thalamus than were cells positive for PV (E14), and the adult somatosensory thalamus was characterized by complementary CB and PV distributions with PV dominance in the first-order relaying nuclei and CB dominance in the higher-order regions. We then labeled proliferating cells with [(3) H]-thymidine from E11 to 19 and found that the onset of neurogenesis began later (E12) in the first-order relaying nuclei than in the higher-order regions (E11). Using double-labeling with [(3) H]-thymidine autoradiography and CB or PV immunohistochemistry, we found that CB neurons were born earlier (E11-12) than PV neurons (E12-13) in the studied areas. Thus, similar to auditory nuclei, the first and the higher-order somatosensory nuclei exhibited significant distinctions in CB/PV immunohistochemistry and birthdates during embryonic development. These data, combined with the results of a cladistic analysis of the thalamic somatosensory nuclei, are discussed from an evolutionary perspective of sensory nuclei.

Gbx2 is essential for maintaining thalamic neuron identity and repressing habenular characters in the developing thalamus

The thalamus and habenula, two important nodes of the forebrain circuitry, are derived from a single developmental compartment, called prosomere 2, in the diencephalon. Habenular and thalamic neurons display distinct molecular identity, neurochemistry, and connectivity. Furthermore, their progenitors exhibit distinctive neurogenic patterns with a marked delay in the onset of neurogenesis in the thalamus. However, the progenitors in prosomere 2 express many common developmental regulators and the mechanism underlying the specification and differentiation of these two populations of neurons remains unknown. Gbx2, coding for a homeodomain transcription factor, is initially expressed in thalamic neuronal precursors that have just exited the cell cycle, and its expression is maintained in many mature thalamic neurons in adults. Deletion of Gbx2 severely disrupts histogenesis of the thalamus and abolishes thalamocortical projections in mice. Here, by using genome-wide transcriptional profiling, we show that Gbx2 promotes thalamic but inhibits habenular molecular characters. Remarkably, although Gbx2 is expressed in postmitotic neuronal precursors, deletion of Gbx2 changes gene expression and cell proliferation in dividing progenitors in the developing thalamus. These defects are partially rescued by the mosaic presence of wild-type cells, demonstrating a cell non-autonomous role of Gbx2 in regulating the development of thalamic progenitors. Our results suggest that Gbx2 is essential for the acquisition of the thalamic neuronal identity by repressing habenular identity through a feedback signaling from postmitotic neurons to progenitors.

Development of the Avian Dorsal Thalamus: Patterns and Gradients of Neurogenesis

The dorsal thalamus is a region of the diencephalon that relays sensory and motor information between areas of the brain stem and the telencephalon. Although a dorsal thalamic region is recognized in all vertebrates and believed to be homologous, little is known about how the regions within it evolved and whether some or all regions within the dorsal thalamus are homologous among different vertebrate species. To characterize the gradients and patterns of neurogenesis of the avian dorsal thalamus, a single application of a low dose of bromodeoxyuridine (BrdU) was delivered to each chick between embryonic day (E)3 and E8 (stages 21 and 34), and chicks were followed up to E8 or E10 (stage 34 or 36). Comparisons of anti-BrdU labeling patterns across the different injection days suggest that nearly all dorsal thalamic neurons are born early in chick embryogenesis, between E3 and E8. Furthermore, neurons in the lateral, dorsal, and posterior parts of the dorsal thalamus are generally born earlier than those in the medial, ventral, and anterior parts. Analyses of the birth dates for nine regions show that the general pattern of neurogenesis in the avian dorsal thalamus resembles that of homologous regions within the rodent thalamus, with the exception of the auditory region, the nucleus ovoidalis, which is born later than the mammalian auditory medial geniculate nucleus. The similar pattern of neurogenesis in birds and mammals may represent a highly conserved developmental pattern that was present in the common ancestor of living birds and mammals, or may represent independently derived states. Additional studies in reptiles and amphibians are needed to distinguish between these evolutionary histories.

Irx3 and Pax6 establish differential competence for Shh-mediated induction of GABAergic and glutamatergic neurons of the thalamus

During embryonic development, the presumptive GABAergic rostral thalamus (rTh) and glutamatergic caudal thalamus (cTh) are induced by Sonic hedgehog (Shh) signaling from the zona limitans intrathalamica (ZLI) at the rostral border of the thalamic primordium. We found that these inductions are limited to the neuroepithelium between the ZLI and the forebrain-midbrain boundary, suggesting a prepattern that limits thalamic competence. We hypothesized that this prepattern is established by the overlapping expression of two transcription factors: Iroquois-related homeobox gene 3 (Irx3) posterior to the ZLI, and paired box gene 6 (Pax6) anterior to the forebrain-midbrain boundary. Consistent with this assumption, we show that misexpression of Irx3 in the prethalamus or telencephalon results in ectopic induction of thalamic markers in response to Shh, that it functions as a transcriptional repressor in this context, and that antagonizing its function in the diencephalon attenuates thalamic specification. Similarly, misexpression of Pax6 in the midbrain together with Shh pathway activation results in ectopic induction of cTh markers in clusters of cells that fail to integrate into tectal layers and of atypical long-range projections, whereas antagonizing Pax6 function in the thalamus disrupts cTh formation. However, rTh markers are negatively regulated by Pax6, which itself is down-regulated by Shh from the ZLI in this area. Our results demonstrate that the combinatorial expression of Irx3 and Pax6 endows cells with the competence for cTh formation, whereas Shh-mediated down-regulation of Pax6 is required for rTh formation. Thus, thalamus induction and patterning depends both on a prepattern of Irx3 and Pax6 expression that establishes differential cellular competence and on Shh signaling from the ZLI organizer.

Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.

Structural organization, neurochemical characteristics, and connections of the reticular nucleus of the thalamus

This review analyzes current concepts of the structural organization and ultrastructure of the reticular nucleus of the thalamus (RNT) and the neurochemical characteristics of its neurons. The topography, cytoarchitectonics, and neuronal organization of this nucleus are considered in detail, as are questions of its neurogenesis. Neurochemical data clarifying the representation of neurotransmitter systems in the RNT and data on neuropeptides synthesized in its neurons are systematized. The complex ultrastructural organization of the RNT is characterized in terms of recent data from state-of-the-art immunocytochemical methods allowing localization of glutamatergic and GABAergic receptors on synaptic elements. Data on the afferent and efferent connections of the RNT demonstrate its influences on various parts of the brain and the specific features of its interactions with cortical formations.

