Interactions of neurons and astrocytes during growth and development of human fetal brain in vitro

The stimulating influence of the raphé nuclei on the morphofunctional development of the hippocampus during their combined cultivation

It has been demonstrated in combined (raphé nuclei-hippocampus, cerebellum-hippocampus) organotypical cultures of the brain of newborn rats that, unlike cerebellar explants, the raphé nuclei grow in an oriented manner toward the hippocampus and exert a stimulating influence on its morphofunctional development; this is manifested: a) in an intense proliferation and migration of glial cells, which are accompanied by accelerated growth of neuronal processes and an increase in the dimensions of the initial piece; b) in the earlier formation of the background electrical activity of the neurons; c) in an increase in these cultures in the number of active cells; d) in a significant predominance of neurons with periodic (in the form of trains or groups) discharges. These data are in agreement with previously established effects of exogenous serotonin, and attest to an important role of the serotoninergic influences of the raphé nuclei in the morphofunctional development of the hippocampus.

Are neurons of the arcuate nucleus necessary for pathfinding by GnRH fibers arising from third ventricular grafts?

The hypogonadal (hpg) mouse lacks GnRH due to a severe truncation of the gene by which it is encoded. This results in an infertile animal with an infantile reproductive system. When fetal or 1-day postnatal septal/preoptic area of a normal mouse is grafted into the third ventricle of an hpg mouse, GnRH-containing fibers grow out of the grafts and innervate the host median eminence (ME), a normal target of these fibers. GnRH axons exiting the graft course follow a very stereotyped pathway through host tissue. They are observed passing through the ependymal wall of the ventricle directly into the ME or arching through the host arcuate nucleus to terminate in the host ME. Given the fixed pattern of outgrowth, we wanted to determine if the neurons of the arcuate nucleus, which lie between the graft and its target, are exerting an influence on the growth and direction of these fibers. The excitotoxin monosodium glutamate (MSG) has been shown to destroy the vast majority of arcuate neurons when administered neonatally. Mutant host animals treated with MSG received fetal grafts of normal septal/preoptic area. Brains were examined for GnRH fiber outgrowth 30 days later to assess early outgrowth which preferentially uses the arcuate route. We report here that the pattern of outgrowth is virtually identical to that observed in saline-injected, grafted animals. There is also no difference in the success rate of grafts placed in control vs MSG-treated hosts nor in the stimulation of testicular growth. The results of this experiment imply that axonal outgrowth to the ME does not rely on arcuate neurons for guidance information or trophic substances. These functions may be subserved by glia, tanycytes/ependyma, or the target.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

Changes in the molecular structure of mouse fetal astrocyte nucleosomes produced in vitro by methylmercuric chloride

The fluorescent probe N-(3-pyrene)maleimide, which specifically labels the cysteine residues of histone H3 within the nucleosome, was used to monitor changes in the nucleosomal structure of mouse fetal astrocytes exposed to varying concentrations of methylmercuric chloride. Methylmercuric chloride treatment (10 microM) for 6 hr produced a significant decrease in the degree of fluorescence of the probe. The decrease was much smaller following a 4-hr incubation period. These results correlate with recent observations showing that significant changes in the thymidine labeling index occur following 4-6 hr of exposure to methylmercury (MeHg). It is hypothesized that MeHg enters the cells during the growth phase and attaches to the protein moiety of the nucleosome in or near the cysteine groups of histone H3, thus diminishing the binding capacity of the fluorescent probe. Addition of a detergent (sodium dodecyl sulfate) resulted in only a small increase in the degree of fluorescence of the treated nucleosomes as compared to controls, showing that the interaction of MeHg with the nuclear proteins was not dissociated by detergent. In view of the strong interaction between DNA and the histone dimer H3-H4 and the potential importance of the latter in gene regulation, these results suggest an additional means by which MeHg may exert its toxic effects.

The effects of serotonin on the morpho-functional development of rat cerebral neocortex in tissue culture

The morpho-functional development of the visual area of the cerebral cortex of newborn rats and effects of serotonin (5-HT) on these processes were studied in long-term (up to 40 days) organotypic cultures. It was shown that systematic addition of 5-HT in physiological concentrations to the nutrient medium during cultivation of the explants stimulates glia proliferation, neuron differentiation, neuropil formation, axon myelination and synaptogenesis. Electrophysiological studies of cortical cells in 5-HT-treated cultures showed earlier exhibition of spontaneous activity, increased number (by 33%) of spontaneously firing neurons and prevalence of periodic (bursting) type of discharges. Cultivation of neurons in a medium with 5-HT approximates their sensitivity to 5-HT to that observed in situ. The data obtained are indicative of the stimulating effects of 5-HT on the morpho-functional development of the rat cerebral cortex in tissue culture.

