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produced eventually in most if not all astrocytes. Glial
fibrillary acidic protein (GFAP) appears in a small
population of astrocytes beginning at day 16 of
development. The oligodendrocyte specific marker myelin
basic protein (MBP) appears on day 12 followed by S-100 on
day 16. This is in striking contrast to the neuronal
marker, neurofilaments, which appears beginning on day 3 of
development when neurons begin to exit the mitotic cycle
(Bennett and DiLullo, 1985). Unfortunately, however,
another widely used neuronal marker, neuron-specific
enolase, does not appear in chick brain until much later
(day 17) (Ledig et al., 1985).
The monoclonal antibody A2B5 which was used to
tentatively identify and remove neurons immunomagnetically
seemed at first to be a relatively stable marker with
freshly dissociated cells. It reacted with approximately
50% of dissociated cells from days 7-13. If it was assumed
that A2B5 was reacting with the same population of cells
throughout this range of ages, it would be interesting to
examine whether induced deficiencies of A2B5(+) cells and/
or neurons would be compensated for if day 12 or 13 A2B5(-)
cells were purified as they were with day 7 and 8 cells.
Since two known glial differentiation markers have appeared
in the tissue by this time (GS and GC), it might be
hypothesized that the capacity for neuronal "recruitment"
from other cells would be very limited. In several other