Formation of structural and ultrastructural organization of striatum rat postnatal ontogenesis upon changes in their embryonal development

By light microscopy (by Nissl and Golgi), electron microscopy, and immunohistochemistry methods, formation of structure of the brain striatum dorsolateral part from birth to the 3-month age was studied in rats submitted to acute hypoxia at the period of embryogenesis. It has been established that hypoxia at the 13.5th day (E13.5) leads to a delay of neuronogenesis for the first two weeks of postnatal development as compared with control animals, while the majority of large neurons at this period are degenerated by the type of chromatolysis with swelling cell body and processes and lysis of cytoplasmic organoids. By the end of the 3rd week, shrunk hyperchromic or picnomorphic neurons with the electron-dense cytoplasm and enlarged tubules of endoplasmic reticulum and Golgi complex were also observed. An increased number of swollen processes of glial cells was detected in neuropil around degenerating neurons. By the 30th day as well as in adult rats there was observed destruction of mitochondrial apparatus, an increase of the number of lysosomes, and the appearance of bladed nuclei - signs of apoptotic cell death, which was also confirmed by an increased expression of proapoptotic p53 protein and its colocalization with caspase-3 in a part of neurons. Morphometrical analysis has shown a decrease of density of striatum cell arrangement and a change of ratio of different cell types in the rats submitted to hypoxia as compared with control group. At early stages of postnatal ontogenesis there was the greatest decrease (42.3% at the 5th day, 14.2% at the 10th day, p < 0.01) of the number of large neurons with the area more than 80 microm2. After 3 weeks of postnatal development the number of middlesize neurons (30-95 microm2) decreased (by 11.8-19.2%) as compared with control. The obtained data show that a change of conditions of embryogenesis (hypoxia) at the period of the most intensive proliferation of the forebrain neuroblasts leads to disturbances of the process of formation of the striatum nervous tissue. This can be the cause of delay of development and disturbances of behavior and learning observed in rats submitted to prenatal hypoxia.     

Structural changes in the neocortex nervous tissue in rat ontogenesis after hypoxia at various terms of embryogenesis

The performed study has shown that in rats submitted to hypoxia (3 h, 7% O₂) at the 14th day of embryogenesis (E14) as compared with control animals, density of distribution of cells in the brain cortex decreased for the first month of postnatal ontogenesis (maximally by 40.8% by P20). In dying neurons, swelling of the cell body, lyses of or ganoids, and disturbance of the cytoplasm membrane intactness were observed. Two waves of neuronal death by the mechanism of capsize-dependent apoptosis were revealed; the first involved large pyramidal neurons of the layer V (P10–20), the second-small pyramidal and non-pyramidal neurons of the layers II–III (P20–30). In neurosis of molecular layer, a decrease of the mean amount of labile synaptopodin-positive dendrite spines was observed, as compared with control. In rats exposed to hypoxia at E18, no changes of cell composition and structure of the nervous tissue were found in the studied brain cortex areas. Thus, formation of the cortex nervous tissue in postnatal ontogenesis of rats submitted to hypoxia at the period of neuroblast proliferation-migration is accompanied not only by a change of the cell composition of various cortex layers in early ontogenesis, but also by a decrease of the number of the synaptopodin-positive spines in the molecular layer, the decrease being preserved in adult animals.     

Change of the neocortex nervous tissue in rat ontogenesis after hypoxia at various terms of embryogenesis

The performed study has shown that in rats submitted to hypoxia (3 h, 7% O2) at the 14th day of embryogenesis (E14) as compared with control animals, density of disposition of cells in the brain cortex decreased for the first month of postnatal ontogenesis (maximally by 40.8% by P20). In dying neurons, swelling of the cell body, lysis of organoids, and disturbance of the cytoplasmic membrane intactness were observed. Two waved of neuronal death by the mechanism of caspase-dependent apoptosis were revealed; the first involved large pyramidal neurons of the V layer (P10-20), the second--small pyramidal and non-pyramidal neurons of the II--III layers (P20-30). In neuropil of molecular layer, a decrease of the mean amount of labile synaptopodin-positive dendrite spines was observed, as compared with control. In rats exposed to hypoxia at E18, no changes of cell composition and structure of the nervous tissue were found in the studied brain cortex areas. Thus, formation of the cortex nervous tissue in postnatal ontogenesis of rats submitted to hypoxia at the period of neuroblast proliferation-migration is accompanied not only by a change of the cell composition of various cortex layers in early ontogenesis, but also by a decrease of the number of the synaptopodin-positive spines in molecular layer, the decrease being preserved in adult animals.     

