Isthmic origin of neurons of the rat substantia nigra

Control of neuronal diversity by the floor plate: contact-mediated induction of midbrain dopaminergic neurons

The notochord and floor plate contribute to patterning the ventral neural tube in part by expressing a diffusible factor that induces motoneurons. To determine the mechanisms that direct the development of other classes of ventral neurons, we studied the development of dopaminergic neurons that reside near motoneurons in the ventral midbrain. We provide evidence that dopaminergic neurons develop in the vicinity of the floor plate and that they can be specified by the floor plate in vitro and in vivo. Unlike motoneurons, efficient induction of dopaminergic neurons requires contact with floor plate cells. These results suggest that neuronal diversification along the dorsal-ventral axis may be achieved partly through the concerted action of diffusible and contact-dependent signals from a single organizing center, the floor plate.

Development of catecholamine systems in the brain of the lizard Gallotia galloti

For a better insight into general and derived traits of developmental aspects of catecholaminergic (CA) systems in amniotes, we have studied the development of these systems in the brain of a lizard, Gallotia galloti, with tyrosine hydroxylase (TH)- and dopamine (DA) immunohistochemical techniques. Two main groups of TH-immunoreactive (THi) perikarya appear very early in development: one group in the midbrain which gives rise to the future ventral tegmental area, substantia nigra and retrorubral cell groups, and another group in the tuberomammillary hypothalamus. Somewhat later in development, TH/DA-immunoreactive cells are observed in the thalamus, rostrodorsal hypothalamus and spinal cord, and, with another delay, in the suprachiasmatic nucleus, the periventricular organ, and the pretectal posterodorsal nucleus. CA cell groups that appear rather late in development include the cells in the olfactory bulb, the locus coeruleus and the caudal brainstem. As expected, the development of immunoreactive fibers stays behind that of the cell bodies, but reaches the adult-like pattern just prior to hatching. The present study revealed considerable variation in the relation between the state of cytodifferentiation and first expression of TH/DA immunoreactivity between CA cell groups. Catecholamine cells in the midbrain and tuberomammillary hypothalamus are still migrating, immature (absence of dendrites) and express only TH immunoreactivity at the time of first detection. Cells which appear at later developmental stages lie already further away from the ventricle, possess two or more dendritic processes, and generally express both TH- and DA immunoreactivity.

Postnatal changes in the distribution and morphology of rat substantia nigra dopaminergic neurons

Significant changes in the neurophysiology and neuropharmacology of nigral dopaminergic neurons take place in the first postnatal month. In order to correlate these changes with the postnatal development of dopaminergic neuron morphology and substantia nigra cytoarchitecture, brains from Sprague-Dawley rat pups of age postnatal days 1, 7, 14, 21 and 28 and adult rats were sectioned and processed for tyrosine hydroxylase immunocytochemistry. At postnatal day 1, pars compacta and pars reticulata were not clearly delineated; tyrosine hydroxylase positive neurons and a dense plexus of fibers were scattered throughout the substantia nigra. By day 7 the density of tyrosine hydroxylase positive neurons decreased markedly in ventral substantia nigra, and a dopaminergic pars compacta and a non-dopaminergic pars reticulata could be more clearly distinguished. By day 14 the substantia nigra appeared essentially as it does in the adult. Cell counts during development revealed that the number of tyrosine hydroxylase positive neurons/section in both pars compacta and pars reticulata decreased significantly from postnatal day 1 to postnatal day 14, while those in pars lateralis did not change. Tyrosine hydroxylase-positive somatic size increased modestly but significantly from postnatal day 1 to day 14 as did the diameter of the proximal and distal dendrites. However, even at day 1, the morphology of tyrosine hydroxylase positive neurons appeared essentially the same as in adults. Dendritic arborizations were well developed. The dendrites were non-varicose and modestly branched, with some of the longer ventrally directed dendrites passing through pars reticulata into the crus cerebri.(ABSTRACT TRUNCATED AT 250 WORDS)