The rostral and caudal boundaries of the diencephalon

Knowledge of nature and features of the boundaries between the main neural regions seems to be essential to understand the rules of brain regionalization. On the light of several current and classical criteria used to define cerebral boundaries, we examine the features of the places recognized as rostral and caudal boundaries in the developing diencephalon and provide new images about the glial features of these boundaries. One demonstrated property of some embryonic boundaries is the prevention of the crossing cells in the early ventricular zone (clonal restriction), while the intermediate zone seems to lack it. Data available so far indicate that the early boundary between diencephalon and mesencephalon (d/m) is a clonal restriction limit, but not between diencephalon and telencephalon (d/t). Later, while diencephalic nuclei form, cellular dispersion does not occur through the alar part of d/m, but it achieves in the corresponding d/t alar portion. The relationship between origin, migration, and cell-type specification of neural cells is being the object of special attention in the telencephalon, where specific cellular fenotipes can migrate to distant regions following non-radial routes. Such is the case of most GABAergic interneurons of avian and mammalian pallium and oligodendrocytes of the forebrain. In this regard, little attention has been devoted to the diencephalon, where this type of migration, specially those through the rostral boundary, has been reported by different authors. We introduce increasing evidence about non-conventional neuronal migration in the developing diencephalon and compare the reported migratory behavior with respect to both boundaries.

Differential vulnerability of the rat retina, suprachiasmatic nucleus and intergeniculate leaflet to malnutrition induced during brain development

We investigated in young rats the effects of malnutrition on the main structures of the circadian timing system: retina, hypothalamic suprachiasmatic nuclei (SCN), thalamic intergeniculate leaflet, retinohypothalamic- and geniculohypothalamic tracts. Control rats were born from mothers fed a commercial diet since gestation, and malnourished rats from mothers fed a multideficient diet since gestation (GLA group) or lactation (LA group). After weaning, pups received the same diet as their mothers, and were analysed at postnatal days 27, 30-33 and 60-63. Brain sections were processed to visualise in the SCN neuropeptide Y immunoreactivity and terminal labeling after intraocular tracer injections. Nissl staining was used to assess cytoarchitectonic boundaries of the SCN and cell features in retinal whole mounts. Cell counts, morphometric and densitometric analysis were performed. Compared with controls, the total retinal surface was reduced and the topographical distribution of retinal ganglion cells was altered in malnourished rats, with changes in their density. Alterations were also detected in the SCN dimensions in the GLA and LA groups at one and two postnatal months, as well as in the SCN portion occupied by the retinal input in the GLA group at days 30-33, but not in the NPY-containing geniculohypothalamic tract. The present data point to subtle changes, with a low and differential vulnerability to early malnutrition, of structures involved in circadian timing regulation. Furthermore, the present findings suggest that the altered circadian rhythmicity previously documented in malnourished rats cannot be ascribed to impaired development of the retino- and geniculohypothalamic projections to the SCN.

Optical mapping of the functional organization of the rat trigeminal nucleus: initial expression and spatiotemporal dynamics of sensory information transfer during embryogenesis

We examined the functional organization of the rat trigeminal nuclear complex and its developmental dynamics using a multiple-site optical recording technique. Brainstem preparations were dissected from embryonic day 12 (E12)-E16 rat embryos, and stimulation was applied individually to the three branches of the trigeminal nerve (V1-V3). The action potential activity of presynaptic fibers was detected from E13, and the glutamate-mediated postsynaptic response was significantly observed from E15 on. At E14, the evoked signals usually consisted of only the action potential-related fast component. However, when extracellular Mg2+ was removed, a significant dl-2-amino-5-phosphonovaleric acid-sensitive slow component appeared. These results suggest that postsynaptic function mediated by NMDA receptors is latently generated as early as E14. The response area of the three branches of the trigeminal nerve showed some functional somatotopic organization, with the ophthalmic (V1) nerve area medially located and the mandibular (V3) nerve area laterally located. The center of the trigeminal nuclear complex in which the activity of neurons and synaptic function was greatest shifted caudally with development, suggesting that the functional architecture of the trigeminal nuclear complex is not fixed but changes dynamically during embryogenesis. By electron microscopy, we could not observe clear correlations between functional data and morphological information; when we surveyed E16 preparations, we could not identify typical synaptic structures between the 1,1'-dioctyldecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-labeled trigeminal nerve terminals and the neurons in the trigeminal nuclear complex. This implies that postsynaptic function in the trigeminal nuclear complex is generated before the appearance of the morphological structure of conventional synapses.

Two epochs in the development of gamma-aminobutyric acidergic neurons in the ferret thalamus

These studies chart the development of gamma-aminobutyric acid (GABA)-ergic neurons in the three divisions of the thalamus (ventral thalamus, dorsal thalamus, and epithalamus). GABAergic neurons were identified by in situ hybridization to localize mRNA for 67-kDa glutamic acid decarboxylase (GAD(67)) and related to the morphological maturation of the thalamus in fetal and postnatal brains and to expression of transcription factors Gbx-2 and Tbr-1. Origins of GABAergic neurons were sought in in vitro slice preparations incubated in bromodeoxyuridine or injected with a carbocyanine dye. GABA neurons of ventral thalamus (reticular nucleus, ventral lateral geniculate nucleus, zona incerta, and nucleus of the fields of Forel) and of epithalamus appear at least 14 days before those intrinsic to dorsal thalamus. Ventral thalamus GABA cells are derived from a region connecting the ventricular zone of the third ventricle to the caudal ganglionic eminence. This region is delimited ventrally by the Tbr-1-expressing prethalamic eminence and dorsally by the Gbx-2-expressing part of the dorsal thalamus. GABA neurons of epithalamus are derived from the embryonic pretectum. Neurons continue to be added to the ventral thalamus, perireticular nucleus, entopeduncular nucleus, and substantia nigra from the ganglionic eminence as development proceeds. GAD(67)-expressing cells of dorsal thalamus become detectable only at birth and populate the thalamus from posterior to anterior over the first week of life. Although a very small number reaches the dorsal lateral geniculate nucleus from the caudal ganglionic eminence, there is no obvious new source of proliferating neurons at this stage. Intrinsic GABA cells of dorsal thalamus may, therefore, derive from an early generated population of cells that turns on a GABAergic phenotype only late in development.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Membrane-associated molecules guide limbic and nonlimbic thalamocortical projections

Membrane-associated signals expressed in restricted domains of the developing cerebral cortex may mediate axon target recognition during the establishment of thalamocortical projections, which form in a highly precise manner during development. To test this hypothesis, we first analyzed the outgrowth of thalamic explants from limbic and nonlimbic nuclei on membrane substrates prepared from limbic cortex and neocortex. The results show that different thalamic fiber populations are able to discriminate between membrane substrates prepared from target and nontarget cortical regions. A candidate molecule that could mediate selective choice in the thalamocortical system is the limbic system-associated membrane protein (LAMP), which is an early marker of cortical and subcortical limbic regions (Pimenta et al.,1995) that can promote outgrowth of limbic axons. Limbic thalamic and cortical axons showed preferences for recombinant LAMP (rLAMP) in a stripe assay. Incubation of cortical membranes with an antibody against LAMP prevented the ability of limbic thalamic fibers to distinguish between membranes from limbic cortex and neocortex. Strikingly, nonlimbic thalamic fibers also responded to LAMP, but in contrast to limbic thalamic fibers, rLAMP inhibited branch formation and acted as a repulsive axonal guidance signal for nonlimbic thalamic axons. The present studies indicate that LAMP fulfills a role as a selective guidance cue in the developing thalamocortical system.