Disturbances in Formation of the New and Old Cortex at Changes of Conditions of Embryonic Development

Using a model of acute hypoxia during pregnancy of rats, changes in the development of old (hippocampus) and new (sensorimotor) cortex associated with disturbance of neuronogenesis have been revealed in the studied brain structures at the period of action of a pathological factor. It was found that in rats submitted to hypoxia at the 13–14th days of embryogenesis, the number of degenerating neurons (including the pyramidal ones) at various levels of chromatolysis increased since the 5th day after birth; the increase was present for the entire first month of postnatal development. In the cortex of rat pups submitted to prenatal hypoxia there were observed deformation of neuronal bodies, vacuoles in the cytoplasm, shrinkage of apical dendrites of pyramidal neurons and delayed development of the structure (time of the appearance of spikes, formation of structural elements and the size of the cells) of the nervous tissue of the brain of the rat pups exposed to prenatal hypoxia. The columnar structure of the cortex was disturbed. In hippocampus, the process of degeneration of neurons started by 2–3 days later than in the cortex; by two weeks of postnatal development a massive degeneration and death of a part of neurons were also revealed. The morphometrical analysis showed a decrease in the number of neurons and their total area in the sensorimotor cortex (the layer V) and an increase in the number of glial elements at the 10–17th days after birth. In the hippocampus a decrease in the area occupied by neurons and in their size was detected in adult animals. The adult rats submitted to prenatal hypoxia were found to have disturbances of memory and learning. A correlation was shown between the disturbances of the conditions of embryonic development and the changes in the ability of learning and storage of new skills in the offspring.     

Postnatal physiological development of rats after acute prenatal hypoxia

The aim of study was to investigate the physiological development of the brain and behaviour in rats subjected to prenatal hypoxia on the 13.5th day of embryogenesis. We have found that such rats manifested a delayed physiological development and a change in nervous tissue of the sensorimotor cortex, as well a disturbed formation of motor responses during the first month of postnatal ontogenesis. During maturation these modifications were in part compensated, however we observed a decrease of the rats' ability to learn new forepaw movements. The destruction of the brain tissue and the modification of neurons composition in the sensorimotor cortex correlated with changes of behaviour at different stages of ontogenesis. Thus, changes of the conditions under which an organism develops during embryogenesis, predetermine a disturbance in ontogenesis and the learning ability.     

Effects of prenatal hypoxia on expression of amyloid precursor protein and metallopeptidases in the rat brain

Summary In the present study we have investigated the effect of prenatal hypoxia on expression of amyloid precursor protein (APP) and some metallopeptidases, which regulate β-amyloid peptide (Aβ) levels (neprilysin (NEP) and endothelin-converting enzyme (ECE-1)) in the cortex of rats during different periods of postnatal development. We have found that the level of APP in the sensorimotor cortex (SMC) of rats, analysed by Western blotting, increases from days 1 to 5 of postnatal development and then steadily decreases with age, with the most dramatic decline in the period from day 180 to 600. In the cortex of rats subjected to prenatal hypoxia on day 13.5 of embryogenesis, the postnatal levels of APP were higher than in the control. Secretion of the soluble form of APP (sAPP) by α-secretase was found to be the most active on day 30 of postnatal development and there was a significant decrease in the production of sAPP after prenatal hypoxia. NEP was found to be expressed in the cortex of rats only at the early stages of postnatal development and it was barely detectable in adult rats. The decline of NEP levels during ageing might contribute to accumulation of Aβ in later life in humans. Prenatal hypoxia resulted in a significant decrease of NEP expression on day 10, but its level was recovered when animals were preconditioned to mild hypoxia. A similar phenomenon was observed when the expression of ECE-1 was analysed. Overall, prenatal hypoxia leads to significant changes in the levels of APP and expression of metallopeptidases involved in amyloid metabolism during all postnatal life and preconditioning to hypoxia appeared to be neuroprotective.     