Pattern formation in the developing superior colliculus: ontogeny of the periodic architecture in the intermediate layers

The superior colliculus of mammals contains a striking neurochemical architecture in which histochemically identifiable compartments are distributed in an iterative arrangement in the intermediate layers. We used stains for acetylcholinesterase activity as a compartment marker to trace ontogenesis of this architecture during pre- and postnatal development in the domestic cat. We found that compartmentation in the intermediate collicular layers is virtually absent at birth, and only gradually emerges during the first weeks of postnatal life. Over the same postnatal period, acetylcholinesterase activity shifts from a predominantly perikaryal expression pattern immediately postnatally to a nearly exclusive localization in the neuropil at maturity. Remarkably, a striking compartmentation of the superior colliculus was readily apparent with acetylcholinesterase histochemistry prenatally. The first appearance of a periodic architecture in the superior colliculus was observed at embryonic day 34, a time at which the collicular plate had not yet become laminated. The compartments characterized by high levels of acetylcholinesterase activity then gained in prominence until late in the prenatal period, when they receded and disappeared. The loss of the acetylcholinesterase-positive compartments in the perinatal period did not reflect a loss of compartmentation altogether. Neonatally, there was a distinct compartmental architecture visible with enkephalin immunohistochemistry. The virtual absence of acetylcholinesterase-positive compartments in the superior colliculus at birth therefore reflects developmental regulation of enzyme expression in the compartments, not regulation of the compartments as structural entities. We conclude that the periodic architecture, which characterizes the intermediate collicular layers in the adult cat, arises early in ontogenesis. These observations raise the possibility that the histochemical compartments are ontogenetic units that undergo remodeling as the superior colliculus matures.

Prenatal ontogeny of tyrosine hydroxylase gene expression in the rat ventral mesencephalon

We have examined the development of dopaminergic (DA) neurons in the embryonic mesencephalon with regard to the expression of the gene coding for tyrosine hydroxylase (TH). Mesencephalic DA neurons from rat embryos aged E13 to E21 were analyzed using a quantitative in situ hybridization protocol featuring a 35S-labeled RNA probe complimentary to TH mRNA. In the early-to-mid stage embryonic brains, the expression of the TH gene was examined relative to the position of individual, migrating DA cells in the caudal-rostral and dorsal-ventral axes of the mesencephalon. In the later embryonic subjects, neurons were analyzed according to their position in one of the midbrain DA nuclei. The ontogeny of TH gene expression in the rat mesencephalon exhibited two phases: during the early phase (E13-E15), we observed major fluctuations in the level of TH gene expression accompanying the differentiation and maturation processes of the DA cells. Later, in the mid-to-late gestation fetus (E18-E21), TH gene expression generally stabilized as TH mRNA-expressing neurons reached their final anatomical positions within the mesencephalic DA complex. Our data demonstrate the complex dynamics which characterize the ontogeny of TH gene expression in the prenatally developing mesencephalon, and suggest a connection between the maturational level of DA neurons and the expression of the key gene regulating their principle neurotransmitter.

Neonatal hypoxic-ischemic or excitotoxic striatal injury results in a decreased adult number of substantia nigra neurons

It has been shown that morphologic and biochemical presynaptic markers of dopaminergic terminals are preserved in a unilateral experimental model of neonatal hypoxic-ischemic injury to the striatum. As the substantia nigra is spared direct injury in this model, we anticipated that the number of tyrosine hydroxylase-positive dopaminergic neurons projecting to the striatum would also be normal. We have found, however, that following unilateral neonatal striatal injury the number of ipsilateral tyrosine hydroxylase-positive neurons is decreased, as is the mean area of the substantia nigra pars compacta. The decrease in neurons is correlated with the decrease in striatal size (r = 0.7, P = 0.01). Neuron loss is most pronounced in the substantia nigra pars reticulata, where it is 50%. Calbindin-positive neurons in the dorsal tier of the substantia nigra pars compacta appear to be preserved. We also examined effects on the nigra following a neonatal excitotoxic striatal lesion made with quinolinic acid. We observed a decrease in the number of substantia nigra tyrosine hydroxylase-positive neurons in the absence of direct nigral injury, and the decrease was closely correlated with reductions in striatal area (r = 0.91, p < 0.01). While there are a number of possible explanations for these observations, one major possibility is that there has been a reduction in tyrosine hydroxylase-positive neurons due to a diminution in developmental target-derived trophic support from the striatum. If striatum-derived trophic support plays a role in the developmental regulation of substantia nigra neuron number, then abnormalities in this supportive relationship may play a role in the loss of these neurons in some animal models of developmental nigral degeneration, and some forms of human parkinsonism.

