Correspondence between Glutamine Synthetase Activity and Differentiation in the Embryonic Retina in Situ and in Culture

The development of glutamine synthetase (GS) in the neural retina of chick embryos has recently been employed in studies on the control of specific biosynthesis in differentiating cells (Kirk and Moscona, 1963; Moscona and Kirk, 1965). Retinal GS activity is characterized by a sharp increase that begins about the 16th day of embryonic development, continues until after hatching, and coincides with the general period of functional maturation of the retina (Rudnick and Waelsch, 1955; Rudnick, 1959; Lindeman, 1947). This rapid increase in GS activity can be reproduced in vitro, in organ cultures of retina from 16-day embryos (Moscona and Hubby, 1963; Kirk and Moscona, 1963; Moscona and Kirk, 1965); furthermore, it can be elicited precociously in cultures of retinas from early embryos, resulting in the premature enhancement of this aspect of retinal differentiation (Moscona and Kirk, 1965). These findings made it of interest to determine in further detail whether increases in retinal GS activity, in the embryo and in culture, coincided with other changes characteristic of differentiation in the retina. Since, in the embryo, histological development progresses differently in different areas of the retina (Weysse and Burgess, 1906; Coulombre, 1955; Shen et al., 1956), we looked for differences in the distribution of GS activities and for possible topographical and chronological correspondences between enzymatic and cytologic changes. The distribution patterns of GS activity were mapped throughout retinas from chick embryos, chicks, and young adults; these maps were compared with regional differences in the structural maturation of retinal photoreceptor cells, as indicated by the development of the paraboloid bodies in the visual processes. In addition, the appearance of these structures was examined in retinal cultures with precociously elicited high GS activities to determine whether histological development was also enhanced.