Effect of serotonin on the formation of neurons producing gonadotropin-releasing hormone in Wistar rats

The effect of serotonin on the formation of neurons producing gonadotropin-releasing hormone (GnRH) during embryogenesis of Wistar rats was studied. The neurons producing GnRH were detected immunocytochemically on days 18 and 21 of embryonic development and on day 15 of postnatal development of rats with normal serotonin metabolism and rats in which the synthesis of serotonin was inhibited by p-chlorophenylalanine. The total number of GnRH neurons in serotonin deficiency was larger than in the case of its normal metabolism at all developmental stages studied. This is an indirect evidence for the inhibitory effect of serotonin on the formation of GnRH neurons. To confirm the morphogenetic effect of serotonin, we studied the rate of formation of GnRH neurons by injecting bromodeoxyuridine in the formation period of these neurons. It was found that serotonin deficiency had no effect on the time of formation of GnRH neurons: over 97% of neurons formed on days 11 to 15 of embryonic development both in the experimental and control groups. Note that, in serotonin deficiency, the maximum number of GnRH neurons formed one day later than in the normal state. Thus, serotonin inhibits the proliferation of GnRH neuron progenitor cells and thereby has a morphogenetic effect on the development of these neurons.     

BDNF is required for the survival of differentiated geniculate ganglion neurons

In mice lacking functional brain-derived neurotrophic factor (BDNF), the number of geniculate ganglion neurons, which innervate taste buds, is reduced by one-half. Here, we determined how and when BDNF regulates the number of neurons in the developing geniculate ganglion. The loss of geniculate neurons begins at embryonic day 13.5 (E13.5) and continues until E18.5 in BDNF-null mice. Neuronal loss in BDNF-null mice was prevented by the removal of the pro-apoptotic gene Bax. Thus, BDNF regulates embryonic geniculate neuronal number by preventing cell death rather than promoting cell proliferation. The number of neurofilament positive neurons expressing activated caspase-3 increased on E13.5 in bdnf^−/− mice, compared to wild-type mice, demonstrating that differentiated neurons were dying. The axons of geniculate neurons approach their target cells, the fungiform papillae, beginning on E13.5, at which time we found robust BDNF^LacZ expression in these targets. Altogether, our findings establish that BDNF produced in peripheral target cells regulates the survival of early geniculate neurons by inhibiting cell death of differentiated neurons on E13.5 of development. Thus, BDNF acts as a classic target-derived growth factor in the developing taste system.     

Study of distribution of the spine apparatus protein synaptopodin in cortical brain parts of rats exposed to hypoxia at different periods of embryogenesis

A comparative study of the nervous tissue and distribution of the spine apparatus protein synaptopodin was performed in all layers of the brain sensorimotor cortex and hippocampal CAl area in control rats and in the rats exposed to hypoxia at E14 and E18. It was found that beginning from the 20th day of postnatal development, a statistically significant decrease of the mean number of labile synaptopodin-positive spines in the stratum radiatum moleculare of the hippocampal area CAl was observed in rats exposed to hypoxia both at E14 and E18. The decrease of the number of labile spines in the sensorimotor brain cortex was revealed only in the I layer beginning from the 20th day after birth in the rats exposed to hypoxia at E14. Maximal differences in the studied brain areas were observed in adult rats exposed to hypoxia at E14 in the neocortex—a decrease by 23 ± 10%, in hippocampus—by 24 ± 8%, respectively. However, no increased degeneration of neurons was detected in adult animals. It is suggested that disturbances in cognitive functions and in the capability for learning observed in rats after prenatal hypoxia can be due to a decrease of the amount of the labile synaptopodin-positive spines, which leads to a change of the structural-functional properties of neuronal networks and to a decrease of their plasticity.     

Immunohistochemical Localization of α-Mannosidase During Postnatal Development of the Rat Cerebellum

The localization of alpha-D-mannosidase in the rat cerebellum was studied by using indirect immunohistochemistry at both optical and electron microscopic levels. In the adult the enzyme is particularly concentrated in the dendrites and cell bodies of Purkinje cells, basket cells, and Golgi neurons in the cerebellar cortex and in the cytoplasm and dendrites of deep nuclei neurons. The cytoplasm of granule cells is poorly stained, whereas parallel fibers, white matter, Bergman fibers, and Golgi epitheloid cell perikarya show virtually no staining. Electron microscopy suggests that most of the staining is found in the cytosol, although some staining is found in the postsynaptic densities of the synapses between parallel fibers and Purkinje dendrites. The pattern of staining was followed throughout the postnatal development of the rat cerebellum. At bith an intense and diffuse staining is found in all cells except those of the external germinative layer. At the 6th postnatal day, Purkinje cell bodies and apical cones are strongly labeled. From the 13th day on the pattern is very similar to that found in the adult. However, at the 18th postnatal day (when compared with the other structures), the staining of Purkinje cell dendrites seems to be higher than at all other ages. These data are correlated with biochemical studies and discussed in relation to the possible role of this enzyme during the postnatal development of the rat cerebellum.     