Postnatal cytoarchitecture of the rat medial geniculate body

The medial geniculate body (MGB) is a thalamic structure that provides vital information flow to the forebrain for complex acoustic processing. The development of cytoarchitectural features of the MGB was examined in rat to identify age-related patterns of growth in major geniculate compartments that have been described previously (Clerici and Coleman [1990] J. Comp. Neurol. 297:14-31; Clerici et al. [1990] J. Comp. Neurol. 297:32-54): the ventral (MGv), dorsal (MGd), and medial (MGm) divisions. Results show that, on the day of parturition, all major nuclei of each division are characterized, including the ovoid (OV) and ventral (LV) nuclei of MGv; the dorsal, deep dorsal (DD), caudodorsal, limitans, and suprageniculate nuclei of MGd; and the MGm. The MGv and MGd, which display comparable areas at birth, show rapid growth to postnatal day 7 (PND7), which then slows until PND11, around the time of ear canal opening; subsequently, MGv accelerates growth to reach larger adult size. From PND11 to PND16, thionin facilitates parcellation by extensive staining of dendritic processes of MGd, MGm, and lateral posterior nucleus neurons but not neurons of the MGv or the dorsal lateral geniculate nucleus. Golgi stains after birth reveal restricted dendritic arborizations in MGv cells and dichotomous branching patterns of MGd neurons. Somal size in MGB increases dramatically subsequent to afferent innervation and again following onset of auditory function. Somal growth occurs between all postnatal age groups tested for OV, LV, and DD nuclei, although LV segments related to high and low frequencies do not differ. Cell packing density predicts the expanse of major MGB divisions better than somal size. These results demonstrate the integrity and growth patterns of MGB nuclei and divisions from nascence and provide a substrate for subsequent study of anatomical and physiological development of the MGB.

Calretinin immunoreactivity in the developing thalamus of the rat: a marker of early generated thalamic cells

The present work was aimed to study the immunocytochemical localization of the calcium-binding protein, calretinin, in the rat thalamus from embryonic day 14 to the third postnatal week. In the adult rat thalamus, calretinin immunoreactivity is intensely expressed in some intralaminar and midline nuclei, as well as in selected regions of the reticular nucleus. At embryonic day 14, calretinin was expressed by immature and migrating neurons and fibres laterally to the neuroepithelium of the diencephalic vesicle in the region identified as reticular neuroepithelium. At embryonic day 16, immunoreactive neurons were present in the primordium of the reticular nucleus and in the region of the reticular thalamic migration, where neurons showed the morphology of migratory cells. At the end of embryonic development and in the first postnatal week, calretinin-positive neurons were observed in selected region of the reticular nucleus and it was intensely expressed in some intralaminar and midline nuclei. Bands of immunopositive fibres were also observed crossing the thalamus. During the second postnatal week, the immunolabelling in the reuniens, rhomboid, paraventricular and central medial thalamic nuclei remains very intense while a decrease of immunoreactivity in mediodorsal, centrolateral and laterodorsal nuclei was observed. The immunostaining of fibres, particularly evident in the perinatal period, progressively decreased and it was no longer visible by the end of the second postnatal week when the distribution and intensity of calretinin immunostaining was similar to that observed in the adult rat thalamus. The present findings indicate that the immunolocalization of calretinin can be used to identify subsets of thalamic neuronal population during pre- and postnatal maturation allowing also the detection of the migratory pattern of early generated reticular thalamic neurons.

Genesis and fate of the perireticular thalamic nucleus during early development

A striking feature of the internal capsule during early development is that it is full of small neurones. Later, this group of neurones, called the perireticular thalamic nucleus, appears to have reduced in size, and only a few scattered cells are seen. In an effort to understand better the developmental history of the perireticular nucleus this study examines: i) the period of cell generation in the nucleus, ii) the magnitude of cell loss in the nucleus, and iii) the subsequent fate of cells in the nucleus during development. The perireticular cells are generated very early in development, being among the first generated in the thalamus (rats: E13-14; cats: E21-30). In rats, the first perireticular cells are generated at about the same developmental stage as the first subplate cells, which are among the first generated cells of the cortex: in cats, the first perireticular cells are generated well before those in the subplate (E24-30). In rats, the number of perireticular cells during developmental peaks at P5 (approximately 30,000) and then declines sharply (approximately 98%) by P15 (approximately 750), when adult-like patterns are seen. This dramatic loss of perireticular cells is due to both cell death and a migration of cells into the adjacent globus pallidus. The majority of the perireticular cells which migrate into the globus pallidus, however, are likely to die also. The presence of pyknotic profiles (indicators of dying cells) in the rat perireticular nucleus points to cell death as a contributor to the reduction in cell number during development. In this study, a period of relatively high pyknotic profile incidence (number of pyknotic cells per 1,000 "living" cells) is recorded in the perireticular nucleus over a 5 day period, from P2 to P7 (13.5-15.5). Similar values and patterns are recorded in the reticular nucleus and globus pallidus, except that in these structures, a period of relatively high pyknotic profile incidence (15-20) occurs over a shorter period (3 days; P2-5). Previous studies have suggested that some perireticular cells migrate into and settle within the adjacent globus pallidus. This study, with the use of long-term survivals after tracer injections in rats, shows that none (or very few) of these perireticular cells which migrate into the globus pallidus survive into more mature postnatal stages. Tracer (biotinylated dextran) was injected into the sensory nuclei of the dorsal thalamus at early stages (P7) and the rats were allowed to survive for either a day thereafter (to P8) or until well after the period of cell death was complete (to P16 or P21). In the short-term survivals (to P8), there are many dextran-labelled cells seen in the globus pallidus and in the perireticular nucleus. In the long-term survivals (to P16 or P21), by contrast, there are no dextran-labelled cells apparent in the globus pallidus or in the perireticular nucleus. It is likely that these cells in the globus pallidus, as with those in the perireticular nucleus, undergo cell death during development.

Individual axon morphology and thalamocortical topography in developing rat somatosensory cortex

The morphology of individual thalamocortical axons in developing rat primary somatosensory cortex was studied using lipophilic tracers. Anterograde labeling with lipophilic dyes demonstrated a topographical organization of thalamocortical projections exiting the thalamus as early as embryonic day (E) 16; retrograde labeling studies demonstrated topography of these projections as they reached the cortex as early as E18. At E17, axons course tangentially within the intermediate zone and turn or branch near the deepest layer of cortex (layer VIb), suggesting the presence of guidance cues in this region. Axons appear to grow and branch progressively within layers VIb and VIa during the following days; axons in the intermediate zone may give rise to radially directed branches. Individual axons appear to grow steadily and progressively into the cortex, with the leading front of axons at the transition zone between the cortical plate (CP) and the differentiating cortical layers. At birth (P0), thalamocortical axons extend radially through layers VIa and V and emit branches within these layers; some axons reach the CP. By P1, layer IV has begun to differentiate and axons begin to form a few simple branches in the vicinity of the layer IV cells. Over the ensuing week, axons generate more branches within layer IV, but the tangential extent of individual axon arbors does not exceed the width of a barrel. By P7, individual axons overlap within barrel clusters, and individual axons span the width of a cluster. These observations indicate that thalamic afferents develop by progressive growth of arbors that remain spatially restricted, rather than by overbranching and retracting arbors.

