Morphogenesis of photoreceptor outer segments in the developing kitten retina.

The sequence of photoreceptor outer segment morphogenesis has been studied in the developing kitten retina by the examination of serial ultrathin sections and stereopairs of semithin sections. After the outer segment arose as an expansion of the outer end of the cilium, lamellae which had vertical or near-vertical orientations were formed over the whole surface of the outer segment. Subsequently, lamellae of horizontal orientation were formed at the base of the outer segment. The latter gave risk to a stack of horizontal lamellae which increased in height, so displacing the vertical lamellae further and further from the base of the outer segment. These vertical lamellae were eventually ingested by the pigment epithelium, resulting in outer segments which contained horizontal lamellae only.