An ultrastructural and radioautographic study of the migrating neuroblast in hamster neocortex

Parallel Development of Chromatin Patterns, Neuron Morphology, and Connections: Potential for Disruption in Autism

The phenotype of neurons and their connections depend on complex genetic and epigenetic processes that regulate the expression of genes in the nucleus during development and throughout life. Here we examined the distribution of nuclear chromatin patters in relation to the epigenetic landscape, phenotype and connections of neurons with a focus on the primate cerebral cortex. We show that nuclear patterns of chromatin in cortical neurons are related to neuron size and cortical connections. Moreover, we point to evidence that reveals an orderly sequence of events during development, linking chromatin and gene expression patterns, neuron morphology, function, and connections across cortical areas and layers. Based on this synthesis, we posit that systematic studies of changes in chromatin patterns and epigenetic marks across cortical areas will provide novel insights on the development and evolution of cortical networks, and their disruption in connectivity disorders of developmental origin, like autism. Achieving this requires embedding and interpreting genetic, transcriptional, and epigenetic studies within a framework that takes into consideration distinct types of neurons, local circuit interactions, and interareal pathways. These features vary systematically across cortical areas in parallel with laminar structure and are differentially affected in disorders. Finally, based on evidence that autism-associated genetic polymorphisms are especially prominent in excitatory neurons and connectivity disruption affects mostly limbic cortices, we employ this systematic approach to propose novel, targeted studies of projection neurons in limbic areas to elucidate the emergence and time-course of developmental disruptions in autism.

Subcellular localization of Patched and Smoothened, the receptors for Sonic hedgehog signaling, in the hippocampal neuron

Cumulative evidence suggests that, aside from patterning the embryonic neural tube, Sonic hedgehog (Shh) signaling plays important roles in the mature nervous system. In this study, we investigate the expression and localization of the Shh signaling receptors, Patched (Ptch) and Smoothened (Smo), in the hippocampal neurons of young and mature rats. Reverse transcriptase-polymerase chain reaction and immunoblotting analyses show that the expression of Ptch and Smo remains at a moderate level in young postnatal and adult brains. By using immunofluorescence light microscopy and immunoelectron microscopy, we examine the spatial distribution of Ptch and Smo within the hippocampal neurons. In young developing neurons, Ptch and Smo are present in the processes and are clustered at their growth cones. In mature neurons, Ptch and Smo are concentrated in dendrites, spines, and postsynaptic sites. Synaptic Ptch and Smo often co-exist with unusual structures-synaptic spinules and autophagosomes. Our results reveal the anatomical organization of the Shh receptors within both the young and the mature hippocampal neurons.

Altered postnatal development of the visual cortex in trisomy 19 mice

The development of the visual cortex was studied in 30 Trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. Morphogenesis of the Ts19 visual cortex, though delayed in development, followed the regular sequence observed in control littermates. Early morphogenetic events, such as obliteration of the ventricular lumen, disappearance of the ventricular zone, formation of a visible apical dendrite, as well as disappearance of both migrating neurons and the columnar organization of bipolar preneurons were delayed by 1 day; maturation steps occurring later such as appearance and disappearance of perikaryal basophilia were delayed by 2 days. Myelination of the white matter was similarly retarded by 2 days. The fronto-occipital length of the cerebral hemispheres and the thickness of the visual cortex were decreased by about 20%, consistent with a hypoplasia of the Ts19 neocortex. Unlike in the cerebral cortex of human Ts21, morphometric analysis of the visual cortex of Ts19 mice did not give any indication of a selective deficit in a particular neuron population; the increased cell density and the reduced nuclear volume observed during early postnatal development are attributable to a maturational delay. The relevance of these results with respect to the mechanisms underlying neuropathological alterations in human Ts21 is discussed.

Neuron migration within the radial glial fiber system of the developing murine cerebrum: an electron microscopic autoradiographic analysis

The present analysis provides direct evidence in the mouse that in the course of course of neocortical histogenesis, contact between migrating neurons and the surfaces of radial glial fibers is both invariant and relatively selective. The analysis characterizes in detail the migratory behavior of the individual migrating cell with respect to the overall radial glial fiber system as this system varies systematically in its structure with ascent through the strata of the cerebral wall. A quantitative study of the relationships between the radial glial fibers confidently identified by their glycogen content and the migrating neurons marked autoradiographically by injection of [3H]thymidine was also performed at the ultrastructural level on tangential sections at different pallial levels in E16 and E17 embryos. The overall set of observations lend support to the hypothesis that radial glial fibers act specifically as guides to neuronal migration and illustrate the nature of the cell-to-cell interaction which serves this cellular process critical to neocortical histogenesis.

