The formation and distribution of intercellular junctions in the rhesus monkey optic cup: The early development of the cilio-iridic and sensory retinas

The eyes of prenatal monkeys from 30 to 102 ± 2 days old were examined by light microscopy, conventional electron microscopy, and the freeze-fracturing technique. At 30 days, invagination of the optic vesicle has begun, and the inner and outer walls of the forming optic cup are closely apposed anteriorly; invagination is complete at 45 days. By 58 days, the rudiment of the ciliary body and iris has appeared; at 71 days, primitive ciliary processes are present and retinal photoreceptors begin to differentiate. The distribution of intercellular junctions varies both in different regions of the optic cup and at different stages of development. At 30 days, adjacent ventricular and adjacent pigmented cells are joined throughout the optic cup by zonulae adhaerentes and gap junctions. The anterior region of the cup, however, contains two additional junctional specializations: (1) fasciae occludentes between ventricular cells and (2) intermediate and gap junctions between the apposing luminal surfaces of ventricular and pigmented cells. By 36 days the fasciae occludentes between ventricular cells in the anterior optic cup become zonular, signaling the morphological development of the blood-aqueous barrier. In the posterior optic cup, zonulae occludentes appear between adjacent pigmented cells at 36 days; furthermore, with the continuing obliteration of the optic ventricle, luminal junctions spread toward the optic stalk but do not reach the optic disc until 45 days, when invagination is complete. Between 58 and 102 days there are no further changes in the distribution of the junctions anteriorly between the primitive cilio-iridial epithelial cells, whereas in the posterior optic cup the luminal gap and intermediate junctions between pigmented cells and differentiating photoreceptors decrease in number and finally disappear. Two main conclusions can be drawn from this study. (1) In the optic cup, intermediate junctions are consistently present in regions of the plasma membrane which later contain junctional complexes. The temporal and spatial pattern of junctional development suggests that intermediate junctions are necessary for the establishment of tight and gap junctions. (2) Twenty days before the ciliary body-iris anlage becomes visible in the light microscope, the distribution of junctions in the anterior part of the optic cup is identical to that in the adult cilio-iridic retina. The time-honored view that the cilio-iridic retina appears late in development is, therefore, no longer tenable. In the monkey, the optic cup is divided into a cilio-iridic and a sensory region soon after the onset of invagination.