An immunohistochemical study of the telencephalon of the senegal bichir (Polypterus senegalus)

The telencephalon in ray-finned fish (actinopterygians) is everted, in contrast to the evaginated telencephalic hemispheres in all other vertebrates. In the more derived ray-finned fish, the teleosts, proliferation of neurons and their migration from the ependymal zone of the pallium renders comparisons between telencephalic cell groups of the teleosts and members of other vertebrate groups extremely difficult. The telencephalon of Polypterus (a primitive living ray-finned fish), although everted, is cytoarchitecturally much simpler than that of teleosts. We have thus applied immunohistochemical techniques to the study of the telencephalon of Polypterus to help clarify the evolution of the telencephalon in teleosts and facilitate comparisons between the telencephalon in ray-finned fish and other vertebrates. Antisera against the following neuroactive substances were used: 1) serotonin (5HT), 2) tyrosine hydroxylase (TH), 3) substance P (SP), 4) leucine-enkephalin (ENK), 5) neuropeptide Y (NPY), and 6) the neurotensin-related hexapeptide LANT6. Several features of the labeling patterns obtained suggested that the dorsal and ventral subdivisions of the area ventralis are homologous as a field to the basal ganglia and septum plus other basal telencephalic regions of land vertebrates, sharks and lungfish: 1) an abundance of SP+, NPY+, and ENK+ fibers; 2) an abundance of TH+ fibers, possibly of posterior tubercle/tegmental origin; 3) the presence of an SP+ fiber bundle that appeared to descend from basal telencephalic levels and terminate in the posterior tubercle/tegmentum, which contain TH+ (possibly dopaminergic) neurons; and 4) an abundance of 5HT+ fibers, presumably of posterior tubercle/tegmental origin. It was not possible, however, to recognize distinct pallidal and striatal subdivisions within the area ventralis of Polypterus. The olfactory pallium (P1) was generally poor in most of the substances examined, except for the presence of LANT6+ fibers. The P3 pallial field was conspicuously rich in SP+ and ENK+ fibers throughout its extent, and the caudal and lateral parts of the P2 field were rich in SP+ fibers and ENK+ fibers. Since this is characteristic of the medial pallial and/or dorsomedial pallial walls of the telencephalon in lungfish, sharks, frogs, and reptiles, the P3 field and caudolateral part of the P2 field may be homologous to these portions of the telencephalon in other vertebrates. More rostromedial parts of P2 may correspond to those parts of the pallium in land vertebrates that are in receipt of specific sensory input from the thalamus, since low neuropeptide levels are characteristic of these regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Mesencephalic dopaminergic cells exhibit increased density of neural cell adhesion molecule and polysialic acid during development

