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As suggested by Del Rio Ortega a long time ago, it is now widely accepted that microglia are the resident macrophages of the central nervous system. Microglia represent about 10% of the adult brain cell population. We have previously shown that the late embryonic and adult mouse brain contain potential microglial progenitors. We report here that microglial progenitors can be detected in neural folds from embryonic day 8. They originate from the yolk sac in which macrophage progenitors are found from embryonic day 7. We also report that the bulk of microglial cells (about 95%) appear during post-natal development. A major finding is that microglia arise by an intense in situ proliferation comparable to that of neural cells. Taken together, these results show that adult mouse microglia originate from cells migrating from the yolk sac and whose progeny actively proliferates in the brain during development.

Immunoreactive neuropeptides have been found in cultures of neuroretina cells obtained from 7 day Chick embryos. Methionine-enkephalin and thyroliberin (TRH) were present in a large number of neurons, while somatostatin, beta-melanotropin (MSH) and alpha-endorphin were occasionally seen in a few neurons. In cultures devoid of neurons and probably made of Müller (astroglial) cells, methionine-enkephalin, alpha-MSH and TRH were each detected in 60 to 80% of the cells. Immunoreactive methionine enkephalin was also localized in Müller cells in retina sections of 10 and 20 day old embryos.

Primary cultures of glial cells from newborn rat forebrain were tested after 3 to 4 weeks. Oligodendrocytes and astrocytes were characterized by immunofluorescence with monoclonal antibodies to galactocerebroside and glial fibrillary acidic protein, respectively. The cytoplasm of oligodendrocytes was specifically and intensely immunostained with monospecific polyclonal antibodies to the cytochrome P-450scc involved in the synthesis of pregnenolone from cholesterol. This observation brings additional support to the concept of "neurosteroids".

Cells dissociated from the ventral medulla of embryonic or adult mice were cocultured on monolayers of an immortalized cerebellar astroglial clone. After 10-12 days, rare single cells gave rise to foci that formed colonies of 200,000 cells with a mixed macroglial and microglial phenotype.

Immunoreactive neuropeptides have been found in cells of permanent lines originating from nervous (cerebellum, retina) and non nervous (mesenchyme, kidney, bladder, colon, lung, melanocyte) tissues. The pattern of neuropeptides varied from one cell type to another, but all the cell lines studied contained immunoreactive alpha-endorphin and methionin-enkephalin.