Development of direct GABAergic projections from the zona incerta to the somatosensory cortex of the rat

The postnatal development of direct thalamocortical projections from the zona incerta of the ventral thalamus to the whisker representation area of the rat primary somatosensory cortex was investigated. Cytoarchitectonic analysis based on Nissl staining, cytochrome oxidase histochemistry and immunohistochemistry for glutamic acid decarboxylase, GABA, parvalbumin and calbindin D28K revealed that the zona incerta can be clearly distinguished from surrounding diencephalic structures from the day of birth. Moreover, four distinct anatomical subdivisions of this nucleus were identified: the rostral, dorsal, ventral and caudal. Of these, the ventral subdivision is by far the most conspicuous, containing the highest density of neurons, and the highest levels of cytochrome oxidase, glutamate decarboxylase, GABA, parvalbumin and calbindin D28K. In contrast, the dorsal, rostral and caudal subdivisions contain fewer cells, lower levels of glutamic acid decarboxylase and GABA and very few parvalbumin-positive and calbindin-positive neurons. Small injections of rhodamine coated microspheres or Fluoro-gold in the primary somatosensory cortex of animals at different stages of development revealed the existence of retrogradely labeled neurons in the rostral and dorsal subdivisions of the zona incerta from postnatal day 1. At this age, retrogradely labeled cells were also found in the ventral lateral, ventral posterior medial, posterior medial, centrolateral, ventral medial and magnocellular subdivision of the medial geniculate nuclei of the dorsal thalamus. The density of the incertocortical projection reaches its maximum between the first and second postnatal weeks, decreasing subsequently, until an adult pattern of labeling is achieved. Tracer injections combined with immunohistochemistry revealed that the majority of the incertocortical projection derives from GABAergic neurons, implying a potentially inhibitory role for the incertocortical projection. These results demonstrate that the rat trigeminal system contains parallel thalamocortical pathways of opposite polarity, emerging from both the dorsal (glutamatergic, excitatory) and ventral (GABAergic, inhibitory) thalamus since the day of birth. As such, these findings suggest that, contrary to the classical notion, not only the dorsal but also the ventral thalamus may play a special role in both cortical maturation and function.

The formation of glutamatergic synapses in cultured central neurons: selective increase in miniature synaptic currents

The formation of synapses between cultured rat thalamic neurons was studied with electrophysiological and immunocytochemical methods. Thalamic neurons in culture form predominantly glutamatergic synapses. Already after 3 days in vitro glutamatergic miniature EPSCs occurred spontaneously and their frequency was strongly increased after K+ depolarization, while GABAergic mIPSCs were found after K+ depolarization at lower frequency. This demonstrates that both, excitatory glutamatergic and inhibitory GABAergic synapses were functional in close succession to initial neurite outgrowth. Synapses formed independent of spontaneous electrical activity, which was absent during the first week in culture. Spontaneous action potentials appeared during the second week and chronic action potential blockade by addition of tetrodotoxin reduced neuronal survival and the number of glutamatergic synapses per neuron. During in vitro differentiation the number of synapsin I immunoreactive presynaptic terminals and the frequency of spontaneous glutamatergic miniature EPSCs increased closely correlated, while the frequency of GABAergic mIPSCs after K+ depolarization did not increase. Thus, the continous formation of presynaptic terminals, including possible maturation of transmitter release, appeared to underlie the increase in mEPSC frequency. Analysis of miniature EPSC amplitudes at different stages in vitro revealed an increase in amplitudes, suggesting synaptic differentiation after initial establishment of functional transmission in glutamatergic synapses. This process was synapse specific as amplitudes of GABAergic mIPSCs were invariant.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the thalamic reticular and perireticular nuclei in rats and their relationship to the course of growing corticofugal and corticopetal axons

This study examines the connections of the thalamic reticular and perireticular nuclei during development. In addition, because these nuclei lie directly in the path of corticofugal and corticopetal axons during development, we have examined the relationship of these growing axons to the reticular and perireticular cell groups. Neurones were labelled by applying DiI, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), or HRP to the dorsal thalamus and/or cerebral cortex of rats at different stages of development. The axons of neurons in the reticular nucleus reach the dorsal thalamus as early as embryonic day (E) 14. At this age, and during later prenatal development, a small DiI implant limited to the presumptive lateral geniculate nucleus labels reticulothalamic and thalamocortical axons which travel in a clearly defined bundle through the thalamus. During late gestation, thalamocortical (approximately E15) and corticothalamic (approximately E17) axons pass directly through the reticular nucleus toward their targets. It is not until birth that collaterals are seen extending into the nucleus from the parent axons. Neurones in the perireticular nucleus, in contrast to those in the reticular nucleus, are not labelled from the lateral geniculate nucleus until after birth. The perireticular nucleus is very large at a stage when the first thalamocortical axons leave and when the first corticothalamic axons approach the thalamus. These axons are seen to change course sharply in the region of the internal capsule, where there are many perireticular cells. Corticothalamic axons turn toward the reticular nucleus, and thalamocortical axons turn toward the cortical subplate. Corticospinal and corticobulbar axons, on the other hand, pass directly through the perireticular region toward their more caudal targets. After these axons have reached their targets, the perireticular nucleus reduces dramatically in size.

Acetylcholinesterase as an early marker of the differentiation of dorsal thalamus in embryonic rats

The enzyme acetylcholinesterase (AChE) is transiently expressed in rats by neurons of the principal sensory thalamic nuclei, although these neurons do not use acetylcholine as a neurotransmitter. Reports that AChE expression begins at late embryonic stages led to the proposal that AChE may function in the establishment of connections, but not in earlier events. However, we find AChE reactivity in rat dorsal thalamus 5 days earlier than previously described. Cells that form the ventrobasal complex (VB), the dorsal lateral geniculate nucleus (dLG) and the medial geniculate nucleus, express AChE as they migrate and aggregate into definitive nuclei. AChE-positive cells are occasionally observed in the dorsal thalamic neuroepithelium, but are more common in others regions of the diencephalic neuroepithelium. AChE reactivity delineates VB and dLG earlier than Nissl-stained cytoarchitecture. These findings indicate that AChE is an early marker of neuronal differentiation. Certain properties of AChE, together with its early detection, are consistent with a proposed role in the migration of principal sensory neurons and their organization into discrete nuclei.