In the developing mesencephalon of the rat, the dopaminergic neurons are generated in the ventricular zone of the basal plate between E11 and E15 and then migrate along radial glia to the ventral surface of the developing mesencephalon. To study the factors that control migration and maturation of the dopaminergic neurons, we immunolabeled embryo and pups, ages E12-P21, for neural cell adhesion molecule (NCAM), polysialic acid (PSA) - a polysaccharide found in high amounts on NCAM during development, tyrosine hydroxylase (TH) - a marker of mesencephalic dopaminergic cells, and vimentin - the major cytoskeletal protein in radial glia in the rat. At E13, we noted that cells throughout the mesencephalon contained NCAM-immunoreactive (NCAM-IR) material but that cells along the ventral surface of the mesencephalon contained an increased amount of NCAM-IR material and PSA-immunoreactive (PSA-IR) material. At this age, we first noted a small number of TH-immunoreactive (TH-IR) cells adjacent to the marginal zone of the ventral surface of the mesencephalon. Many of the TH-IR cells contained an increased density of NCAM-IR material. At age E14, the pattern of increased density of NCAM-IR material on cells along the ventral surface of the mesencephalon persisted and a conspicuous amount of PSA-IR material was also noted on cells in this region. TH-IR cells were more numerous, and a striking number of the TH-IR cells also contained an increased amount of NCAM-IR material and PSA-IR material. With increasing age the distribution of NCAM-IR material and PSA-IR material in the mesencephalon became more uniform. Our work suggests that NCAM may be involved in control of migration and synthesis of TH in the dopaminergic cells of the developing mesencephalon.

Development of dopaminergic neurons in the human substantia nigra

A series of 29 human embryonic brains were examined in order to characterize the ontogeny of dopaminergic neurons within the developing substantia nigra. Embryos from Postconception Weeks 4.0 to 11.2 (last menstrual period 6.0-13.2) were studied. Immunohistochemical staining was performed using a polyclonal antibody to tyrosine hydroxylase. Tyrosine hydroxylase-like immunoreactivity was first seen in cells of the ventral mesencephalon at 6.5 weeks adjacent to the ventricular zone. Ventral migration of TH-positive cells began at 6.7 weeks. Neural process extension was first identified in tyrosine hydroxylase-positive neurons at 8.0 weeks. The ascending nigrostriatal bundle was also first demonstrated at 8.0 weeks. Tyrosine hydroxylase containing neurites were seen initially in the developing putamen at 9.0 weeks. Only a few tyrosine hydroxylase-positive cells remained adjacent to the ventricular zone at Week 10.0 and all had disappeared from the ventricular zone by 11.2 weeks. At this latter stage, a large number of dopaminergic cells had elaborated neural processes. The sequence of developmental events of human mesencephalic dopaminergic neurons is similar to the equivalent period of ontogeny in other mammals. The duration of the developmental period is, however, significantly protracted.

Characterization of [3H]dopamine uptake sites and [3H]cocaine recognition sites in primary cultures of mesencephalic neurons during in vitro development

[3H]Dopamine uptake and [3H]cocaine binding sites were studied in primary cultures of ventral mesencephalon from 14-day-old rat embryos. Specific binding sites for [3H]cocaine and [3H]mazindol were detected only in intact cell cultures of ventral mesencephalon, and were absent in sonicated, washed membranes prepared from these cell cultures. [3H]Cocaine was not taken up by the cells through an active transport process because [3H]cocaine binding occurred also at 4 degrees C. Moreover, the possibility of [3H]cocaine entering the cells by passive diffusion and ion trapping was also excluded because extensive washing failed to remove [3H]cocaine from the cells. [3H]Cocaine binding was reduced to 6% of control when cells were permeabilized with streptolysin O (0.2 U/ml, 5 min). Taken together, these results suggest that in cultured mesencephalic neurons, [3H]cocaine may enter the cell by passive diffusion and then be sequestered by a cytosolic compartment that is lost in the process of permeabilization or sonication and washing of membrane preparations. Permeabilization of cultured neurons failed to alter the storage of [3H]dopamine. When cells were permeabilized with streptolysin O (0.2 U/ml; 5 min) after [3H]dopamine was taken up, [3H]dopamine was retained by the cells and did not leak into the incubation medium, indicating that [3H]dopamine was stored in sites that could not pass through the perforated membranes. In contrast, [3H]dopamine uptake into already permeabilized cells was reduced by 33%, suggesting that a cytosolic protein that had leaked out may play a functional role in the uptake process. In contrast to striatal membrane preparations of adult rats, [3H]cocaine binding in intact mesencephalic cell cultures was Na+ independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.