Non-neuronal cell proliferation in tissue culture: implications for axonal regeneration in the central nervous system

A tissue culture model has been developed to examine the hypothesis that proliferating non-neuronal cells may constitute a physical and/or chemical barrier to regenerating neurons in the central nervous system. Explants from the sensorimotor cortex of 20-day-old fetal rats were cultured in serum medium (control) or serum medium containing 10(-5) M cytosine arabinoside (AraC), a mitotic inhibitor, for varying periods: 2-10, 4-12, 4-10, 4-8 and 4-7 days in vitro (DIV). The center and outgrowth zone of the explants were examined by phase-contrast microscopy at varying intervals between 3 and 18 DIV. The extent of central degeneration was greatest in explants treated with AraC from 2 DIV, and was least in the 4-7 day treated group in which only minimal degeneration was evident at 13 and 18 DIV. In the outgrowth zone at 18 DIV non-neuronal cell proliferation was controlled in the 4-10 day treated explants, although this was accompanied by extensive degeneration of neurites. Further examination of neurite viability, using a neurite viability ratio, revealed that degeneration was first evident at 6 DIV in the 2-10 day treated explants, but not until 9 or 13 DIV in any of the explants exposed to AraC from 4 days onwards. There was minimal degeneration in the 4-7 day treated explants. Electron microscopic examination revealed the presence of atypical inclusions in non-neuronal cells of 4-8 day treated explants, suggesting that the cytotoxic effect of AraC may be due to a disturbance in lipid and/or ganglioside metabolism. Quantitative electron microscopic analysis of the outgrowth zone at 18 DIV revealed a significant increase in the summated area of neuronal tissue (from 7 to 18 microns2/100 microns2) and a decline in the summated area of non-neuronal cells (from 83 to 61 microns2/100 microns2) for explants treated with AraC from 4 to 7 DIV compared to control. Diminishing the potential of non-neuronal cells to act as a barrier by controlling their proliferation may, therefore, be of importance in enhancing the regenerative response of central neurons.

The comparative effects of methylmercuric chloride and mercuric chloride upon DNA synthesis in mouse fetal astrocytes in vitro

The relative effects of direct exposure to methylmercuric chloride (MMC) and mercuric chloride (MC) upon [3H]thymidine incorporation were determined using cultured mouse fetal astrocytes. The labeling indices of cells exposed to MMC were significantly lower than those of cells exposed to equimolar concentrations of MC. These results indicate that on a mole-for-mole basis, MMC is much more deleterious upon DNA synthesis of mouse fetal astrocytes than those of MC.

Cyclic morphological changes of glial cells in long-term cultures of rat brain

Brain cells from embryonic rats were dissociated with trypsin, cultivated under constant conditions in Falcon flasks, and studied for periods of one year or more. Antisera against glial fibrillary acidic protein (GFA) and myelin basic protein (MBP) were used to identify glial cell types. For scanning electron microscopical (SEM) observation an embedding method in resin was developed that allows good preservation of the fine ultrastructural features of the cultivated cells and precise characterization of the cell types. Under our culture conditions, after four subcultures and 8-10 weeks of cultivation, the following cell types can be distinguished. (a) Flat epitheloid cells. From an immunocytological point of view these cells form a heterogeneous population composed of GFA- and MBP-positive and negative cells. They are the precursors of the following cell types. (b) Astroglial cells. SEM observations show a characteristic network of radially orientated prolongations. 92% of these cells are GFA-positive. (c) Oligodendroglial cells with characteristic dichotomously dividing branches. Secondary and tertiary branches end in flat amoeboid prolongations. These cells are MBP-positive. After approximately six weeks the most prominent cells are the flat epitheloid cells. The astroglial cells originate continuously from the epitheloid cells during the whole cultivation time. The formation of oligodendroglial cells, on the other hand, takes place at relatively precise intervals of time (approximately every 20-30 days) over the entire cultivation period (of more than one year).