Vibrissal stimulation affects glucose utilization in the trigeminal/somatosensory system of normal rats and rats prenatally exposed to ethanol

The effect of gestational ethanol exposure on stimulus-induced sensory activity in the trigeminal/somatosensory system was determined. The mature offspring of mothers fed an ethanol-containing diet (Et) or pair-fed a nutritionally matched control diet (Ct) were examined. The C-row mystacial whiskers were stimulated. Glucose utilization in the principal sensory nucleus of the trigeminal nerve (PSN), ventrobasal thalamus, and somatosensory cortex was determined with [14C]2-deoxyglucose autoradiography. In Ct- and Et-treated rats, whisker stimulation increased glucose utilization in C-row barrel(oid)s in the left PSN, the right ventrobasal thalamus, and the right somatosensory cortex. The rate of glucose utilization in the C-row barrel(oid)s and in nonstimulated regions was lower in the Et-treated rats than in controls. In the cortices of Ct-treated rats, the activity in the C-row barrels on the right side was greater than in the right nonbarrel somatosensory cortex. Et-treated rats also exhibited an increase in glucose utilization, albeit smaller than that in the Ct-treated rats. In contrast, the glucose utilization in the left B- and C-row barrels of Ct-treated rats was decreased. No such decrease was evident in the left cortices of Et-treated rats. Thus, stroking whiskers stimulates the activity of sites in the trigeminal/somatosensory system. In cortex, the definition of these sites is emphasized by depressed activity, i.e., "surround" inhibition, in sites connected via callosal or corticocortical projections. Prenatal exposure to ethanol depresses the metabolic activity regardless of the physiological state; however, the "surround" inhibition of cortical activity is eliminated by prenatal exposure to ethanol through an exuberant projection.

Development of geniculocortical projections to visual cortex in rat: evidence early ingrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Patterns of antigenic expression in the thalamic reticular nucleus of developing rats

The present study describes the development of the thalamic reticular nucleus in rats with the use of Nissl staining and antibodies to parvalbumin and pro-alpha-thyrotropin-releasing hormone (alpha TRH). Two major subdivisions of the reticular nucleus are apparent: 1) the main body, which is itself heterogeneous and lies for the most part between the fibres of the internal capsule and external medullary lamina, and 2) the perireticular nucleus, which lies lateral to the main body and medial to the globus pallidus. In the main body of the reticular nucleus of adults, most cells in all regions are immunoreactive to parvalbumin and alpha TRH. During development there are two waves of parvalbumin and alpha TRH expression. The first wave occurs between postnatal day (P) 0 and P10, and labelled cells are apparent in rostrolateral areas of the main body of the nucleus only. At P10, such cells are not apparent. From P7 to adult, there is a second wave of parvalbumin and alpha TRH expression: labelled cells emerge first in central, then in caudal, and finally in rostral areas of the nucleus. In adults, the perireticular nucleus is made up of a few small cells which are immunostained for parvalbumin and alpha TRH. These cells are more frequent in areas of the internal capsule adjacent to the ventral regions of the main body of the reticular nucleus, rostrodorsal to the entopeduncular nucleus. From E (embryonic day) 17 to about P10, the perireticular nucleus consists of a surprisingly large population of neurones, many of which are parvalbumin and alpha TRH immunoreactive. By about P10, as in adults, there are few perireticular cells.

Prenatal development of calbindin immunoreactivity in the dorsal thalamus of the rat

The distribution of calbindin immunoreactivity was studied in the developing rat dorsal thalamus at embryonic days 14, 16, 18 and 20. At early stages (days 14-16), calbindin is expressed throughout the dorsal thalamic cell mass. Most intense labeling occurs in cells adjacent to the ventricular surface, in a spatial gradient reflecting the well-known outside-in generation pattern. Between days 16 and 20, calbindin-positive periventricular cells are redistributed in the dorsal thalamus according to two different patterns. They first become oriented tangentially within the periventricular layer, and diminish in number at the central locus where midline thalamic fusion occurs at 18 days. Periventricular calbindin immunoreactivity becomes restricted to a ring of late-born cells surrounding the gray commissure. Recognizable portions of this ring-shaped primordium will mature forming n.paratenialis, n.reuniens, n.paraventricularis, and n.subparafascicularis magnocellularis. Simultaneously, a massive contingent of radially-oriented, fusiform, calbindin-positive young neurons extends from the periventricular ring-shaped aggregate to the lateral brain surface at the caudoventral pole of the dorsal thalamus at embryonic days 17/18. These cells surround the primordium of the medial geniculate body, participating in the constitution of its marginal zone, and invade the lateral posterior nucleus, accumulating within its caudomedial part. Other portions of this stream form the parvocellular subparafascicular nucleus and the peripeduncular nucleus. The observed patterns of calbindin expression suggest that dorsal thalamic postmitotic neurons transiently express the marker during initial phases of axogenesis, whereas a specific, late-born population expresses calbindin continuously into adulthood. This late subpopulation displays migratory behavior, and finally subdivides into several nuclei of the mature midline, superficial and posterior thalamus.

Transneuronal degeneration of thalamic neurons following deafferentation: quantitative studies using [3H]thymidine autoradiography

Transneuronal degeneration of thalamic neurons following partial deafferentation was studied using [3H]thymidine autoradiography. Timed-pregnant female Sprague-Dawley rats received systemic injections of [3H]thymidine on embryonic day (E) 13, 14 and/or 15. On the day of birth, pups were anesthetized by hypothermia and subjected to unilateral enucleation, unilateral removal of the inferior colliculus or sham lesion. Animals were sacrificed on postnatal day 10 or 30 and the brains processed for autoradiography. Material from sham-lesioned animals demonstrates that neurons destined for the dorsal lateral geniculate nucleus (LGd) undergo final mitoses on E13, 14 and 15. Neurons in the ventral medial geniculate nucleus (MGv) undergo final mitoses on E13 and 14. Thirty days following neonatal unilateral eye removal, the contralateral LGd displays a loss of approximately 30-35% of [3H]thymidine labeled neurons. Neonatal unilateral removal of the inferior colliculus results in a loss of approximately 30-40% of labeled neurons in MGv. For both LGd and MGv, shorter survival times reveal less severe cell loss. Late generated (E15) LGd neurons show less severe loss following enucleation than do earlier generated neurons. These results document the degree of cell loss in sensory thalamic nuclei following deafferentation and demonstrate that [3H]thymidine autoradiography provides a useful quantitative method for assessing anterograde transneuronal cell loss in targeted populations of neurons in the developing central nervous system.

Generation and death of cells in the dorsal lateral geniculate nucleus and superior colliculus of the wallaby, Setonix brachyurus (quokka)

To study postnatal cell generation in primary visual centres of the quokka, tritiated thymidine was injected into pouch-young aged postnatal day (P)1-P85. Brains were examined at P100, just before eye-opening, when primary visual projections are essentially mature. Neurons in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) were generated at P1-P10 and P1-P18 respectively. Peak numbers of labelled cells were seen at P3 and P5 in the dLGN and SC. Cell death was assessed in the dLGN and SC of young aged P10-P150. Low numbers of dying cells were seen in the dLGN throughout this period, with a small peak at P85. A more substantial peak of cell death was seen in the SC, also at P85. In the quokka, the time interval between the peaks of cell generation and of cell death in the dLGN and SC is 70-80 days, considerably longer than the interval of 40 days between birth and death of retinal cells.

