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antigen. Also, the A2B5(+) cells did not require new DNA
for their differentiation, whether it was into apparently
normal neurons or into cells that also expressed GFAP
and/or 5A11. In this respect these cells were like neurons
(post-mitotic) and not the majority of A2B5(-) flat cells
that were apparently glia and proliferated in culture. The
lack of labelled nuclei in oligodendrocytes also indicates
that the only cell population that may have proliferated in
culture was the astrocytes.
The possibility of the existence of "type 2"
astrocytes (Raff et al., 1983a) in the cultures must be
discussed. These cells express both GFAP and A2B5 antigen
and are thought to be a normal cell phenotype in vivo. To
my knoweldge, these cells have never been documented in the
chick system. Even in the rat system where they have been
reported they are peculiar to the white matter fiber
tracts, optic nerve and corpus callosum, and were not found
in the brain (Raff et al., 1983b). Furthermore, when
A2B5(+) cells were lysed via complement in that system, no
new A2B5(+) cells appeared in culture. GC(+) cells were
found to develop on time in cultures made from dissociated
rat brain cells 13-14 days before they normally appeared in
vivo (Abney et al., 1981). But, GC(+) oliodendrocytes did
not develop in cultures of purified A2B5(-) cells from day
13-15 cells (Abney et al., 1983). Thus, the rat system
appears to be different from chick in several respects.