Analysis of normal and abnormal amniotic fluid cells in vitro by cinemicrography

Time-lapse cinemicrography has been used to study cell behaviour and movement in long- and short-term amniotic fluid cultures from various types of fetus. These included several with a neural tube defect (9 anencephalics, 5 spina bifidas, 2 encephaloceles), 1 with an abdominal wall lesion (omphalocele) and 9 controls with no open lesion. In short-term (less than 3 days) amniotic fluid cultures from normal fetuses, non-adherent squamous cells dominate, but there is a small population of cells which, in the longer term (approximately 4 weeks), will form colonies of epithelioid and fibroblastic cells. In addition, there is a further group, called 'AF' by Hoehn et al. (1974), which have the ability to form syncitial strands and are often multinucleated; these cells seem to be trophoblastic in origin. In contrast to the controls, short-term anencephalic cultures contained many adherent cells; these were mainly latex-particle-phagocytosing macrophages and neural cells. In long-term cultures, the neural cells dominate and differentiate into a range of recognizable forms whose type and behaviour seem to depend on cell density, time in culture and extent of cell-cell contacts. In the spina bifida and encephalocele cultures, there were initially far fewer adherent cells than in the anencephalic ones, but, after approximately 2 weeks in vitro, a range of neural cell types could be recognized in addition to those seen in the controls. In the context of prenatal diagnosis, the presence of neural cells certainly indicates that the fetus has a NTD, but the difficulty of standardizing culture conditions implies that any differential diagnosis on the basis or morphology and movement will be difficult. If, as seems likely, the AF cells of Hoehn et al. (1974) are indeed trophoblastic, amniotic fluids from the second trimester may provide a useful source of these cells.

Laminin is produced by early rat astrocytes in primary culture

The production of laminin by early rat astrocytes in primary culture was investigated by double immunofluorescence staining for laminin and the glial fibrillary acidic protein (GFAP), a defined astrocyte marker. In early cultures (3 d in vitro; 3 DIV) cytoplasmic laminin was detected in all the GFAP-positive cells which formed the major population (80%) of the nonneuronal cells present in cultures from 20-21-d embryonic, newborn, or 5-d-old rat brains. Monensin treatment (10 microM, 4 h) resulted in accumulation of laminin in the Golgi region, located using labeled wheat germ agglutinin. Laminin started gradually to disappear from the cells with the time in culture, was absent in star-shaped, apparently mature astrocytes, but remained as pericellular matrix deposits. The disappearance of cellular laminin was dependent on the age of the animal and the time in culture so that it started earlier in cultures from 5-d-old rat brains (5 DIV) and approximately following the in vivo age difference in cultures from newborn (12 DIV) and embryonic (14 DIV) rat brains. Our results indicate that laminin is a protein of early astrocytes and also deposited by them in primary culture, thus suggesting a role for this glycoprotein in the development of the central nervous system.

Cell morphology in long-term cultures of normal and abnormal amniotic fluids

The cell morphology of long-term cultures of amniotic fluid cells from 10 fetuses with a neural tube defect (NTD) and three with omphalocele was examined and compared to 30 long-term cultures of normal amniotic fluids as well as a long-term culture of human fetal brain. Cultures from the amniotic fluids of the fetuses with NTD and omphalocele showed cells with the same general characteristics as normal amniotic fluid cells. However, the cultures of amniotic fluid cells from NTD pregnancies had an additional cell type also seen in fetal brain culture. This was a neuroblast-like cell, with small rounded refractile morphology and long branching processes forming clusters of varying sizes which lay on top of large flat cells. These neuroblast-like cells diminished in number with time in culture and were not present in subcultures. Their possible neuronal origin is discussed.

Radial glia of developing human fetal spinal cord: Golgi, immunohistochemical and electron microscopic study

Radial glia of the developing spinal cord were examined by Golgi, electron microscopic and immunohistochemical methods in 40 human embryos and fetuses of ovulation age 6-20 weeks. As in developing cerebrum and cerebellum, gliogenesis in the spinal cord occurs much earlier than traditionally believed. The presence of transitional forms suggests that subependymal glia originate from detached radial glia that divide and differentiate in their new location. Well-differentiated astroglia evolve from radial glia as fetal age advances.

