CHAPTER II
SEPARATION OF DAY 7-8 CELLS
Introduction
During development of the chick optic tectum the
various differentiated cell types emerge in a temporally
stepwise but overlapping manner. The neurons are generally
the first cell type to exit the mitotic cycle (LaVail and
Cowan, 1971b) and to express a differentiation product such
as neurofilaments (Bennett and DiLullo, 1985). The glia are
generally later in becoming post-mitotic and show overt
signs of differentiation (Linser and Perkins, 1987a)
practically coincident with the completion of neurogenesis
(LaVail and Cowan, 1971b; Fujita, 1964). This timing of
overt differentiation, however, does not necessarily
reflect the timing of when different cells are determined
to become one cell type or another. Additionally, this
general pattern does not necessarily imply when a cell is
restricted in its ability to become anything else if its
microenvironment were to change.
The mechanisms that govern cell determination and
differentiation during brain development seem multiple but
are poorly understood. Interactions between cells appear to
influence development at several levels from physical
positioning of neurons (Levitt and Rakic, 1980) to the
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