67
13 optic tectum suggest that the recruited cells in
cultures stem from the astrocyte lineage (Chapter 3). The
majority of the recruited A2B5(+) cells in these purified
cultures showed immunoreactivity for GS thus identifying
them as glia. There is also evidence in the chick
peripheral nervous system that certain glial precursors are
capable of being diverted to a neuronal lineage under
certain transplantation conditions (Le Lievre et al.,
1980) .
The mechanism of recruitment is also unclear. One
possibility is that there exists either a negative feedback
system between A2B5(+) and A2B5(-) cells or a positive
feedback system between A2B5(-) cells only. With the
negative feedback system the A2B5(-) cells would have
recognized the loss of the A2B5(+) cells by some mechanism
and then reacted as a result of this. With the positive
feedback system the A2B5(-) cells would have sensed an
increase in the density of A2B5(-) cells in the purified
cultures and reacted to replenish A2B5(+) cells. The cell
isolation experiments, however, clearly rule out the latter
possibility since recruitment of A2B5(+) cells occured as a
result of a loss of contact between all cells. What remains
in question then is whether the communication between
A2B5(+) and A2B5(-) cells is via cell contact or soluble
factors.