Subcultured astrocytes suppress the proliferation of neuroblasts in vitro

The effects of subcultured astrocytes on the proliferation of neuronal precursor cells (neuroblasts) from rat embryonic cerebral hemispheres were examined. The survivability of neurons and the neurite outgrowth were significantly enhanced by the subcultured astrocytes compared to those of neurons plated on poly-L-lysine-coated coverslips. The incorporation of [3H]thymidine into neuroblasts was remarkably suppressed by the subcultured astrocytes indicating that the astrocytes inhibit the proliferation of neuroblasts. These results suggest that astrocytes enhance the maturation of neuroblasts possibly via either cell-cell contact or trophic substances.

Prenatal development of nucleus basalis complex and related fiber systems in man: a histochemical study

To provide parameters for study of the "cholinergic" innervation of a human fetal cerebrum, we have analyzed the prenatal development of histochemical reactivity in the nucleus basalis complex (a magnocellular complex known to contain a high concentration of cholinergic perikarya). Brains from fetuses and premature infants ranging between 8 and 35 weeks of gestation were frozen cut and processed by the thiocholine method for the demonstration of acetylcholinesterase activity. Since no consistent results were obtained with inhibitors on the material younger than 15 weeks, the histochemical reactivity for early stages was expressed as the total cholinesterase reactivity. The first sign of histochemical differentiation of the basal telencephalon is the appearance of a dark cholinesterase reactive "spot" situated between the developing lenticular nucleus and basal telencephalon surface as early as 9 weeks of gestation. The first cholinesterase reactive bundle connects this reactive area (nucleus basalis complex anlage) with the strongly reactive fiber system situated along the dorsal side of the optic tract. During the next "stage" (10.5 weeks), there is a significant increase in the size of the nucleus basalis complex and strongly cholinesterase reactive neuropil occupies the sublenticular, diagonal and septal areas. At this stage we have seen two new cholinesterase-reactive bundles: one well developed cholinesterase reactive fiber stratum approaching (but not penetrating) the neocortical anlage through the external capsule and another minute bundle running towards the medial limbic cortex through the precommissural septum. The supraoptic fiber system can be traced now to the pregeniculate area and the tegmentum. At 15 weeks, the first acetylcholinesterase reactive perikarya appear and the nucleus basalis complex anlage becomes segregated into several strongly reactive territories, corresponding in position to the medial septal, diagonal and basal nuclei as defined on adjacent Nissl stained sections. At this stage, fibers from the nucleus basalis complex enter the "white" matter of frontal, temporal, parietal and occipital parts of the cerebral hemisphere via the external capsule. Between 15 and 18 weeks, acetylcholinesterase fibers spread throughout the "white" matter of the cerebral hemisphere. In the next "stage" (18-22 weeks), strongly reactive fibers can be followed from the nucleus basalis below the putamen and through the external capsule to the transient, synapse-rich subplate zone of frontal, temporal, parietal and occipital cortices.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of GABAergic neurons in chick brain: studies in vivo and in vitro

The ontogeny of presynaptic elements of GABAergic neurons has been studied in the cerebrum of the chick embryo both in vivo and in vitro. The specific activity of glutamate decarboxylase (GAD) in tissue extracts followed a rising curve and approached a plateau value after 28 days in vivo. One-half of the adult levels of GAD were achieved by day 20. The specific activity of Na+-gamma-aminobutyric acid (GABA) cotransport in membrane vesicles followed a similar pattern and reached maximal levels by 28 days in vivo. One-half of the adult levels of GABA uptake were observed at day 17. The development of these markers was also studied in cultured neurons prepared from the cerebrum of 8-day-old chick embryos. The GAD activity in neuronal extracts increased linearly with time in culture up to 14 days. At this point the specific activity had reached 20% of that observed for the adult cerebrum. The specific activity of GABA uptake by intact neurons followed a pattern similar to that for GAD from days 2 to 9 in culture. Both activities increased 4-5-fold during this period, but the level of GABA transport declined thereafter. In order to compare GABA uptake values for cultured cells with those for embryonic and adult brain, membrane vesicles were prepared from cultures. At the maximal level (9-10 days in culture) the vesicular GABA uptake represented 33% of that in the 18-day embryo and 20% of adult levels. Thus the presynaptic GABAergic components developed according to similar schedules both in vivo and in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic development of the nervus terminalis in toothed whales. Evidence for its non-olfactory nature