Evolution of Bergmann glia in developing human fetal cerebellum: a Golgi, electron microscopic and immunofluorescent study

Astrogliogenesis in the human fetal cerebellum was examined in 46 cerebella obtained from hysterotomy specimens ranging between 9 and 20 weeks of ovulation age. By correlating the results obtained by rapid Golgi and Golgi-Cox methods, the indirect immunofluorescence technique for glial fibrillary acidic protein, and electron microscopy, it was possible to ensure identification of cells and obtain a comprehensive view of the ontogenesis of cerebellar astroglia, in particular Bergmann fibers. Radial fibers were present at 9 weeks of ovulation age, with features of astroglial differentiation. In the cerebellar hemisphere radial fibers arising near the ventricular zone did not reach all the way to the pial surface but terminated in vascular walls of the intermediate zone. A second set of glial cells located in the intermediate zone gave rise to long, tapering processes oriented radially to the pia, some reaching to the pial surface and terminating there in conical swellings. Radial glia with these features were observed in cerebella at all fetal ages examined, indicating their availability for guidance of external granular cells as they migrate inward. With advancing fetal age, the segment of those radial glia traversing the molecular layer demonstrated an increasing resemblance to Bergmann fibers, though the cell bodies giving rise to these processes were still located below the Purkinje cells. Transitional forms between radial glial processes and fibers beginning to resemble Bergmann fibers were observed in numerous specimens impregnated with the Golgi methods. Astrogliogenesis in human fetal cerebellum occurs earlier than formerly believed, and Bergmann fibers are a final stage in the development of a defined group of radial glia in the cerebellum.

A light microscopic study of the developing human neural retina

Human embryonic and fetal eyes from 4 mm to 200 mm crown-rump length (26 days to 21 weeks of gestation) were studied in 0.75-micron sections impregnated with silver and toned with gold. The layer of Chievitz was formed by an inward migration of ganglion-cell and Müller-cell nuclei from the outer neuroepithelium and then, after further changes in nuclear location, the layer of Chievitz was cleared of ganglion-cell nuclei to become the definitive inner plexiform layer. Müller-cell nuclei later populated all retinal layers in the fetus. The argyrophilic Müller-cell cytoplasm associated with neuronal development is discussed in relation to glia-neuron interactions, with emphasis on the particular needs of retinal tissue.

Studies of cultured human and simian fetal brain cells. I. Characterization of the cell types

Explant cultures of the cerebral subventricular zone and cerebellar external germinal layer were established from fetal human, rhesus and cynomolgus monkey brains. Using comparable gestational ages, the morphogenesis of the cultures from these three sources was almost indistinguishable. Four cell types were distinguished by electron microscopy. Germinal cells or neuroblasts were confined largely to the primary explant and extended into a transitional outgrowth region. Astrocytes, which stained for glial fibrillary acidic protein, grew out of the explants and these could be distinguished from the large, mesenchymal epithelioid cells. A fourth cell type, not identified in previous studies, had the ultrastructural characteristics of an oligodendrocyte, but did not produce myelin in these culture conditions.

Radial glia in the human fetal cerebrum: a combined Golgi, immunofluorescent and electron microscopic study

Golgi techniques, immunofluorescence for glial fibrillary acidic (GFA) protein, and electron microscopy (EM) were used to determine the nature of radial glia in the cerebrum of human fetuses ranging from 7 to 20 weeks of ovulation age. Successful Golgi impregnation of radial fibers was achieved in fetuses 12 weeks of age and older. These fibers spanned the entire thickness of the hemisphere. At the pial surface many of them branched and terminated in pyramidal end feet expansions. Indirect immunofluorescent preparations utilizing antiserum to GFA protein, a protein specific for astrocytes, demonstrated numerous radially oriented nearly parallel fluorescent fibres between the ventricular zone and pia mater. GFA protein-positive fibers were demonstrated in all fetal specimens examined with this technique (10 weeks of age and older). Along the outer border of the marginal zone they formed a horizontal GFA protein-containing subpial membrane. By EM there were numerous linear electron lucent astrocytic processes containing 8-9 nm filaments and occasional glycogen granules at all levels of the cerebrum. They were interspersed among smaller and darker neuronal processes containing 20-25 nm neurotubules, and were demonstrable at all fetal ages between 7 and 18 weeks. They formed pericapillary investments and subpial terminal expansions closely abutting basal lamina of pia mater in every specimen examined. On the basis of these combined analyses, we conclude that radial glial fibers in early human fetal cerebrum represent processes of immature astrocytes. Although subsequently undergoing further maturation, radial glia already possess fundamental immunocytochemical and morphological characteristics indicative of astrocytic differentiation. A significant implication of our findings is that the development of astrocytes in the human fetal brain occurs much earlier than formerly believed.

Evidence for the presence of viable endothelial cells in cultures derived from dissociated rat brain

The morphology and histochemistry of dissociated newborn rat brain was studied in tissue culture. Direct microscopy of developing cells, electron microscopy and the alkaline phosphatase activity were used to identify the capillary endothelial cells.