Changes in Small Blood Vessels During Cyclical Luteal Regression in Sheep

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Abstract
Changes in small blood vessels of corpora lutea (CL) undergoing cyclical regression were studied by light and electron microscopy, and the progesterone content of some CL was assayed. Vascular changes were first identified at Day 14 of the estrous cycle, were consistently present by Day 16, and were observed in all CL in which progesterone content was <10 µg/g luteal tissue. Vascular changes involved both the luminal contents and walls of capillaries and small venules. Many capillaries contained degenerating cellular fragments, incorporating nuclear and cytoplasmic remnants, together with blood cells. The walls of many vessels containing cellular fragments appeared normal. In others, abnormalities observed included folding of the basal lamina, increase in the size and number of gaps in the endothelium, increased endothelial cell projection into the lumen, the inclusion of hetero- and/or autophagic vacuoles in the cytoplasm of many endothelial cells and pericytes, and endothelial cell degeneration leading to ultimate disintegration of affected vessels. By Day 1 of the following cycle the number of capillaries was much reduced. It was concluded that, at a relatively early stage of luteolysis, many luteal capillaries show changes involving obstruction of their lumina by large amounts of cellular debris, at least a part of which is of endothelial origin. The relationship of these changes to other phenomena associated with luteolysis is discussed.