For the first time in cetaceans, the development of the terminalis system and its continuity between the olfactory placode and the telencephalon has been demonstrated by light microscopy. In the early development of toothed whales (Odontoceti) this system is partially incorporated within the fila olfactoria which grow out from the olfactory placode. As the peripheral olfactory system is reduced in later stages, a strongly developed ganglionlike structure (terminalis ganglion) remains within the primitive meninx. Peripherally it is connected via the cribriform plate with ganglionic cell clusters near the septal mucosa. Centrally it is attached to the telencephalon (olfactory tubercle, septal region) by several nerve fibre bundles. In contrast to all other mammalian groups, toothed whales and dolphins are anosmatic while being totally adapted to aquatic life. Therefore the remaining ganglion and plexus must have non-olfactory properties. They may be responsible for the autonomic innervation of intracranial arteries and of the large mucous epithelia in the accessory nasal air sacs. The morphology, evolution and functional implications of the terminalis system in odontocetes and other mammals are discussed.

Mapping of the early neural primordium in quail-chick chimeras. I. Developmental relationships between placodes, facial ectoderm, and prosencephalon

Defined fragments of the anterolateral neural ridge and of the associated region of the neural plate of presomitic to three-somite stage quail embryos were grafted isotopically and isochronically into chick hosts. This resulted in the development of apparently normal brain and facial structures to which the contribution of the grafted tissue could be observed by means of the quail nuclear marker. It was shown that the anterolateral neural ridge contains the progenitor cells of the adenohypophyseal and olfactory placodes and also of the superficial ectoderm lining the nasal cavity and conchae and the superficial ectoderm of the beak. When the appropriate region of the neural ridge was involved in the quail-chick substitution, the egg tooth was made up of graft-derived cells. Grafting of the neural plate area adjacent to the "ridge" territory containing the placodal ectoderm revealed that the presumptive region of the hypothalamus is in contiguity with that of the adenohypophyseal placode. The same observation was made for the olfactory placode and the floor of the telencephalon from which the olfactive bulb later develops.

Differential growth of the cell production systems in the lateral wall of the developing mouse telencephalon

Three major cell production systems were identified in the lateral wall of the mouse telencephalon. These were morphologically evident as the medial and lateral elevations and the pallial crescent. Each was originally derived from a small, circumscribed, subset of ventricular cells. These formed the parent populations of large and proliferatively complex precursor pools which gave rise to the large numbers and considerable variety of neuron populations of the telencephalon. An attempt was made to identify the major neuron groups derived from each system by using the ventricular cell processes as a guide to the site of neuron origin. The proliferative changes occurring in the two elevations, the variety of their neuronal output, the early loss of a radial structure and the diversity of the final adult configurations were considered to represent a more complex series of changes than the corresponding events in the cortical tissue, which was generated from the pallial crescent where neurons accumulate within an adaptation of the original radial structure. It was considered that the number and complexity of the changes in the genome required to produce and organise such subcortical diversity was of a greater order of magnitude than those evident in the cortical areas. It was suggested that the genesis of the mammalian basal telencephalon should be considered as one of the major evolutionary achievements in the conglomerate of changes which occurred during the transition from the reptilian to the mammalian grade of organisation in the forebrain.

Human telencephalic angiogenesis

We provide new observations regarding the histogenesis and regional development of the human telencephalic microvasculature during the last half of gestation. Endothelium-lined trunks extending from pia to the subventricular plexus, evident early in gestation, persist during the last half of gestation. The courses of these trunks are modified and become complex as the bulk of the telencephalon, striatum, and thalamus increases and as gyri grow. New cerebral tissue is supplied by increasing numbers of shorter penetrating vessels. All extrastriatal vessels have many lateral right- and acute-angle branches that join nearby trunks and shorter vessels. Striatal vessel branches predominantly have acute angles. Most extrastriatal channels remain devoid of apparent muscularis until the final weeks of gestation. In contrast, striatal arteries begin to muscularize at about 24 weeks of gestation. Muscularization appears to occur in a centripetal direction and is apparent in the caudate at approximately 30 weeks' gestation. We did not identify transventricular, paraventricular, or recurrent arteries ending in deep white matter.