Compartmental organization of the thalamostriatal connection in the cat

The compartmental organization of the thalamostriatal connection in the cat was studied by labelling thalamic fibers in anterograde axonal transport experiments and comparing their striatal distributions with the arrangement of striosomes and matrix tissue identified by histochemical staining methods. When analyzed according to their principal compartmental targets in dorsal striatum, the thalamic deposits indicated the existence of medial and lateral divisions within the thalamostriatal projection. Nuclei of the medial division, which includes parts of the thalamic midline, projected primarily to striosomes. The lateral division, which embraces the anterior and posterior intralaminar groups, the rostral ventral tier nuclei, and parts of the posterior lateral nuclear complex, predominantly innervated matrix tissue. In the dorsal division of the nucleus accumbens, the medial system preferentially terminated in zones that stain heavily in butyrylcholinesterase and substance P preparations, but fibers from both the medial and the lateral systems largely avoided the histochemically marked compartments such as the border islands of the nucleus accumbens that are seen elsewhere in the ventral striatum. Medial division: Thalamic deposits involving the paraventricular and rhomboid nuclei of the thalamic midline elicited labelling of striosomes and, invariably, ventral extrastriosomal matrix, the nucleus accumbens, and the amygdala. This projection was topographically organized: rostral thalamic deposits elicited labelling in the medial caudate nucleus and the medial nucleus accumbens. More caudal injections produced more lateral labelling. Lateral division: The lateral division is composed of at least three projection systems distinguished by their patterns of matrix innervation. Deposits involving the anterior intralaminar nuclei and the striatally projecting cells located lateral to the stria medullaris (anterior intralaminar complex) produced an even, diffuse labelling of the matrix tissue and weak labelling of the striosomes. Injections placed in the ventroanterior, ventrolateral, and ventromedial nuclei (rostral ventral complex) elicited fibrous labelling of matrix tissue that often showed nonstriosomal inhomogeneities. Deposits involving the centromedian and parafascicular nuclei (posterior intralaminar complex) produced a highly variable pattern of matrix labelling that included both homogeneous and decidedly patchy innervations of the extrastriosomal matrix. Each of these lateral thalamostriatal systems showed a similar spatial organization, whereby dorsoventral and mediolateral thalamic axes were roughly preserved in the projection to striatum.

Distribution of phosphoprotein p19 in rat brain during ontogeny: stage-specific expression in neurons and glia

p19 is an evolutionarily highly conserved 19-kDa cytosolic protein that undergoes hormonally regulated phosphorylation in a variety of mammalian cells. Its expression is abundant in brain and testis and is developmentally regulated. Here we have used immunocytochemistry to define the cell types expressing p19 in the rat CNS during pre- and postnatal development. p19-like immunoreactivity appears in young postmitotic neurons in the mantle zone of the neural tube on embryonic day 12-13. Subsequently, it is abundant in most, if not all, early immature forms of both neurons and glia and declines to undetectable levels in fully differentiated cells. In adult brain, strong p19-like immunoreactivity remains detectable in selective regions, primarily where production of glia and neurons is known to persist, such as the subventricular zone of olfactory bulb and lateral ventricle, and the dentate gyrus. The abundance of p19 mRNA, determined by Northern blot analysis of selected brain regions, parallels the distribution of p19 assessed by immunocytochemistry, suggesting that control of p19 expression is pretranslational. Together with previous findings on the transient expression of p19 during spermatogenesis, the present data suggest that expression of p19 occurs in a number of cell lineages in a differentiation stage-dependent manner. In brain, p19 represents a new marker that may prove valuable for defining immature cell populations.

Early ingrowth of thalamocortical afferents to the neocortex of the prenatal rat

The initial ingrowth of thalamocortical afferents into the presumptive somatosensory cortex was examined in the fetal rat. Thalamic fibers were labeled in fixed brains with the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). On embryonic day 16, thalamocortical afferents arrive in the neocortex and course tangentially within the intermediate zone immediately underneath the cortical plate. By embryonic day 17, thalamocortical fibers have begun their radial growth into cortex and their arbors span the cell-sparse zone between layer VIb and the bottom of the cortical plate. By the day of birth (embryonic day 21), thalamocortical fibers from a dense plexus within layers VI and V below the dense cortical plate. Our observations indicate that in the rat thalamic afferents arrive in the cortex at a very early age and arborize within the forming cortical layers without an apparent "waiting" period.

Histochemical study of RNA content of neurones in the dorsal lateral geniculate nucleus during postnatal development

Nuclear and cytoplasmic dLGN neurons were investigated by cytophotometric measurements of RNA. This study has been carried out in rats from birth to adulthood. In order to quantify the RNA content a cytophotometer was used. Extinction mean values were obtained which indicated RNA concentrations per surface unit. The nuclear and cytoplasmic surface were calculated simultaneously and from the product of the mean extinction and the surface the RNA total content was calculated. Our results have suggested that the changes are age-related. From day 1 to day 21 the neuronal size and RNA content increase; this may somehow be involved with the differentiation process. Around post-natal day 21 neuronal maturation may begin, reaching its optimal phase around day 42, on which the RNA concentration per surface unit, surface neuronal content and RNA total content are stable.

A major direct GABAergic pathway from zona incerta to neocortex

Retrograde fluorescent tracers were used to demonstrate a previously unknown but sizable direct gamma-aminobutyric acid (GABA)-containing neuronal pathway from the zona incerta to the neocortex in rats. This incertocortical pathway was found to project bilaterally to the entire neocortex and exhibited a rough corticotopic organization. Many of the zona incerta neurons projecting to the parietal and occipital cortices could also be immunohistochemically stained with antibodies to glutamic acid decarboxylase and GABA. Few of these neurons were immunoreactive to tyrosine hydroxylase antibodies, which identify dopamine-containing neurons. Injections in the frontal and entorhinal cortices labeled many neurons near or within the dopaminergic A13 subdivision of the zona incerta. In addition, the incertocortical system was found to be significantly larger during early postnatal (2 to 3 weeks) development. The projection pattern of this newly discovered pathway resembles that of the monoaminergic and cholinergic systems, arising from the brainstem and forebrain, suggesting possible similarities of function.

