Monoclonal antibody 3CB2 recognizes an antigen expressed in discrete cell types derived from ectoderm and mesoderm. Biochemical and immunohistochemical studies indicate that the antigen recognized by the antibody is a 55 kDa cytoplasmic protein that may be an intermediate filament associated protein (IFAP). Developmental studies show that 3CB2 antigen is intensely expressed very early in the chick embryo, in the neural tube, myotomes, and in mesenchymal cells of visceral arches and the presumptive facial area. As development proceeds, antigen expression becomes restricted to astrocytes and radial glia cells throughout the brain. A detailed immunohistochemical study of the developing chick retina shows that the expression of 3CB2 antigen at embryonic day 8 (E8) is restricted to Müller cells, with the pattern of expression closely related to their degree of differentiation. We show, by immunocytochemistry labeling of entire Müller cells dissociated from retinas of E16-E20, that 3CB2 monoclonal is labeling the whole cell. 3CB2 selectively labels Müller cells in the rat and chameleon, but not their retinal horizontal cell axons. 3CB2 monoclonal is a very sensitive marker for early differentiating Müller cells. Our results provide evidence that 3CB2 antigen is a cytoskeletal component which may be involved in the morphogenesis of these cells, and also perhaps in the outgrowth of some axons.

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The profile of glutamine synthetase (GS) activity in the neural retina of chicken embryos and adults was studied alongside the in vivo spatio-temporal patterns of generation and morphogenesis of Müller cell and of retinal synaptogenesis. The rise of GS activity during development is not related to Müller cell differentiation but to synaptogenesis in the outer plexiform layer (opl). GS expression was investigated by immunoreaction with GS-specific antiserum. Three spatial gradients of decreasing labeling were observed between embryonic (E)15 and E18, from central to peripheral retina, dorsal to ventral, and temporal to nasal, which are in spatio-temporal relationship with synaptogenesis in the opl. GS is localized in Müller cells and apparently also in a population of astrocyte-like glial cells, located in the ganglion cell layer throughout the retina. Precocious induction by hydrocortisone, in ovo, at E10, does not show the spatial pattern of GS immunoreactivity observed in control retinas at the time of natural induction (E15). We also show that dissociated (non-cultured) Müller cells of E18-20 retinas, to which only photoreceptors or photoreceptors and neurons remain joined, maintain an immunodetectable level of GS, while those in isolated state lose GS immunoreactivity rapidly. Our results suggest that the induction of GS expression might be mediated by Müller cell-neuron interactions at the opl and also perhaps at the outer nuclear layer (onl). An analysis of our results and those of previous authors suggests that the level of GS in differentiated Müller cells could be determined by conjoint cell interactions at the onl, opl, and inner plexiform layer.

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