Optic fissure closure in the normal cinnamon mouse. An ultrastructural study.

The purpose of this study was to determine the ultrastructural features of optic fissure closure. Serial coronal sections of fetal eyes from the eleventh to the thirteenth gestational day were examined by light and electron microscopy. Fusion was associated with inversion of the retinal pigment epithelium at the optic fissure and it occurred first between undifferentiated cells at the junction of the retina and retinal pigment epithelium. It later extended to involve the entire thickness of the pigment epithelium and neuroretina, with the inner aspect of the latter being the last area to fuse. There was some evidence that closure does not always start at the center of the fissure as generally described, but may sometimes start near the developing papilla. At an ultrastructural level, there was multifocal disintegration of the basement membrane associated with the formation of cytoplasmic processes at these sites. Simple appositional contacts between processes on either side of the fissure comprised the first stages of fusion. Later intermediate-type junctions were formed between adjacent outer retinal cells (presumptive photoreceptors) and junctional complexes were formed at the apices of pigment epithelial cells at the site of fusion. This suggests an increase in mechanical adherence between cells. While basement membrane disintegrated at the center of the fusion site, there was a continuous layer of basement membrane at the internal and external limits of fusion. Cell death together with two morphological types of phagocytic cells were a constant feature at the fissure margins before, during and after fusion. The possible origins and roles of these cells in the fusion process is discussed.