Dissimilar responses of adult thalamic monoaminergic and somatosensory afferent fibers to implantation of thalamic fetal cells

It is generally accepted that transplanted fetal neurons can, after several weeks to months, establish connections with the host CNS. Host afferent systems seem, however, to show different types of responses to the presence of grafted fetal neurons. The present study is a preliminary step to identify mechanisms involved in the reactions of adult axons to transplanted fetal neurons. The right ventrobasal thalamus of adult rats was depleted of neurons by in-situ injection of kainic acid and cell suspensions from homotopic thalamic embryonic primordia which were injected into the lesioned area. After various post-implantation delays, ranging from five to 30 days, two types of experiments were performed: (i) noradrenaline and serotonin immunohistochemistry with specific antibodies on alternate sections; and (ii) anterograde tracing using wheat germ agglutinin conjugated to horseradish peroxidase from the dorsal column nuclei and the principal sensory trigeminal nucleus. Five days after transplantation, host monoaminergic fibers (either noradrenergic or serotoninergic) had already grown into the transplants. Ingrowing fibers were thin and poorly varicose, exhibiting endings morphologically similar to the growth cones observed during axogenesis. Seven days after grafting, growth cones were no longer visible and monoaminergic fibers exhibited either normal-sized or very large varicosities. Large varicosities progressively decreased in number and, after three weeks, the fibers displayed a normal adult morphology, forming a dense network all over the transplants. In contrast, host somatosensory afferents, labeled by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, did not grow into the transplants. Intermingling of somatosensory afferents and transplanted cells was observed only after 10 days, when grafted neurons extended outside the original transplantation site into the neuron-depleted area containing the somatosensory afferents. The present results demonstrate that adult monoaminergic and somatosensory afferents, when deprived of their usual target, do not react in a similar way to the addition of fetal neurons. It is proposed that adult monaminergic fibers have the ability to regain morphological (and probably functional) immature forms which were considered to be restricted to the period of axogenesis or to lesion-induced regeneration. In contrast, fetal transplants do not seem to induce, by themselves, a similar alteration of genetic expression in adult somatosensory neurons. It has been proposed that "diffuse" and "point-to-point" axonal systems may be differentiated in the CNS on anatomical bases. The present results add to the identification of two different systems by demonstrating that, in the thalamus, they present dissimilar responses to the implantation of fetal cells.

Patterns of transiently expressed acetylcholinesterase activity in cerebral cortex and dorsal thalamus of developing rats with cytotoxin-induced microencephaly

Previous studies have demonstrated that acetylcholinesterase (AChE) activity is expressed transiently by thalamocortical neurons of primary sensory systems in developing rat pups. In the present study, prenatal treatment with methylazoxymethanol acetate (MAM) on embryonic day 15, 16, or 17 resulted in rat pups with cerebral cortices markedly reduced in thickness and areal extent. Histochemical studies demonstrated that AChE staining occurs in fiber-like plexuses in primary visual, auditory, and somatosensory regions of developing cerebral cortex of MAM-treated animals, just as in normal developing rats, but that the transient patterns of AChE are found more superficially than normal and they occur in an abnormal patchy distribution. Neuronal somata in thalamic lateral geniculate, medial geniculate and ventral basal nuclei of MAM-treated animals show transient AChE staining indistinguishable from that seen in normal animals. These data indicate: (1) AChE is expressed transiently by thalamocortical neurons in MAM-treated animals, (2) intensity of the transiently expressed AChE is not affected by MAM-induced loss of cortical neurons, and (3) the abnormal AChE patterns in cortex likely reflect the abnormal distributions of thalamocortical terminal fields that are characteristic of MAM-treated animals.

Development of the rat thalamus: IV. The intermediate lobule of the thalamic neuroepithelium, and the time and site of origin and settling pattern of neurons of the ventral nuclear complex

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, settling pattern, migratory route, and site of origin of neurons of the ventral nuclear complex of the thalamus. Quantitative examination of long-survival radiograms established that the bulk of the neurons of the ventral nuclear complex are generated between days E14 and E16 but with statistically significant differences between its three nuclei. The ventrobasal nucleus is the oldest component (97% of the cells are generated on days E14 and E15); the ventrolateral nucleus is next (82% of the cells are generated on days E14 and E15); and the ventromedial nucleus is last (51% of the cells are generated on days E14 and E15). In addition to this caudal-to-rostral (from the ventrobasal nucleus to the ventrolateral nucleus) and lateral-to-medial (from the ventrobasal nucleus to the ventromedial nucleus) internuclear gradients, there are lateral-to-medial and ventral-to-dorsal intranuclear neurogenetic gradients within the ventrobasal and ventrolateral nuclei. Qualitative examination of short and sequential survival thymidine radiograms indicate that the neurons of the ventral nuclear complex originate in the unique intermediate thalamic neuroepithelial lobule, which is distinguished from the rest of the thalamic neuroepithelium by the presence of a mitotically active secondary neuroepithelial matrix. Two sublobules can be distinguished in the intermediate lobule during the early stages of thalamic development. On the basis of their location and chronological pattern of cell production and differentiation, it is inferred that the neurons of the ventrobasal nucleus originate in the earlier differentiating, posteroventrally situated inverted sublobule, and the neurons of the ventrolateral nucleus are produced in the later differentiating, anterodorsally situated everted sublobule. The neurons of the ventromedial nucleus appear to originate from the intermediate neuroepithelial lobule after its two sublobules are no longer distinguishable. The heavily labeled neurons generated soon after injection on day E15 form a wave front that translocates in a lateral direction at a steady rate of 215 microns/day. Examination of methacrylate-embedded materials showed that, in day E15 rats the actively migrating cells are spindle-shaped, with their long axis oriented horizontally. The far-laterally situated differentiating cells (the oldest neurons) become vertically oriented by day E16. Associated with this change in polarity, vertically oriented fibers appear among the cells. These fibers can be traced to the inte

Development of the rat thalamus: VI. The posterior lobule of the thalamic neuroepithelium and the time and site of origin and settling pattern of neurons of the lateral geniculate and lateral posterior nuclei

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to determine the time of origin, site of origin, migratory route, and settling pattern of neurons of the dorsal lateral geniculate (LGD), ventral lateral geniculate (LGV), and lateral posterior (LP) nuclei of the thalamus. Quantitative examination of long-survival radiograms established that the neurons of the LGD are produced on days E14 and E15. Within the LGD there is an external-to-internal neurogenetic gradient; the majority (77%) of neurons of the external half are generated on day E14, while in the internal half the majority (64%) of neurons originate on day E15. The late-generated LGD neurons are located in the termination field of the uncrossed fibers of the optic tract. Examination of short-survival radiograms indicated that the neurons of the LGD originate in a discrete neuroepithelial eversion situated ventral to the pineal rudiment and dorsal to the putative neuroepithelium of the ventral nuclear complex. In sequential radiograms from rats injected with 3H-thymidine on day E15 and killed on days E16 and E17, the migration of young LGD neurons was followed in a posterolateral direction to the formative lateral geniculate body. By day E17, the day when the optic tract fibers begin to disperse over the lateral surface of the posterior diencephalon, the distribution of early and late-generated neurons of the LGD resembles that seen in young pups. As a whole, the neurons of the LGV are produced earlier than the neurons of the LGD. The bulk of LGV neurons are generated on days E14 and E15 in a caudal-to-rostral intranuclear neurogenetic gradient. Caudal LGV neurons are generated mainly on day E14 (82%), while a substantial proportion of rostral neurons (32%) are generated on day E15. Examination of short-survival and sequential radiograms suggest that the LGV neurons originate in an inverted sublobule situated beneath the putative neuroepithelium of the LGD. At anterior levels the putative inverted sublobule of the LGV merges imperceptibly with the neuroepithelium that produces the neurons of the lateral habenular nucleus. Like the neurons of the LGD and LGV, so also those of the LP are generated on days E14 and E15, but the neurogenetic gradients are different. There is a lateral-to-medial gradient within the LP as a whole. Peak production of neurons is on day E14 laterally (58%) and on day E15 medially (59%).(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the rat thalamus: V. The posterior lobule of the thalamic neuroepithelium and the time and site of origin and settling pattern of neurons of the medial geniculate body

