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nothing to do with the amount of GS produced or the proper
ratio of neurons to glia is somehow obtained. Although it
was shown that GS is produced in glia under clusters of
A2B5(+) neurons it could not have been determined whether
more GS activity was induced under large clusters than
under small clusters. The proper ratio of neurons to glia
could have been accomplished by division of the glia since
these cells have been shown to incorporate 3H-thymidine in
culture.
There appears to be an absolute correlation between
having cell surface A2B5 antigen and not synthesizing new
DNA in cultures of tectum cells. The lack of 3H-thymidine
labelled nuclei in neurons in cultures from unpurified day
7 or 8 cells was expected since other workers have found
that the majority of neurogenesis in the tissue has already
occured by this time (LaVail and Cowan, 1971b; Puelles and
Bendala, 1978). The fact that no labelled neurons were
found suggests that all of the neurons that survived in
culture had completed their terminal S-phase by day 7.
Similarly, the lack of any labelled nuclei in recruited
A2B5(+) cells that appeared in purified cultures
demonstrated that new DNA synthesis was not required for
cell surface expression of A2B5 antigen. The antigen may
have been expressed via a mechanism as discussed in the
section above. The neurons that developed in these
cultures, likewise, did not require new DNA synthesis for