Characterization of photoreceptor cell differentiation in the rat retinal cell culture

Photoreceptor cell differentiation in the rat retina was studied in vivo and in vitro, using an immunohistochemical method to demonstrate opsin-like immunoreactivity. Cells in a dissociated monolayer culture expressed some properties characteristic of rat rod cells developing in vivo, including a ciliary structure and opsin-like immunoreactivity. Immunoblot analysis revealed that cultured retinal cells synthesize a polypeptide with the same molecular weight as that synthesized by the intact retina. Although the outer segment (OS) was not present in the culture, immunoreactive cells possessed a ciliary structure. Opsin-like immunoreactivity was found on the plasma membrane, including the cilia. The neuritic extensions were also intensely stained. In mature rod cells of the intact rat retina, opsin was detected only on the OS but, during development, it was found both in the somatic region of the rod cells and on the differentiating OS. During maturation of rod cells opsin immunoreactivity seemed to shift to the OS from other locations. However, some "displaced" photoreceptor cells, found in the inner nuclear layer and extending fibers bipolarly, retained immunoreactivity throughout their structure. The absence of polarized distribution of opsin in these cells is considered to be due to an abnormal environment, which may also be the case with cultured retinal cells. The present culture conditions will offer a useful model system to understand the cellular mechanism of the hereditary retinal dystrophy of rodent animals in which photoreceptor cells selectively degenerate.