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 se lective transport of proteins required in the outer segments of the rat pho toreceptor cell. As a first step, the three-dimensional architecture of the periciliary region and its developmental changes were examined. Freeze-dee p-etching and freeze-substitution methods combined with rapid freezing tech nique were used. The apical surface of the inner segment was swollen and pa rtially enclosed the base of the connecting cilium in early postnatal stage s, so that the basal region of the connecting cilium was inevitably surroun ded by a groove. However, a specialized periciliary ridge complex as seen i n frog photoreceptor cells has never been observed in rat photoreceptor cel ls. The cytoplasmic surface of the plasma membrane of the apical inner segm ent in the vicinity of the connecting cilium was covered with loose fine fi laments. However, it was unlikely to be a possible structural candidate for selective transport of membrane proteins. This study also revealed the int erior structure of the connecting cilium. Actin filaments in the distal axo nem formed a complicated meshwork together with an unknown substance. Since S1 decorated filaments were not detected in the middle region of the conne cting cilium, actin filaments at the base of outer segment: seem to be inde pendently polymerized locally from G-actin that is transported from the inn er segment.