The human brain at stages 21-23, with particular reference to the cerebral cortical plate and to the development of the cerebellum

The development of the human brain during the eighth embryonic week was studied in serial sections of 22 embryos, and graphic reconstructions were prepared. The cortical plate appears in stage 21 in the area of the future insula and is an excellent feature for staging. The internal capsule contains neocortical fibres. Its three main outlets begin to be present in stage 22 and lead to epithalamus, to dorsal thalamus, and to mesencephalon. At this time a well developed lateral olfactory tract can be seen. The anterior commissure appears in stage 23. A clear developmental relationship between claustrum and olfactory area is described for the first time in human embryos. The optic tract reaches the ventral area of the lateral geniculate body. Scattered fibres of the lateral lemniscus reach at least as far as the caudal mesencephalon, in which superior and inferior colliculi can be distinguished at stage 23; two caudal Blindsäcke containing ventricular recesses form in stage 23. The cerebellum is still present as a plate, but its internal bulge is considerably enlarged. It possesses radially- and tangentially-arranged cells; the latter form the external germinal layer. The dentate nucleus, as well as the inferior and superior cerebellar peduncles and some of the cerebellar commissures, are present. Compared with the highly developed and probably already functional remainder of the hindbrain, the cerebellar plate shows far less differentiation. Two caudal migratory streams (marginal and submarginal) are present and represent the corpus pontobulbare. The decussation of the pyramids appears in stage 23. This article concludes the study of the developing human brain during the embryonic period, from stage 8 to stage 23. The series was based on 340 serially-sectioned embryos and graphic reconstructions from 89 brains. No comparable investigation of the fetal brain is available.

Ontogeny of tyrosine hydroxylase and cholecystokinin gene expression in the rat mesencephalon

The ontogeny of tyrosine hydroxylase and cholecystokinin gene expression was studied in the rat mesencephalon using hybridization histochemistry. Both transcripts appeared on E13 in the ventrocaudal mesencephalon. The levels of both transcripts increased synchronously during the second half of gestation. The locations of neurons containing either transcript changed similarly during development with a rostral transposition and a lateral extension of the respective areas covered with grains. On the day after birth, the patterns of expression for both genes, although at lower transcript levels, were similar to the patterns seen in the adult.

Dopaminergic cells align along radial glia in the developing mesencephalon of the rat

Studies were performed to examine the relation of dopaminergic cells and radial glia in the developing mesencephalon of the rat at ages E12-E20. Dopaminergic cells were immunolabelled with an antiserum which recognizes tyrosine hydroxylase, and radial glia were immunolabelled with a monoclonal antibody which recognizes vimentin. The vimentin-immunoreactive fibres of radial glia were noted at E12. At E12, and more clearly at later time points, the radial glia extended from the aqueduct to the pial surface, and this pattern persisted throughout the prenatal period. Tyrosine hydroxylase-immunoreactive cells were located along the ventral surface of the mesencephalon at age E13. At age E15, E16, and E18 the tyrosine hydroxylase-immunoreactive cells were present from the aqueduct to the ventral pial surface of the mesencephalon and were aligned along radial glia. Our study suggests that radial glia provide paths for migration of dopaminergic cells in the mantle layer from E15 to E18 of the developing mesencephalon. It also suggests that some dopaminergic cells between E15 and E18 may express tyrosine hydroxylase during their migration through the mantle layer and prior to reaching the location they occupy in the adult brain.

