The discussion so far has focused on the potential roles of Müller cells in retinal development, and it is clear that Müller cells produce extrinsic
agents that affect neuronal differentiation and survival. Do retinal neurons influence Müller cell differentiation? There is some evidence that the biochemical differentiation of Müller cells depends on contact interactions with retinal neurons. This idea is best illustrated by the pioneering studies of Moscona and his associates who examined the role of neuron-Müller cell interactions in the induction of glutamine synthetase, a Müller cell-specific protein, in the chick retina (cf. Moscona, 1987; Linser and Moscona, 1979, 1983; Linser, 1987; Vardimon et al., 1988,1983; Gorovits et al., 1994). In the developing chick retina, the levels of glutamine synthetase are fairly low until days 15-16, but the enzyme can be induced precociously by administering glucocorticoids to the embryo (Fig. 2.16). Thus glucocorticoid receptors
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