Studies of the earliest generated cells of the cat's visual cortex: cogeneration of subplate and marginal zones

The earliest generated cells of the cat's telencephalon that may play a role in the formation of the primary visual cortex are the subject of this study. Using [3H]thymidine autoradiography, we have found that these cells are generated between embryonic day 24 (E24) and E30 (gestation is 65 days) and that they are present in very low numbers in the white matter of the adult brain. These cells are rarely labeled by injections made after E30, when the cells destined for the cortical layers are generated. Examination of the labeling pattern in the fetal brain 10 days or more after administration of [3H]thymidine between E24 and E30 revealed a bistratified distribution of these early generated cells. Labeled cells were found in large numbers in two embryonic zones flanking the developing cortical plate: above in the marginal zone and below in the subplate. (Some if not all of the marginal zone cells constitute the population of Cajal-Retzius cells of the cat's telencephalon.). These experiments indicate that cells of the subplate and marginal zones are cogenerated in time during the days just preceding the genesis of the cortical plate. We also examined the distribution of the early generated cells shortly after their genesis--on E30, a time when cells of the cortical plate are just being generated at the ventricular zone. In this case, the labeling pattern at the occipital pole was not bistratified. Rather, labeled cells were situated within a single zone extending from the pial surface inward to the border of the ventricular zone. This finding indicates that the cells of the subplate and marginal zones are generated as a contiguous population that is subsequently split apart by the insertion of cells forming the cortical plate. A comparison between the number of early generated cells found in fetal and newborn brains with that found in adult brains suggests that these cells are generated initially in substantial numbers but then largely disappear during early postnatal life, since injections of [3H]thymidine between E24 and E30 yielded large numbers of labeled cells in the white matter and layer 1 at birth, but very few at 2 months postnatal. This significant loss contrasted with the results from injections made just a few days later (E33) that resulted in large numbers of labeled cells in cortical layer 6 not only at birth but also in adulthood.(ABSTRACT TRUNCATED AT 400 WORDS)

Myelogenesis and estimation of the number of axons in the anterior commissure of the chick (Gallus gallus)

By use of light- and electron microscopy the anterior commissure of the chick was studied at different times during development. Between the 19th day of incubation and the 35th day after hatching the cross-sectional area of the anterior commissure, as determined from mid-sagittal sections, undergoes a 6-fold increase in size. Thereafter the area remains fairly constant. The total number of fibres in the anterior commissure was estimated to be 89000. The full complement of fibres is already present by the 19th day of incubation. Myelogenesis occurs mainly between the 19th day of incubation and the 35th day after hatching, concomitant with the increase in cross-sectional area. From the 35th day after hatching, myelinated fibres comprise approximately 40% of the total number of fibres. The median diameter of unmyelinated fibres is about 0.35-0.40 micron. The median diameters of myelinated axons and fibres are 0.8-1.0 micron and 1.1-1.3 micron, respectively.

The distribution of plasma proteins in the neocortex and early allocortex of the developing sheep brain

The histogenesis of the cerebral neocortex and early allocortex of the sheep has been described and, using an immunohistochemical technique, five plasma proteins have been identified in the telencephalic wall and their distribution followed during its differentiation. The development of the neocortex was studied from 18 days gestation, when the neural tube was still open, to 120 days, when the adult structure was established. A primordial plexiform layer was formed above the ventricular zone by 25 days and by 35 days this layer was divided by the differentiating cortical plate into an outer marginal zone and an inner subplate zone. The appearance of the subventricular and intermediate zones by 50 days gestation completed the formation of the neocortical layers. The differentiation of the allocortex was generally less advanced than the neocortex up to 40 days gestation, when the primordium of the pyramidal layer was beginning to develop. The five plasma proteins identified, fetuin, alpha-fetoprotein, albumin, transferrin and alpha 1-antitrypsin, are quantitatively the most important in the csf and plasma of the sheep fetus. Fetuin was the earliest plasma protein to be detected in the brain and it was also the most widespread; positive staining for this protein was seen in cells and fibres of all layers as they differentiated and could still be identified in some mature neurons at 120 days. alpha-Fetoprotein and albumin had a limited distribution, appearing in cells in the developing cortical plate for a short period early in gestation (35-40 days), but mainly confined to the ventricular zones later and barely detectable by 80 days gestation. Transferrin appeared to have a different distribution, being detected in fibres first in the primordial plexiform layer and then in the marginal and subplate zones, only later being identified in cells of the cortical plate. From their distribution it is suggested that fetuin and transferrin may play an important role in the differentiation of the cortex and the establishment of correct connections between fiber systems and migrating cells at certain stages of development. alpha 1-Antitrypsin was only found in a few cells during a restricted period of gestation. All five plasma proteins were identified in precipitated csf and plasma at most ages examined, although at 18 days gestation albumin, transferrin and alpha 1-antitrypsin and at 120 days, alpha-fetoprotein, could not be detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient expression of a neurofilament protein by replicating neuroepithelial cells of the embryonic chick brain

