Cells from the early chick optic nerve generate neurons but not oligodendrocytes in vitro

We have recently described neuronal potentialities in neuroepithelial cells of the embryonic chicken optic nerve (Giess et al., Proc. Natl. Acad. Sci. USA, 87 (1990), 1643-1647). To further investigate the developmental repertoire of optic nerve cells, oligodendroglial development was studied in cultures of optic nerve explanted at various developmental stages. Oligodendrocyte differentiation was analyzed using antibodies directed against galactocerebrosides (Gal-C) and against sulfatides. Optic nerves removed at embryonic days 5 and 6 (E5-E6) never gave rise in culture to differentiated oligodendrocytes, even after 3 weeks in vitro. In contrast, in cultures of optic nerves removed from E7 or older embryos, cells expressing both oligodendrocyte markers were rapidly and invariably observed. Absence of oligodendrocytes before E7 was not due to culture conditions being inadequate to support the differentiation of early precursors along this pathway, since neuroepithelial cells from E2 and E4 trunk neural tube cultivated in the same conditions expressed Gal-C after respectively 16 and 10 days. These results demonstrate that the optic nerve territory is initially devoid of oligodendrocyte potentialities. Whether oligodendrocyte precursors that, around E7, populate the optic nerve are induced by a specific developmental signal occurring at this stage or migrate from outside the optic nerve remains to be determined.