Regulation of the postnatal development of dopamine neurons of the substantia nigra in vivo by Akt/protein kinase B

Following mitosis, specification and migration during embryogenesis, dopamine neurons of the mesencephalon undergo a postnatal naturally occurring cell death event that determines their final adult number, and a period of axonal growth that determines pattern and extent of target contacts. While a number of neurotrophic factors have been suggested to regulate these developmental events, little is known, especially in vivo , of the cell signaling pathways that mediate these effects. We have examined the possible role of Akt/Protein Kinase B by transduction of these neurons in vivo with adeno-associated viral vectors to express either a constitutively active or a dominant negative form of Akt/protein kinase B. We find that Akt regulates multiple features of the postnatal development of these neurons, including the magnitude of the apoptotic developmental cell death event, neuron size, and the extent of target innervation of the striatum. Given the diversity and magnitude of its effects, the regulation of the development of these neurons by Akt may have implications for the many psychiatric and neurologic diseases in which these neurons may play a role.     

Neuron survival in vitro is more influenced by the developmental age of the cells than by glucose condition

The objective of this study was to determine whether the sensitivity to varying glucose conditions differs for the peripheral and central nervous system neurons at different developmental stages. Ventral horn neurons (VHN) and dorsal root ganglion neurons (DRG) from rats of different postnatal ages were exposed to glucose-free or glucose-rich culture conditions. Following 24 h at those conditions, the number of protein gene product 9.5 positive (PGP⁺) DRG neurons and choline acetyltransferase positive (ChAT⁺) VHN were counted and their neurite lengths and soma diameters were measured. For both DRG and VHN, the highest number of cells with and without neurite outgrowth was seen when cells from postnatal day 4 donors were cultured, while the lowest cell numbers were when neurons were from donors early after birth and grown under glucose-free conditions. The length of the neurites and the soma diameter for VHN were not affected by either glucose level or age. DRG neurons, however, exhibited the shortest neurites and smallest soma diameter when neurons were obtained and cultured early after birth. Our results indicate that survival of neurons in vitro is more influenced by the developmental stage than by glucose concentrations.     

Participation of catecholamines and NO in regulation of apoptosis of nonapeptidergic neurons of neonatal rat pups

Effects of catecholamines (CA) and the character of interaction of CA and NO in regulation of apoptosis were studied in vasopressinergic (VP-ergic) neurons of supraoptic (SON) and paraventricular (PVN) nuclei of rat pups in early postnatal ontogenesis. To study role of CA in regulation of programmed cell death in SON and PVN in the course of embryonal development, pregnant female rats were intraperitoneally injected daily from the 13th to the 20th day with αMPT—a blocker of CA synthesis. The second group of pregnant rats was injected from the 13th to the 20th day with the same volume of saline. The third group was composed of intact animals. The born rat pups were sacrificed at the 3rd and 15th days of life. Caspase 9, Bcl-2, tyrosine hydroxylase, and neuronal NO-synthase (nNOS) in SON and PVN neurons were revealed immunohistochemically, and the amount of immunoreactive substance in neuronal bodies was estimated using the computerized digital analyzer of TV image and Video Test software. Caspase-9 was shown to play an important role in postnatal formation of cellular composition of hypothalamic nonapeptidergic centers by leading to initiation of apoptosis and rejection of “useless” postmitotic SON and PVN neurons. Survival of “useless” nonapeptidergic neurons in early postnatal ontogenesis seems to be connected with antiapoptotic action of Bcl-2. Death of postmitotic neurons, and therefore formation of cellular composition begins earlier and, accordingly, is completed earlier in SON, in which neurons were noted to have a considerable decrease of the caspase-9 expression and, therefore, also a decrease of intensity of neuronal death via caspase-9-dependent pathway. In PVN, neurons continue to die also at the 15th day of rat life, i.e., almost two weeks later than in SON. The observed high correlation between the content of nNOS, caspase-9, and Bcl-2 in the SON and PVN neurons of intact rats of both age groups allows suggesting participation of NO in realization of apoptosis in the course of early postnatal development. The increase of nNOS expression in hypothalamic neurons as a result of disturbances in CA-ergic innervation in embryogenesis might be a possible cause of the long preserved enhancement of expression of apoptosis signal proteins. It can be suggested that CA participate in morphogenesis of hypothalamic neurons by increasing expression of nNOS in neurons and thereby affecting expression of apoptotic proteins.     

