Possible roles for cognin in chick embryo neural retina

In this review, we discuss current information about a cell-cell recognition protein present in chick embryo neural retina. This protein, retina cognin, has cell adhesion or aggregation promoting propertiesin vitro. We discuss five questions. First, what is retina cognin (R-cognin)? Second, what do we know about cognin in chick retina? We discuss its histological distribution in retina and how that distribution changes during embryonic and early post-hatching development. Third, where is cognin within cells? We review light microscopy evidence for its localization in plasma membranes of somas and neurites of selected retinal neurons as an intrinsic membrane protein. Fourth, how is cognin distributed in membranes? We summarize evidence that cognin might not be uniformly distributed over cellsurfaces and that it might bind to specific proteins on the surfaces of other retina cells. From the available information, we ask what we can deduce about cognin's biological role in the neural retina.