Early appearance of tyrosine hydroxylase immunoreactive cells in the mesencephalon of mouse embryos

The development of mesencephalic catecholaminergic neurons in the embryonic and fetal mouse was analysed in tissues fixed with 5% acrolein using polyclonal rabbit antibodies against tyrosine hydroxylase (TH), the first enzyme in catecholamine synthesis. The first TH positive cells were identified as early as day 8.5-9 of gestation and some expressed TH while apparently still migrating from the proliferative layer. The number of catecholamine cells increased dramatically by embryonic day 9.5-10; at gestation days 10.5-11 numerous TH positive cells bearing many neurites were localized in the ventral part of the mesencephalon but they were not yet separated into two different groups (A9 and A10). After 13 days of gestation two separate catecholaminergic groups could be visualized, although many TH positive cells with long neurites (putative dopaminergic neurons) could still be seen at the edges of the ventricle, and appeared to be moving towards the ventral mesencephalon. On the basis of these results the possibility that catecholamine cells that are produced early during the development of the midbrain may have neurotrophic and/or morphogenetic roles is discussed.

A fluorescence histochemical method for the demonstration of central catecholamine neurons in young embryonic tissues

A simple fluorescence histochemical technique is described for the production of catecholamine histofluorescence in the developing brain of rat embryos. The procedure combines the magnesium-formaldehyde-glutaraldehyde perfusion technique, with polyethylene glycol dehydration and embedding. Several examples illustrate the sensitivity of the technique and the good preservation of tissue sections. The tissue can also be stored for several weeks without an appreciable loss of fluorescence.

Pattern formation in the striatum: developmental changes in the distribution of striatonigral projections

The mammalian striatum (the major subcortical structure in the telencephalon) can be divided into two compartments, the patch and the matrix, on the basis of various neurochemical and hodological markers expressed in the adult. The primary efferent target of striatal neurons is the substantia nigra. We have previously shown that the patch compartment sends projections to the substantia nigra embryonically; whereas the matrix does not form a similar projection until the early postnatal period (Fishell and van der Kooy, J. Neurosci., 7 (1987) 1969-1978). The projection of patch neurons to the substantia nigra is the earliest developmental marker for the patch compartment. Here we ask about the early distribution of patch projections and their possible relation to striatal compartmentalization. Embryonic anterograde axonal tracing of the striatonigral pathway can take advantage of the temporal separation of patch versus matrix projections to reveal the terminal distribution of patch striatonigral neurons independent of the nigral terminal distribution from the striatal matrix. The anterograde tracer rhodamine isothiocyanate was shown in a model system to persist in labeled neurons for more than a week, but to be available for uptake into these neurons for a few days after injection at the most. These properties of rhodamine isothiocyanate were combined experimentally with short and long term survival periods. This allowed assessment of the changing developmental distribution of nigral fibers from specifically the striatal patch compartment. In all experimental cases the anterogradely labeled sections of the substantia nigra were also stained with antibodies to tyrosine hydroxylase, which permitted differentiation of the dopamine cell rich pars compacta from the dopamine cell poor pars reticulata. The results show that in the adult the majority of patch and matrix striatonigral projections are confined to the substantia nigra pars reticulata. Furthermore, their fiber distribution within the pars reticulata is overlapping rather than complementary. Most interestingly, in the late embryonic period (most noticeably at embryonic day 19) there is a marked overlap between patch striatonigral fibers and nigral dopamine perikarya. By early postnatal times, when the matrix compartment begins to form its striatonigral projection, the overlap of patch striatonigral fibers and dopamine cells has largely disappeared. The results suggest that a transient interaction between patch striatonigral fibers and dopamine neurons (which is concomitant with the formation of striatal compartments), may be an important developmental event in the phenotypic maturation of striatal pa

The pre- and postnatal development of the dopaminergic cell groups in the ventral mesencephalon and the dopaminergic innervation of the striatum of the rat

