Ultrastructure and regression of the tunica vasculosa lentis in newborn Wistar rats.

In this study, regression of the hyaloid vessels has been followed in the tunica vasculosa lentis (TVL) of the Wistar rat using light, transmission and scanning electron microscopy. The investigation extended from the 1st to the 32nd postnatal day. On day one, the posterior tunica vasculosa lentis is made up of radiating capillaries connected by side-arm branches, the vascular walls of which possess a continuous endothelium, a basement membrane and an incomplete pericyte covering. Endothelial cell specialization is apparent in the form of extreme thinning and fenestration in capillary regions lying opposite the lenticular capsule. The earliest detectable regressive changes become apparent on approximately day 3 and initially involve the short connecting capillaries surrounding the posterior pole of the lens and proceed from there. Regression takes place in the presence of rarefaction of vessel wall cells and the accumulation of endothelial cells in the adjacent capillaries. This leads to the formation of acellular channels which are made up of only basement membrane tubes. After the complete disappearance of these transitional acellular channels, the capillary meshwork coarsens. Remnants of these capillaries are detectable until the 30th postnatal day.