An immunohistochemical survey was carried out on frozen sections of the early embryonic chick brain between 1 and 6 days of incubation, with antisera to the three neurofilament proteins (NF-L, NF-M, NF-H). Large numbers of replicating neuroepithelial cells were found to express one of these proteins, NF-M, generations before the existence of any postmitotic neuroblasts (Days 1-2 1/2 of incubation). NF-L and NF-H could not be detected. Not all primordial brain regions contained NF-M-positive cells, but in those that did, every cell was positive. These regions included the dorsal forebrain, optic vesicles, and dorsal hindbrain, but not the dorsal midbrain. All cells in all regions of the cephalic neural tube contained vimentin, whether or not they also contained NF-M. This NF-M expression was transient in the sense that later generations of these NF-M-positive neuroepithelial cells became NF-M negative, before finally giving rise to some descendents that ultimately express all three NF proteins. This transient NF-M expression was found in certain other cells of early embryos, including cardiac myoblasts. The identity of the component in these early neural and nonneural tissues, that bound the antibody, was demonstrated to be identical to adult brain NF-M by one- and two-dimensional immunoblots. These findings demonstrate an unusual kind of biochemical heterogeneity among neuroepithelial cells, and they are relevant to considerations regarding lineage analysis and lineage "markers" in the vertebrate central nervous system.

Postnatal development of the telencephalon of the tammar wallaby (Macropus eugenii). An accessible model of neocortical differentiation

The sequence of development of cell layers in the neocortex of the tammar has been followed from 24 days gestation to 213 days postnatal. The tammar is born at 27 days gestation and the major period of its development occurs during the subsequent 250 days, most of this time being spent within the pouch. Although the pattern of differentiation of the cell layers appears to resemble that described for many Eutherian mammals, the neocortex is at an embryonic 2 layered stage at birth and a cortical plate is not present throughout the telencephalon until 10-15 days postnatal. A transient subplate zone, presenting a characteristic appearance with widely spaced rows of cells aligned parallel to the cortical surface, develops between 20 and 70 days postnatal, but no secondary proliferative region is seen in the subventricular zone of the dorso-lateral wall. Preliminary experiments with (3H)-thymidine injections indicate that the cortical plate follows the "inside-out" pattern of development described in many Eutherian mammals and that the oldest neurons are found in the parallel cell rows of the subplate zone. The importance of the late differentiation of the neocortex in relation to the time of birth and the resulting usefulness of the tammar as an experimental model of cortical development is discussed.

Histogenesis of the mesocortical area of the mouse telencephalon

The histogenesis of the mesocortex of the mouse telencephalon was studied by plotting the progress of neuron release on reconstructions of the medial pallial wall. Histological changes were correlated with cell birth date using data obtained from autoradiographs of mice pulse-labelled with tritiated thymidine during the prenatal period of neuron birth. It was found that neuron release for this area began rostrally at about E12-E13 and spread rapidly from this origin in a caudal direction across the medial wall. Neurons accumulating in the intermediate layer were at first more or less equally spaced. About E14, neurons in the outer intermediate layer began to line up to form a 'mesocortical plate'. This plate was formed from older nuclei and therefore overlay a deeper intermediate layer composed of younger cells. The mesocortex continued to develop by progressive withdrawal of younger cells from the deep intermediate layer into more superficial layers of the definitive cortex. In most of the mesocortical area, however, this original pattern was superseded by release of neuron generations which migrated directly to the outer intermediate layer to form a plate of densely packed immature neurons. This population was continuous with a similar population forming the isocortical plate of the lateral telencephalic wall. It was postulated that the wave of neuron birth and release which gave rise to the isocortical plate was propagated beyond the isocortical boundary into mesocortical territory as far as the boundaries of the subiculum, indusium griseum and anterior hippocampal rudiment.