In the adult rat the striatum is a compartmentalized structure, which is reflected in the inhomogeneous distribution of dopamine. As a first step to test the hypothesis that dopamine plays an organizational role in the development of the striatum, the ontogeny of the dopaminergic system was studied in detail with immunocytochemical methods employing antibodies against dopamine. Rat embryos, fetuses, pups and adults were perfusion-fixed with glutaraldehyde on all prenatal days from E11 onward, postnatally on P2, P4, P6, P7, P8, P13, P14, P20, P21, and in adult age. On E13 the first dopaminergic cells are detected in the ventral prosencephalon. On E14 two dopaminergic cell groups are present in the ventral mesencephalon, and fibres of these cells reach the ventrolateral part of the ganglionic eminence. In the next two days both the cell groups and their projections rapidly increase in size. On E17 the afferent dopaminergic fibres to the striatum become aligned and form huge bundles that are closely associated with the fascicles of the internal capsule. Rostrally, the development of the striatal dopaminergic innervation shows a clear ventrolateral to dorsomedial gradient, whereas more caudally the dopaminergic fibres innervate the striatum from a ventromedial position. The lateral parts of the otherwise compact mesencephalic cell groups consist of loosely arranged cells. From E17 onward these cells become arranged into a dorsal and a ventral group. Just before birth, on E21, the primordia of the dopaminergic cell groups in the substantia nigra pars compacta and pars reticulata can be observed. On E19 several centres with extensive fibre ramifications along the dorsolateral margin of the caudate putamen represent the first signs of the inhomogeneous distribution of dopaminergic fibres in the dorsal striatum seen during the next two weeks. In the following pre- and postnatal days these so-called dopaminergic "patches" also appear more medially. By the third postnatal week most of the patches are no longer detectable, and only the most dorsolaterally located ones, i.e. in the region where they first were detected on E19, remain visible through to the adult stage. Prenatally, no varicosities can be observed in the dopaminergic fibres. The first varicosities appear after birth. Their number increase rapidly during the first and second postnatal weeks and reaches near adult levels on P20. The development of the striatal dopaminergic innervation, and that of the "patches" in particular, is discussed in relation to the development of the mesencephalic dopaminergic cell groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Histogenesis of the subthalamic nucleus

Literature presents divergent hypotheses on the histogenesis of the subthalamic nucleus. The basal plate of the mesencephalon, the dorsal hypothalamus (or the longitudinal subthalamic zone) and an area closely related to the mammillary nucleus have all been proposed as the site of origin of the neurons of the subthalamic nucleus. However, based on the selective labelling of neurons of this nucleus by the [3H]thymidine ARG technique, our results clearly show that a germinative zone lying caudally along the dorsal aspect of the mammillary recess is responsible for the formation of the neurons of the subthalamic nucleus. From this site of origin, the neurons migrate radially and then tangentially and dorsally along the marginal layer of the ventral diencephalon. The present study gives a detailed account of the histogenesis of the subthalamic nucleus of the rat through successive stage of development. Our data also show that the same proliferative neuroepithelial matrix that gives rise to the subthalamic neurons is also responsible for the genesis of the neurons of the supramammillary and submammillothalamic nuclei. Some aspects of the cytological maturation of the neurons of the subthalamic nucleus are also given.

Aromatic L-amino acid decarboxylase in the rat brain: immunocytochemical localization during prenatal development

Immunocytochemically labeled cells containing the enzyme aromatic L-amino acid decarboxylase were localized in the brain of rat embryos at gestational age E15-E19. Cell groups that contained aromatic L-amino acid decarboxylase but lacked either the enzyme tyrosine hydroxylase or the indolamine serotonin were referred to as "D" groups. Anatomical landmarks, cytoarchitectonic structure and histochemical staining for acetylcholinesterase were used to delineate the position of "D" groups. In the E15 embryo three "D" groups existed. The first to appear, named D1, was located in the spinal cord and had been demonstrated before. A large "D" cell cluster was found in the walls of the central forebrain deep to the hypothalamic sulcus. This group distributed dorsally in the ventral dorsal thalamic region and ventrally in the dorsal hypothalamus. The rostral-most "D" group, D14, occurred in the ventral telencephalon just medial to fibers of the nigrostriatal projection. D14 was the smallest of the early groups. In E16 and E17 embryos dorsal di- and mesencephalic "D" groups were first detected. During the course of ontogeny a considerable increase of immunoreactive cells occurred and segregation of the large central forebrain cluster into several rostrally and laterally distributed "D" groups took place. Some "D" groups that occur in the adult brain were not present in the E19 embryo. This study provides a first report of the localization of several unique cell groups in the brain of rat embryos and their appearance at different stages of gestation. It also gives further support to the notion that variations of aromatic L-amino acid decarboxylase staining intensities may be characteristic of different monoamine neurons.

