Development of the human sclera. A morphological study.

Differentiation of the sclera in human embryos and fetuses with a gestation age from 6.4 to 24.0 weeks was studied by light and electron microscopy. The developmental route was found to be anteroposterior and directed from inside outwards. Continuous cytological maturation begins anteriorly during week 7.2 with a loss of free ribosomes and polysomes and an increase in the amount of rough-surfaced endoplasmic reticulum (rER) and Golgi complex components. From the region of the future limbus, these cytodevelopmental events progress posteriorly. By week 13 there are no marked differences between anterior and posterior localization. The cytodevelopmental events directed from inside outwards were found to begin during week 7.2, with a marked increase in the addition of glycogen and lipid droplets in the choroidal half of the anterior scleral condensation and from there progressing to the outer (episcleral) half. With the beginning of week 10.9 no more differences were identified between the outer and the inner portions of the sclera. Portions of the ER denuded of ribosomes close to the plasmalemma were observed during the whole gestational period studied. The role of these structures and the involvement of the Golgi complex for the biosynthesis of collagen are discussed. In week 24, the diameter of the collagen fibrils had increased more than three times in comparison with week 6.4. Elastic microfibrils were found beginning with week 7.2, whereas the elastic deposits with central electron-translucent cores were characterized for the first time beginning with week 18.