Development and decision-making in the mammalian cerebral cortex

One of the fundamental tasks of neurobiology is to understand how the precision and specificity of the adult nervous system is achieved during development. This paper reviews the progress that has been made toward this end in studies of the developing mammalian cerebral cortex. Particular attention is focused on the problem of how cortical neurons make decisions during development: the correlation between a neuron's 'birthday' and its final laminar destination and projection patterns has raised the possibility that young neurons may be committed to their adult fates very early on in development, perhaps prior to migration. Indeed, several lines of evidence reviewed here suggest that at least some of the decisions made by cortical neurons are intrinsic properties of the cell itself. These studies include experiments on the reeler mouse mutant, and more recent attempts to manipulate developmental fates by pharmacological interventions and transplantation techniques. It is concluded that early commitment events in the cerebral cortex may specify a neuron's laminar position and restrict the range of potential axonal projections that the cell may form, but that local positional cues direct neurons to select (or maintain) only certain of the possible projections.

Ultrastructural characterization of synaptic terminals formed on newly generated neurons in a song control nucleus of the adult canary forebrain

The fine structure of synaptic terminals contacting neurons generated in the forebrain of adult male canaries was investigated by autoradiography and electron microscopy. The procedure for labeling the new neurons included pretreating adult canaries with 3H-thymidine and sacrificing them 23-45 days later. Neurons were identified as newly generated by the presence of 3H-thymidine in the cell nucleus. The new neurons in the nucleus hyperstriatum ventralis, pars caudalis (HVc) were identified by autoradiography and light microscopy and examined with electron microscopy. Several types of synaptic terminals contacted the cell body and proximal dendrites of the newly formed neurons. Synaptic junctions were formed by terminals that contained spherical, agranular vesicles, large dense-core vesicles and spherical, agranular vesicles, and pleomorphic or flattened synaptic vesicles. Terminals that contained spherical vesicles were most often associated with asymmetric synaptic densities, and terminals that contained pleomorphic or flattened vesicles formed symmetric junctions. New neurons were also contacted by small terminals that contained few vesicles and had little pre- or postsynaptic density associated with the junction; these terminals may be a special type or may be in the process of developing their synaptic contact with the new neuron. In addition, rare terminals that appeared to be degenerating or to contain debris from other degenerating neural elements contacted new neurons. In summary, these data indicate that the new neurons, which are known to be inserted into existing neural networks, receive synaptic input from at least three different sources.

Postnatal development of cat superior colliculus. I. Presumptive neurons, immature neurons and degenerating cells in the intermediate and deep layers

Presumptive neurons, immature neurons and degenerating structures were found in the intermediate and deep layers of the superior colliculus of normal cats over the age range 7-45 days postnatal. Electron microscopical analysis revealed these structures to be unexpectedly frequent in the early postnatal period. There were 34 immature neurons per 1000 neurons at 7-8 days and 78 per 1000 neurons at 14 days postnatal. Few were found in older cats. Two types of degenerative changes in the neurons were observed: nuclear and cytoplasmic. The number of degenerating structures to be unexpectedly frequent in the early postnatal period. There were 34 immature neurons per 1000 neurons at 7-8 days and 78 per 1000 neurons at 14 days postnatal. Few were found in older cats. Two types of degenerative changes in the neurons were observed: nuclear and cytoplasmic. The number of degenerating neurons was higher at 14 days (28 per 1000 neurons) than at 7-8 days (21 per 1000 neurons), but only a few were found at 30 and 45 days. Degenerative changes were noted in synaptic terminals at all ages studied. The greatest number of degenerating synaptic profiles were observed at 14 and 30 days. They were infrequent in younger or older animals. The number of presumptive neurons does not change significantly over 7-14 days. It is speculated that they persist as a reverse pool of neurons. The increasing number of immature neurons in the period between 7 and 14 days indicates that formation of new cells in the intermediate and deep layers of the superior colliculus continues into the early postnatal period. The degeneration of some neurons and synapses over the same developmental period suggest that the final adjustment of the neuronal composition of the superior colliculus occurs postnatally.