Three dimensional growth of the mouse isocortex

The three dimensional growth of the mouse isocortex was examined by plotting the variations in intermediate layer depth on orthogonal projections of the telencephalic surface at successive periods of development; a histological status was assigned to each depth. Thus portrayed, the development of the isocortex was seen as a propagated sequence of histological change, commencing at a rostral focus coextensive with the caudatopallial angle and thence spreading across the telencephalic wall. Growth was asymmetric about the focus of origin and terminated in a rostrocaudal direction as the spread of neuron production reached and extinguished a growth zone along the sagittal perimeter of the hemisphere. The possibility of mouse isocortical histogenesis representing a variation of a general mammalian pattern was noted, as was the evolutionary and methodological significance of the apparent coincidence of the origin of the gradient of isocortical neuron release with the region of cortex representing oropharyngeal structures. An alternative form of representation of the isocortical gradient, as the summation of a number of radial strips of tissue each with a similar history of neuron release and migration, was used to lay a foundation for a three dimensional model.

Developmental gradient of cell cycle in the telencephalic roof of the fetal NMRI-mouse

An analysis of cellular kinetics in the NMRI-mouse after 12, 13, 15, and 17 days of gestation was obtained by means of tritiated thymidine autoradiography. After 12 and 13 days there is no significant difference in generation time between the lateral and the medial districts of the telencephalic roof. From 15 days of gestation onwards, the generation time in the lateral parts is significantly greater than in the medial wall regions. Simultaneously, on day 17 the growth fraction is drastically decreased in the lateral parts, while it remains close to 1 in the medial parts. Lengthening of the generation time during differentiation of the lateral wall is mainly due to an extension of the G1-phase and to a lesser degree also of the S-phase. Another significant contribution comes from the increasing length of the mitotic phase.

Regional surface changes during the development of the telencephalic choroid plexus in the chick. A scanning-electron microscopic study

The surface morphology of the developing chick telencephalic choroid plexus (TCP) was examined by scanning electron and light microscopy. A blunt evagination develops rostro-cranially to the foramen of Monro on the medial telencephalic septum. The pseudostratified TCP epithelium differs in its surface morphology from that of the surrounding ependyma. Subsequently the TCP becomes elongated and branches. On the 9th embryonic day (ED) the pseudostratified epithelium progressively becomes high columnar epithelium in a distal to proximal direction along the branches of the TCP. The apical poles of the high columnar epithelial cells protrude into the ventricular lumen. Later, additional branches sprout at the base of the TCP, which then resembles a tree with a bush growing at its roots. Before the time of hatching, the high columnar epithelium changes to low columnar epithelium again in a distal to proximal direction. The surface of the TCP becomes flatter, in the process of which the number of cilia per unit surface area is reduced. On the developing TCP the epiplexus cells vary in shape, depending upon their functional state. It is proposed that not only the morphological but also the functional differentiation of the TCP proceeds in a distal to proximal direction along the branches of the choroid plexus. The surface differentiation of the TCP has a more regular character than that of the diencephalic CP (DCP), described previously, which seems to be influenced in its development by other anatomical structures.

Kinetics of telencephalic neural cell proliferation during the fetal regeneration period following a single X-irradiation at the late organogenesis stage. II. Cycle times and the size of the functional compartment of neural epithelial cells of distinct lesion districts

Autoradiographic studies were conducted at the cerebral hemispheres of mouse embryos X-irradiated on day 12 of gestation and of normal litter mates during the subsequent developmental period. By counting the percentage of labeled mitoses the generation time, the potential doubling time, the growth fraction, as well as the length of the individual cell cycle stages of the neuroblast cells were determined. A continuous increase of generation time was found in the normal brains, concomitant with a latero-medial gradient in telencephalic wall differentiation progress. After X-irradiation this normal differentiation pattern still prevails, but with some marked topographical peculiarrities. The most important finding was a significant lengthening of the generation time at the medially situated rudiments of the ventricular zone and, similarly at the heterotopic cell islets located within the intermediary zone. Concomitant with this effect, which was seen mainly on days 15 and 17 of gestation, there was a marked increase of mitotic time of these special neuroblasts. The latter finding was regarded as a random event only, which has no causal relationship to the pathogenesis of the heterotopic islets or similar overgrowth anomalies after X-irradiation. In spite of the long generation time of these histological peculiarities, they make a considerable contribution to the regeneration of the injured telencephalic wall: Up to day 15 of gestation the heterotopias had a growth fraction of nearly 1.0 (= 100%), whereas the percentage of proliferating cells within the orthotopic remainders of the ventricular zone was only 44%.