Histogenesis at the level of the basal forebrain: the entopeduncular nucleus

This study shows that the neurons of the entopeduncular nucleus are derived from a longitudinal slab of isochronically generated neurons on day 11 and 15 h of gestation. Many neurons of this longitudinal slab which we have named the basal forebrain cell column, originate from an ependymal matrix closely associated with the ventral diencephalic sulcus and later become associated with the basal forebrain bundle. Other neurons also originate from the ependymal matrix at the site of emergence of the optic recess and keep close relationships with the optic chiasma through the following stages of development to form the retrochiasmatic nucleus. During the second half of day 12 of gestation, the mantle layer of the forebrain shows an early zone of differentiation along its ventrolateral aspect. At this stage, the basal forebrain cell column extends unbroken from the tuberculum posterius to the chiasmatic plate primordium (site of generation of the retrochiasmatic nucleus). At the level of the caudal aspect of the optic stalk however, the basal forebrain cell column divides in two limbs associated to the ventral and dorsal edges of the optic stalk as it emerges from the forebrain. On day 14 of gestation, the neurons of the dorsal limb of the basal forebrain cell column occupy the mantle layer of the neural tube at least as far rostrally as the ventricular elevation in front of the optic stalk in the floor of the foramen of Monro. The neurons derived from the basal forebrain cell column begin breaking up into a series of more definite nuclei at later stages of development. The main finding of this study is the disclosure of the fact that the entopeduncular nucleus as well as other cell groups as dissimilar as the lateral preoptic area, the central, medial and anterior cortical amygdaloid complex and neurons of the dorsal hypothalamic area appear to be embryologically related, as they are all derived from a common longitudinal slab of the matrix of the forebrain.

Histogenesis of the striopallidal system in the rat. Neurogenesis of its neurons

This study discusses the histogenesis of the structures of the extrapyramidal system. The first part based on [3H]thymidine autoradiography provides data on the time of origin of the various neuronal populations that characterize different structures of the extrapyramidal system. Such data are essential to any further study dealing with the localization of the different sites of origin of the neurons along the ependymal matrix and with their migration during the histogenetic sequences leading to their definitive pattern of adult distribution. The neurons of globus pallidus and of the entopeduncular nucleus are generated on days 12-15 and days 11-14, respectively. A peak of neurogenesis occurs on day 12 for the entopeduncular nucleus and on day 14 for the globus pallidus. In the pallidum, the first neurons to form on day 12 settle caudally, while neurons generated on day 15 settle in the rostral extremity. The genesis of the medium-sized neurons of the neostriatum extends from day 12 to at least postnatal day 2. A peak is obvious on day 15. Although the neurogenesis of these neurons follows a mild caudorostral gradient, a more careful examination reveals four different patterns of settlement according to the area involved and the period of gestation. At the level of the caudal neostriatum, the neurons display a clear mediolateral spatiotemporal gradient. More rostrally, the neurons generated on days 13, 14 and 15 show two patterns of settlement. On the one hand, many neurons settle rather densely along the external capsule on day 13, occupying more rostral levels on days 14 and 15. On the other hand, in the body of the neostriatum, clusters of isochronically generated neurons are obvious. Later, newly generated neurons display a rather homogeneous distribution in the structure. A parallel is drawn between these patterns of development and the patterns of distribution of afferent terminals or neurotransmitters. The large chromophilic neurons of the neostriatum appear exclusively during the early period. Two peaks of neurogenesis are apparent. The one on day 13 comprises neurons that settle caudally. It is contemporaneous to the neurogenesis of the adjacent basal nucleus. The second peak occurs on day 15 and corresponds to that of the medium-sized neurons.