Cell proliferation and DNA-synthesis in the external granular layer of the postnatal rat cerebellum: a quantitative study

The external granular layer (EGL) of the rat cerebellum can be subdivided into two morphologically distinct zones, the outer of which consists of proliferating cells, while the inner has generally been considered to contain post-mitotic cells exclusively. Two criteria, the detection of mitotic figures and the observation of nuclei labelled shortly after an injection of [3H]-thymidine, were used to locate proliferating cells. Their distribution across the layer was quantified for several ages of animal. Though most frequent superficially, cell proliferation was found to occur at all levels of the EGL. Possible explanations for the existence of a gradient of proliferative activity across the EGL are discussed.

Morphology and distribution of postnatally generated glial cells in the somatosensory cortex of the rat: an autoradiographic and electron microscopic study

Postnatal cytogenesis in rat somatosensory cortex was examined by the technique combining light microscopic autoradiography with electron microscopy. Tritiated thymidine was injected intraperitoneally to rats on different days (0-21 days). All animals were sacrificed on the twenty-fifth day after birth. Coronal slices including somatosensory cortex were embedded in epoxy resin. Semithin sections for autoradiography and ultrathin ones for electron microscopy were made alternately. The labeled cortical cells were found mainly in the cases injected with [3H]thymidine during the first and the second weeks. Examination of laminar distribution of the labeled cells revealed that the cells in deeper laminae were labeled on earlier postnatal days than those in more superficial laminae. The labeled cells were examined with electron microscope to identify their nature. By this, it was revealed that ultrastructural morphology of the labeled cells were that of glial cells (either astrocyte or oligodendrocyte). Time and space pattern of this neocortical postnatal gliogenesis shows the tendency of the inside-out sequence, though the pattern is not as distinctive as that of prenatal neocortical neurogenesis. The relationships between the pattern of gliogenesis and the maturation of cortical neurons is suggested.

Ontogeny of oral function in hamsters (Mesocricetus auratus)

The oral apparatus of neonatal and juvenile golden hamsters was investigated by clearing and staining of whole crania, videotaping of behavior, and electromyography of several jaw muscles. Chewing developed during the first postnatal week and matured in the second; however, suckling was still the primary mode of feeding. Micromovements of the jaws occurred early when the osseous skeleton and joints developed. Macromovements correlated well with EMG records and were limited to jaw opening at birth. Muscles of the oral floor generated large bursts of activity during jaw opening and tongue protrusion from 0 days postnatal (dpn), when simple and stereotyped gaping was induced, until 14 dpn, when movements were spontaneous and not stereotyped nor inducible. However, adductor muscle activity was brief, low in amplitude, and primarily involved with jaw stabilization until 4 dpn, when these muscles became active during closing the jaws; closing activity increased in frequency and amplitude until the end of the second week. Development of frequent, coordinated macromovements of chewing was associated with the refinement of joint structure and dental occlusion and with the growth of the craniofacial skeleton. Jaw movements and associated EMG's correlated better with available data on development of neural circuitry than with that for musculoskeletal development.

Interlaminer connections of rat visual cortex: an ultrastructural study

Thermal lesions were made in layers I, II, and upper par of layer III of rat visual cortex. The distribution of degenerating axons and axon terminals in layers IV, V, and VI was studied using electron microscopic techniques. Following supragranular thermal lesions, the majority of degenerating axon terminals were found in layer V, with extension into the adjacent part of layer VI. Neural profiles postsynaptic to degenerating axon terminals were found in these layers in the following distribution: 81.7% on spines of small to medium size dendrites; 18.2% on dendrite shafts; and less than 1% on neuronal perikarya. Few degenerating terminals were found on or near apical dendrites. Degenerating terminals were identified on shafts of stellate-type dendrites found in upper part of layer V. Degenerating axons oriented parallel to the cortical surface were found most often in deep layer IV and upper layer V. Degenerating axons were also seen in axon bundles coursing vertically through layer IV. Approximately 10% of the terminals within a grid square have undergone degeneration; no clustering of degenerating terminals was found in vertical or transverse sections through layers V and VI. We suggest that most axon terminals arising from pyramidal neurons in layers II and upper III synapse with spines and shafts of dendrite branches originating from pyramidal neurons in layer V and perhaps VI.

Lack of evidence for glial cells originating from the external granular layer in mouse cerebellum

In 25 day mice cerebella, a quantitative electron microscopic analysis showed that glial cells were not seen among 749 cells counted in the molecular layer. Likewise, a light microscopic autoradiographic study showed that labelled oligodendroglia and/or astroglia in the cerebellum were not derived from the external granular layer (EGL). Previous claims, that these cells derived from the EGL, may have arisen because other cell types, i.e., endothelial cells, pericytes, microglia, and other ectopic granule cells may have been misidentified as oligodendroglia and/or astroglia. It seems likely that the EGL is a unique germinal cell layer in the mammalian nervous system because it gives rise only to neurons, whereas cerebellar astrocytes and oligodendrocytes are derived from the subventricular layer of the fourth ventricle, as first suggested by Cajal in 1911.

