Some fine structural features of the ora serrata region in primate eyes.

The morphological transition between the pars plana and the the neural retina at the ora serrata has been studied in two species of monkey and in a wide range of ages of the human eye. Some difficulty is had in distinguishing the precise cell type at the transition. The most characteristic change appears to be in the type of junctional complexes binding the pigment epithelium to the nonpigmented cells of the pars plana and to the neural epithelium. In the pars plana both zonulae occludentes and desmosomes are found, but on the retinal side of the ora serrata only desmosomes are present. The transition actually occurs between the columnar epithelial cells of the pars plana and two or three cells of somewhat uncertain or undifferentiated type, presumably Miillerian cells, on the retinal side. On both sides of the ora serrata, small intercellular spaces, representing remnants of the optic cavity, are observed. They contain small cytoplasmic processes from the cells on either side of the cavity. Those on the retinal side of the ora serrata are larger and contain longer and more numerous processes. These spaces become confluent posteriorly, into which incompletely differentiated photoreceptors protrude. These consist, at first, only of an inner segment, to which is added, more posteriorly, a cilium and then a primitive outer segment. This transitional zone is approximately 0.28 mm. in width, varying somewhat between the temporal and nasal side in adult human eyes. The series of transitional stages is identical in two species of monkeys and in human eyes of all ages studied and is very similar to embryonic stages of human photoreceptor development.