Periciliary structure of developing rat photoreceptor cells. A deep etch replica and freeze substitution study.

The purpose of this study is to identify the morphological machinery for selective transport of proteins required in the outer segments of the rat photoreceptor cell. As a first step, the three-dimensional architecture of the periciliary region and its developmental changes were examined. Freeze-deep-etching and freeze-substitution methods combined with rapid freezing technique were used. The apical surface of the inner segment was swollen and partially enclosed the base of the connecting cilium in early postnatal stages, so that the basal region of the connecting cilium was inevitably surrounded by a groove. However, a specialized periciliary ridge complex as seen in frog photoreceptor cells has never been observed in rat photoreceptor cells. The cytoplasmic surface of the plasma membrane of the apical inner segment in the vicinity of the connecting cilium was covered with loose fine filaments. However, it was unlikely to be a possible structural candidate for selective transport of membrane proteins. This study also revealed the interior structure of the connecting cilium. Actin filaments in the distal axonem formed a complicated meshwork together with an unknown substance. Since S1 decorated filaments were not detected in the middle region of the connecting cilium, actin filaments at the base of outer segment seem to be independently polymerized locally from G-actin that is transported from the inner segment.