Do oculomotor neuroblasts migrate across the midline in the retal rat brain?

Toluidine blue-stained semithin sections and Cajal-Castro preparations are used to study in rat fetuses whether oculomotor neuroblasts migrate across the midline at a certain period of development. In confirmation of previous studies, a group of oculomotor neuroblasts was detected which first grow cytoplasmic processes into the mesencephalic midline, and afterwards translocate their somata towards the midline, between the 12th and the 15th days of gestation. At this moment a midline mass of neuroblasts characterizes the meeting at this landmark of both left and right migrating neuroblastic groups. No crossing oculomotor axons yet are demonstrable with reduced silver techniques. In further stages of development the neuroblasts continue their migration until they arrive at the contralateral nucleus at the 16th and 17th day of gestation. At the midline the mass of neuroblasts disappears gradually and crossed oculomotor axons become visible. The electron microscope was then used to study ultrastructurally the migrating motoneurons. It was discovered that no preexisting structure guides their movement by contact. Their leading processes show no filopodial activity, and contain abundant microtubules and thick bundles of neurofilaments in eccentric position. The neuroblasts carry their axon across the midline as a trailing process.

The pathogenesis of parvovirus-induced cerebellar hypoplasia in the Syrian hamster, Mesocricetus auratus. Fluorescent antibody, foliation, cytoarchitectonic, Golgi and electron microscopic studies

Cerebellar histogenesis was studied in hamsters infected at birth with a parvovirus, rat virus strain PRE 308. Cerebellar granule cell precursors in these animals were selectively infected and lysed in the external germinal layer before their migration to form the internal granular layer. The effects of the absence of granule cells on cerebellar development and especially on the development of the Purkinje cells and their dendrites was analyzed using fluorescent antibody. Golgi, conventional paraffin, and electron microscopic methods. This study represents the first Golgi and ultrastructural study of the pathogenesis of rat virus infections in the cerebellum. The destruction of the granule cell precursors resulted in a dysplastic cerebellar hypoplasia with total disruption of normal cerebellar stratification and cytoarchitectonics. The Purkinje cells developed misshapen, progressively disoriented dendritic stems lacking tertiary dendrites and studded with numerous spines, devoid of afferent synaptic contacts (naked spines) and encased by glial processes. These developmental studies, together with the mouse mutant studies, demonstrated that the spines of the Purkinje cells were elaborated in the absence of both tertiary dendrites and afferent parallel fiber contacts. Such data suggested that spine formation, once triggered, was intrinsically programmed rather than being dependent on the development of parallel fiber contacts. Despite the loss of a major interneuronal component and disintegration of normal cytoarchitectonic relationships, synapses in the cerebellar cortex developed normally as long as both the pre- and post-synaptic elements were present. Thus synaptic specificity is maintained in the face of gross disruption of cytoarchitectonic relationships. If either the pre- or post-synaptic portion of a contact was absent, then glial processes isolated the persisting element or aberrant contacts formed. In addition to glial encasement of naked spines, there were dendrodendritic articulations between Purkinje cell dendrites, some of which were joined by septate, plaque-like junctions. Aberrant synaptic contacts between mossy and climbing fiber glomeruli and the smooth surface of the Purkinje cell somata were found rarely. In addition to these contacts which also occur in the hypoplastic cerebella produced by other methods, previously undescribed non-synaptic spine-articulations between Purkinje cell dendrites were seen. The role played by granule cells and their axons in Purkinje cell development appeared to be two-fold. First, the development of the orderly array of parallel fibers in the normal animal played a role in orienting and flattening the dendritic trees of Purkinje cells. Second, the formation of tertiary dendritic branches appeared to depend primarily upon the presence of an external germinal layer throughout this stage of Purkinje cell development. By contrast, dendritic spines developed and persisted in the absence of granule cells.

Nerve growth factor: relationship to the cyclic AMP system of sensory ganglia

Nerve growth factor and N(6),O(2)' dibutyryl adenosine 3',5'-monophosphate both stimulate neurite elongation by explanted ganglia. However, the addition of nerve growth factor does not lead to increased amounts of adenosine 3',5'-monophosphate in intact ganglia, nor does it stimulate adenylate cyclase activity in broken ganglia cells.