Expression of thrombospondin-1 and CD36 and CD47 receptors in the rat brain after exposure to damaging factors in the early postnatal period

The expression of thrombospondin-1 (TS-1) and its receptors CD47 and CD36 in the cerebral cortex and hippocampus of rats under damaging factors in the early postnatal period was studied. After hypoxia on the 7th day of postnatal development, an increase in the number of CD47-expressing cerebral endothelial cells (days of postnatal development: P28–P70) and reduction in the number of TS-1-expressing astrocytes in the cortex at P28 were observed. In animals subjected to early postnatal stress at the age of P2–P15, a decrease in TS-1-expressing astrocytes in the cortex and hippocampus was registered (predominantly at the age of P28). It was noted that these changes characterize the period of long-term effects (P28–P70) of early stress that is relevant to the processes of reparative angiogenesis and arresting of neurological deficits.     

On the influence of prenatal hypoxia on formation of the orexinergic system and sleep–wake cycle in early ontogenesis of rats

The role of orexin in the organization of the sleep–wake cycle (SWC) is well known. The aim of this study was to examine the timing of the orexinergic system formation in rat postnatal ontogenesis and to assess the role of orexin A in the SWC organization under normal conditions and after prenatal hypoxia undergone on days 14 and 19 of embryogenesis. The SWC was investigated in 30-day-old rats with electrodes implanted into the somatosensory and occipital cortex. Immunoreactivity within the orexigenic structures of the lateral hypothalamus was analyzed. It was shown that in control 14-day-old animals the orexinergic structures were in their formative stage, whereas in 30-day-old rats they were already as formed as in adults. In 14-day-old rats, prenatal hypoxia evoked retarded formation of the orexinergic system. In 30-day-old animals, hypoxia undergone in the prenatal period increased the activity of the orexinergic system, which was higher in animals exposed to hypoxia on day 19 than on day 14 of gestation. In 30-day-old rats, these changes were reflected in the SWC formation in the form of shorter slow-wave sleep, more fitful sleep and increased number of transitions from slow- to fast-wave sleep. The results obtained are discussed in the light of the adaptive-compensatory role of the orexigenic system in postnatal ontogenesis after prenatal damage to the central nervous system.     

Comparative investigation of the delayed effects of prenatal hypoxia in the period of progestation and organogenesis

The aim of the present study was to compare the survival, physical development and spontaneous behavior of rat pups born from white rats subjected to acute hypobaric hypoxia on the 3rd-5th days of gestation (progestation) period or on the 9-10th day of gestation (period of early organogenesis). It was shown that the delayed effects of progestation hypoxia were less expressed than those following acute hypoxia modeled in the early organogenesis. In latter case, hypoxia led to the increased mortality among rat pups of both sexes while hypoxia-induced delay in physical development and changes in spontaneous behavior and anxiety level were registered up to the 57th day of postnatal period.     

Early Postnatal In Vivo Gliogenesis From Nestin-Lineage Progenitors Requires Cdk5

The early postnatal period is a unique time of brain development, as diminishing amounts of neurogenesis coexist with waves of gliogenesis. Understanding the molecular regulation of early postnatal gliogenesis may provide clues to normal and pathological embryonic brain ontogeny, particularly in regards to the development of astrocytes and oligodendrocytes. Cyclin dependent kinase 5 (Cdk5) contributes to neuronal migration and cell cycle control during embryogenesis, and to the differentiation of neurons and oligodendrocytes during adulthood. However, Cdk5’s function in the postnatal period and within discrete progenitor lineages is unknown. Therefore, we selectively removed Cdk5 from nestin-expressing cells and their progeny by giving transgenic mice (nestin-CreERT2/R26R-YFP/CDK5^flox/flox [iCdk5] and nestin-CreERT2/R26R-YFP/CDK5^wt/wt [WT]) tamoxifen during postnatal (P) days P2-P 4 or P7-P 9, and quantified and phenotyped recombined (YFP+) cells at P14 and P21. When Cdk5 gene deletion was induced in nestin-expressing cells and their progeny during the wave of cortical and hippocampal gliogenesis (P2-P4), significantly fewer YFP+ cells were evident in the cortex, corpus callosum, and hippocampus. Phenotypic analysis revealed the cortical decrease was due to fewer YFP+ astrocytes and oligodendrocytes, with a slightly earlier influence seen in oligodendrocytes vs. astrocytes. This effect on cortical gliogenesis was accompanied by a decrease in YFP+ proliferative cells, but not increased cell death. The role of Cdk5 in gliogenesis appeared specific to the early postnatal period, as induction of recombination at a later postnatal period (P7-P9) resulted in no change YFP+ cell number in the cortex or hippocampus. Thus, glial cells that originate from nestin-expressing cells and their progeny require Cdk5 for proper development during the early postnatal period.     

