Some aspects of corneal and scleral differentiation in the primate

Abstract

Electron microscopic observations were made of corneal and scleral structure in a series of staged primate fetuses from the time the earliest aggregated collagen fibrils appeared. Average diameters of these fibrils range from 31·2–33·3 nm in the cornea and from 26·0–26·6 nm in the sclera, at 43 days of gestation. Though scleral collagen fibrils lag in the initial phases of aggregation, at 54 days of development their average diameter surpasses that of the corneal collagen diameter by about 50%. The scleral collagen fibrils grow and differentiate rapidly, attaining an average thickness of approximately 100 nm with repeating periods of about 80 nm at 76 days of gestation. The diameter of corneal collagen fibrils remain essentially stable during fetal life; their macroperiod, in contrast to that of the scleral fibrils, is poorly defined till late in gestation. Within the fibrocyte the appearance of the organelles during fibrogenesis, the various forms of their contents, and their locations in relation to the cell membrane are noted. The cisternae of the rough endoplasmic reticulum of all ages contain the flocculent or granular material indicative of protein syntheses. Cisternae have sites of ribosomal detachment, and either closely approach the plasma membrane, or they bleb and vesiculate. During the second trimester of gestation some Golgi vacuoles enclose granular or homogenous substances, others long, parallel filaments, often assembled in bundles. Membrane-enclosed, aggregated fibrils, resembling the extracellular collagen fibrils, occur frequently. Similarities between chick and primate collagen-producing cells are pointed out. Within the scleral fibrocytes, near the sites where the first presumed elastic microfibrils appear, are associated glycogen clumps, polysomes and small granules. Close to the outer side of the plasma membrane individual collagen fibrils and the fine filaments of the extracellular space may participate in the formation of pre-elastic foci.