Fine structure of rat corneal vessels in advanced stages of wound healing. I. Permeability to intravenously injected thorium dioxide.

The present work is an examination of the fine structure of corneal vessels in advanced stages of wound healing in the Sprague-Dawley rat. Several weeks after burning the central cornea with silver nitrate or a cautery iron, vascularized corneas classified as ''healed" mildly wounded, or moderately to severely wounded were selected for electron microscopic observation. In all cases the blood vessels peripheral to the wound were mature. In contrast to newly formed blood vessels, they showed no frank gaps between endothelial cells. They had a low continuous endothelial sheet, a continuous basement membrane, one or more layers of pericytes, and tended to develop greatly thickened basement membranes. The surrounding stroma closely resembled that of control avascular corneas except that stromal cells sometimes showed increased amounts of rough-surfaced endoplasmic reticulum, and occasional phagocytes were present. The vessels within the wound of "healed" corneas and mildly icounded corneas were also mature vessels of the low continuous endothelial variety, but they did not contain greatly thickened basement membranes. While vessels from the wound of moderately to severely wounded corneas possessed some properties of immature vessels (i.e., rounded and irregularly shaped endothelial cells, and little or no basement membrane) the profile of organelles within the endothelial cell and the close approximation of endothelial cells at cell junctions were characteristic of mature blood vessels. In all corneas, the stroma from wound areas showed an irregular distribution of collagen and a trend toward increased numbers of activated stromal cells, phagocytes, and blood vessels in the more severely wounded corneas. Qualitative differences such as the presence of lymph channels and the presence of numerous electron opaque particles in the stroma and in phagocytic vacuoles of phagocytes only appeared in the wound areas of moderately to severely wounded corneas. Electron-opaque particles were observed only in corneas burned by silver nitrate and probably represent the morphological basis of argyria.