Changes in the proliferative activity in the brain of offspring rats induced by the influence of131I on the maternal organism

We tested the effects of injections of small doses of¹³¹I (150 μCi) into female rats on brain development in their offspring. The brains of rats whose embryogenesis was influenced by incorporated¹³¹I were studied with the use of an immunohistochemical technique (monoclonal antibodies to nuclear antigen of cell proliferation). Single injections of¹³¹I in the above dose were shown to noticeably suppress the cell proliferative activity in a few structures of the brain of newborn rats. The most significant drop in the proliferative index in the external layers of the cerebellum was observed after¹³¹I injections performed on the 16th day of embryogenesis (when the embryonal thyroid gland begins its functioning).     

Formation of the external zone of the median eminence of rats in the perinatal period

The formation of interrelations of the axons of neurosecretory cells and of ependyme cells with the capillaries of primary portal plexus in rats from the 14th day of embryogenesis till the 9th day of postnatal life was studied using the light and electron microscope methods. During the whole period under study, the basal processes of the ependyme cells reach the primary portal plexus of the capillaries. The terminals of the basal processes are usually separated from the endothelium of the capillaries by two basal membranes and enclosed pericapillary space. After the birth, some basal process penetrate in the pericapillary space and terminate on the endothelium. The surface of contact of the ependyme cell processes with the external basal membrane increases with the age, this being accompanied by the increase of pinocytotic activity. The neurosecretory axons are found in the median eminence already on the 14th day of embryogenesis, but by the 20th day only they reach the external basal membrane and penetrate sometimes in the pericapillary space. After the birth, the number of axons reaching the external basal membrane and the surface of contact between them increase gradually with, apparently, a concomitant intensification of the transport of neurohormones in the portal circulatory system of the hypophysial-hypothalamic complex.     

Ontogeny of the mouse motor cortex. The polymorph layer or layer VI. A Golgi and electronmicroscopical study

Summary The development of neurons and their synapses of the mouse motor cortex has been studied from the first postnatal day up to an age of three weeks both electronmicroscopically and with the Golgi method. Special attention has been paid to the maturation of the different cell types in the sixth cortical layer and their dendritic organization within this layer.The polymorph layer is subdivided into two zones: an internal (VIb) and an external one (VIa). In these zones six different cell types can be identified both electronmicroscopically and with the Golgi method: large, small and inverted pyramidal cells in VIa; horizontal cells, star cells and small pyramidal cells in VIb.Spines of apical dendrites of large pyramidal cells in sublayer VIa can be detected as early as the 6th postnatal day. About the ninth day the basal dendrites as well show emerging spines. Somatic spines are found only on the large pyramidal cells and disappear slowly towards the end of the 3rd postnatal week.The small pyramidal cells show developing spines on their apical dendrite in the first half of the second postnatal week. The final density and distribution of spines is reached by the stem dendrites towards the end of the second week, by the basal dendrites during the third week. The maturation process of the improperly orientated neurons occurs in time in between the large and the small pyramidal cells.The axo-somatic synapses appear in general at a later date than the axo-dendritic ones. In the horizontal cells axo-somatic synapses are visible already at the seventh postnatal day.At the end of the first week especially in layer VIb many immature neurons with an ovoid or round nucleus are present having little if any endoplasmic reticulum organised as ergastoplasm.Towards the end of the second week however most neurons in the polymorph layer have a well developed endoplasmic reticulum.Electronmicroscopical pictures reveal in outgrowing dendrites many enlargements filled with vesicles, these correspond to the varicosities seen in Golgi pictures. At nine days postnatally the first myelinated fibres appear.Aided by grant (R-209-67) from the United Cerebral Palsy Research and Educational Foundation, New York.