Long-survival, sequential, and short-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, site of origin, migratory route, and settling pattern of neurons of the medial geniculate body (MG). Quantitative evaluation of long-survival radiograms established that the bulk of MG neurons are generated between embryonic (E) days E13 and E15, with a pronounced peak on day E14. There is an overall lateral-to-medial and caudal-to-rostral chronological gradient in MG neurogenesis. On the basis of significant regional differences in the birth dates of neurons, the MG was divided into several chronoarchitectonic areas. The earliest-generated neurons (with close to 20% of the cells produced on day E13 and a negligible proportion on day E15) form the dorsal and ventral clusters far laterally. Next in sequential order are the neurons of the lateral shell, intermediate shell, and medial shell of the MG. The medial shell with it latest-generated neurons (with over 30% produced rostrally on day E15) corresponds to the medial (magnocellular) subnucleus of the MG. There were no neurogenetic differences between the traditional dorsal and ventral divisions of the MG. Examination of sequential radiograms in rats labeled with 3H-thymidine on day E14 or E15 and killed on successive days brought supportive evidence for our earlier identification, in short-survival radiograms, of a posteroventral thalamic neuroepithelial evagination as the putative source, or committed cell line, of MG neurons. Wave fronts of apparently migrating unlabeled and labeled cells could be traced from this sublobule in a posterolateral direction to the future site of the MG.

Neonatal enucleations reduce number, size, and acetylcholinesterase histochemical staining of neurons in the dorsal lateral geniculate nucleus of developing rats

Previous studies have demonstrated that transient patterns of acetylcholinesterase (AChE) activity are characteristic of geniculo-recipient regions of rat cortical area 17 during the second and third postnatal weeks of life. Neonatal enucleation results in a marked reduction of this transiently expressed cortical AChE. Parallel studies have demonstrated that the dorsal lateral geniculate nucleus (dLGN) also expresses AChE transiently during development. The present study examines neuronal number and size as well as AChE histochemical staining in the dLGN of normal and neonatally enucleated rat pups to determine whether changes in dLGN neurons could account for the decreased visual cortical AChE staining that results from neonatal enucleation. Changes in 4 parameters in dLGN were noted after neonatal enucleation. First, a 26-37% shrinkage in the volume of dLGN occurred contralateral to enucleation. Second, enucleation resulted in a loss of 16-30% of AChE-stained neuronal somata. Third, remaining AChE-positive neuronal somata appeared shrunken by approximately 40%. Fourth, intensity of AChE histochemical staining of individual dLGN neurons was reduced by approximately 24% following neonatal enucleation. These data suggest that loss of transient AChE activity in cortical area 17 consequent to neonatal enucleation is secondary to enucleation-induced alterations in the dLGN; these alterations include loss of neurons, shrinkage of neurons, and an apparent decrease in the ability of neurons to synthesize AChE. These data support the hypothesis that geniculocortical projection neurons express AChE transiently during development of geniculocortical connectivity and indicate that normal afferent connections and/or activity are important for the transient expression of AChE by these neurons.

Ontogeny of cholecystokinin gene expression in the rat thalamus--a hybridization histochemical study

We examined the ontogeny of cholecystokinin (CCK) gene expression in the rat thalamus using hybridization histochemistry. Maturation of CCK gene expression occurred over the course of a month, beginning in the medial geniculate nucleus on the 17th day of gestation. CCK mRNA was found in the caudal nuclei first, later in the rostral; the ventral were followed by the dorsal; and the lateral by the medial. In some nuclei, such as the medial geniculate, CCK mRNA was present prior to the full differentiation of the neurons; whereas in others, such as the mediodorsal, the neurons differentiated to almost adult morphology before CCK mRNA was detected.

Development of the thalamocortical system: transient-crossed projections to the frontal cortex in neonatal rats

The developmental remodeling of thalamic projections to frontal and prefrontal cortical fields was investigated in the rat by using a double retrograde tracing technique. Bilateral cortical injections of fluorescent tracers were made either in neonatal (first or second postnatal day) or in adult animals. In neonates, the cell populations retrogradely labeled from each cortical injection overlapped in a medial thalamic region that included the midline nuclei and the medial part of the mediodorsal nucleus, ventral medial nucleus, and nucleus gelatinosus. In adults, the overlap region was confined within the boundaries of the midline nuclei. Quantitative analysis showed that this overlap area was three times as wide in neonates as in adults. The neurons located in this region projected unilaterally both in neonatal and adult animals; bilaterally projecting cells were virtually absent. In neonates, a second set of contralaterally projecting neurons was found in more lateral thalamic regions. This population consisted of cell clusters in the dorsal part of the central lateral nucleus and in the lateral part of the ventral medial nucleus; scattered cells were also observed throughout other nuclei. This second cell population was represented in part by neurons bifurcating bilaterally. In adult animals, neurons projecting contralaterally were observed only occasionally in the lateral thalamus. The present results demonstrate that the bilaterality of thalamocortical projections undergoes a reduction during postnatal development. The mechanisms underlying this remodeling and the possible functional role of the transient-crossed thalamocortical system are discussed.

First month of development of fetal neurons transplanted as a cell suspension into the adult CNS

It has been demonstrated elsewhere that fetal thalamic tissue, when transplanted as a cell suspension into the excitotoxically neuron-depleted adult somatosensory thalamus, can grow, differentiate, and receive projections from host afferents. In the present study, we used the same paradigm to analyse the transplanted neurons during their morphogenesis, i.e. during the first month after transplantation. Using various anatomical criteria, at the light and electron microscope levels, we compared the development of transplanted neurons with the normal ontogeny of homologous neuronal populations. Confined solely to the mechanically lesioned area during implantation at seven days post-grafting, the transplant increased in size to occupy most of the previously neuron-depleted area by the third week after grafting. The final size of the transplant thus depended upon the size of the lesion. At seven days post-grafting, the neurons were small in size and the cellular density was high. At this immature stage few synaptic contacts were visible and the ultrastructure was characterized by large extracellular spaces. At 10 days post-grafting, the size of the neurons had increased and the cellular density had decreased. Both an extensive dendritic proliferation and a simultaneous active synaptogenesis could also be observed. All these events continued to evolve and during the third week the neuropil progressively acquired more mature ultrastructural characteristics. Synaptic contacts exhibiting characteristics comparable to those observed in the intact thalamus also became more numerous. At 20 days post-grafting, axonal myelination had started, the development of the graft apparently stopped and the various criteria had stabilized. Until that developmental stage, growth of grafted neurons compared to that of normal thalamic ones. At later stages, however, grafted neurons failed to grow larger and did not reach the size of the homologous population in the adult animal. It seems, therefore, that transplants of thalamic fetal neurons can be used as a tool with which to study thalamic neuronal development, within definable limits.