X-ray induced dysplasia in the developing telencephalic choroid plexus of mice exposed in utero

Pregnant NMRI-mice were X-irradiated with single doses of 0.95 Gy (100 R) and 1.9 Gy (200 R) on day of gestation (dg) 12. For sampling, anesthetized animals were perfused with buffered glutaraldehyde solution or fixed by immersion in Karnovsky solution. LM, SEM, and TEM studies were carried out on brains prenatally and up to the age of 20 months to follow the radiation effects on the developing lateral choroid plexus. Radiation-induced changes were found using all three methods and at all stages studied. The normally sickle-shaped and stretched choroid plexus is shortened and irregular, and the dome-shaped plexus cells are flattened. Their superficial fine structures, i.e., the microvilli and cilia, are altered. Three stages of severity can be distinguished and the internal hydromicrocephalus increases from stage I to III. Intercellular spaces of the treated plexus epithelium are often dilated, but the tight junctions at the ventricular surface seem to be intact. The interstitium shows large dilations in comparison with the controls. Thus, gross changes and alterations in the fine structure can be induced in the choroid plexus by doses of 0.95 Gy and 1.9 Gy, which persist throughout postnatal life.

Abnormal differentiation of selected nuclear centers in the brain of a duck embryo associated with partial duplication of the primitive streak

In a control set of duck embryos, an abnormal duck embryo of 16 days incubation was found which had two beaks as the only outward sign of duplication. The beaks were of equal size, each with upper and lower bills. Bill-clapping movements were absent. The embryo had two normal eyes placed one on either side of the head, and the rest of the body was normal in external appearance. Sections through the head revealed further duplication of the fore-, mid-, and hindbrain divisions. The medial half of each of the embryonic brain divisions, however, was greatly reduced. Two additional optic primordia were also noticed in sections, each of which was reduced to a mass of tissue representing a lens and a much-folded pigment epithelium. The orbital tissues associated with the rudimentary eyes were greatly disorganized. Abnormal differentiation associated with duplication of the brain divisions was determined by cell counts in selected nuclear centers. Cell numbers in each case appeared to be remarkably proportional to the size of the innervation field. Our data, based on cell counts in the nuclear centers chosen for this study in the abnormal embryo and normal control embryos of the same age, are consistent with the hypothesis that cell survival in related parts of the nervous system may be regulated by their peripheral field of innervation.

Growth patterns in the lateral wall of the mouse telencephalon. II. Histological changes during and subsequent to the period of isocortical neuron production

The histogenesis of the isocortical segment of the lateral telencephalic wall at the coronal level of the interventricular foramen was studied in mice between the ages of E10 and the adult. The proliferative activity of the periventricular germinal layers was correlated with changes in cell distributions in the intermediate layer. The appearances were consistent with a wave of differentiation moving across the ventricular layer from lateral to medial and a peak of neuron production occurring about E13. The sequence of changes was analysed using the concept of a radial unit composed of ventricular cells and their related progeny of neurons. The observed histological changes were interpreted as the result of radial units of similar productive history entering and completing the histogenetic sequence at successively later times along a lateromedial gradient. Some of the implications of this approach were examined and discussed in relation to the general evolutionary properties of such a system of histogenesis.

Intracellular plasma proteins in human fetal choroid plexus during development. I. Developmental stages in relation to the number of epithelial cells which contain albumin in telencephalic, diencephalic and myelencephalic choroid plexus

The developmental stages of telencephalic, diencephalic and myelencephalic choroid plexuses in the human fetus and the morphology of choroid plexus epithelial cells in the various plexuses in different development stages were described on basis of PAS- and toluidine blue-stained material. Six different cell types were identified in various combinations in 4 different stages (I-IV). The number and distribution of albumin-containing epithelial cells in various plexuses in the different stages of development were investigated by indirect immunoperoxidase technique. Albumin-containing cells did not belong to a single cell type. The telencephalic plexus exhibited a staining pattern for albumin which was different from that of the diencephalic and myelencephalic plexuses. In the telencephalic plexus positive epithelial cells were frequent in stage I, whereas only a few positive cells were present in stage II and III. In contrast, 30-40% of the epithelial cells in both diencephalic and myelencephalic plexuses in stage I, II and III showed a positive staining reaction. Later in gestation less than 10% were positive. It is suggested that a main function of the diencephalic and myelencephalic plexuses early in gestation is associated with protein transport rather than with glycogen synthesis and storage which might be a major function of the telencephalic plexus.