Differentiation of photoreceptors and horizontal cells in the embryonic mouse retina: an electron microscopic, serial section analysis.

The early differentiation of photoreceptors and horizontal cells in the mouse retina has been studied with serial thin sections and reconstructions in embryos on the fifteenth and seventeenth days of gestation (E15 and E17). The following developmental sequences have been inferred. At E15 photoreceptors develop from ventricular cells when a long vitreal process fails to develop following mitosis, and the end of the ventricular process forms a bulbous enlargement (the future inner segment) which contains a pair of centrioles and a cilium and extends into the optic ventricle. This future inner segment is considerably larger at E17, but otherwise the photoreceptors resemble those seen at E15. At E15 horizontal cells develop from ventricular cells when a long vitreal process fails to develop following mitosis, and the end of the ventricular process detaches from the junctional complex at the ventricular surface. By E17 future horizontal cells are located in the middle of the ventricular layer (neuroblastic layer) and have developed from bipolar shaped cells into cells with multiple branching processes, predominantly radially arranged but rarely with a more tangential orientation. These relatively advanced cells at E17 resemble closely the earliest stage of horizontal cell formation described previously in silver studies by Cajal. A scheme is proposed which explains the initial differentiation of several of the major cell types in the retina in terms of two key features: whether or not the call remains attached to the junctional complex and whether or not a vitreal process grows into the ganglion cell layer. By independent variations in these two features, four classes of cells are produced that, by virtue of their differing environments, differentiate into four cell types: ganglion (and amacrine) cells, horizontal cells, photoreceptors, and Müller cells.