Mechanism of deletion of endothelial cells during regression of the corpus luteum.

Endothelial cells are deleted rapidly, and in large numbers, during cyclical regression of the corpus luteum of the guinea pig. This paper reports a study of the mechanisms, structural and causal, by which this deletion occurs. Corpora lutea from guinea pigs were examined by transmission electron microscopy on day 9 (functional stage) and day 16 (regressing) of the estrous cycle. Corpora lutea were also studied at 1, 3, 12, 24, and 48 hours after administration of a synthetic luteolytic substance (cloprostenol), and after temporary occlusion of the ovarian blood vessels for 15, 30, 60, or 120 minutes. Early signs of endothelial cell degeneration included protrusion of some individual cells into capillary lumina and the formation of adherens junctions across the lumen. Intermediate stages of degeneration included nuclear and cytoplasmic condensation and cellular and nuclear lobation or fragmentation in cells either protruding into, or lying within, the lumen. Terminal changes included loss of plasma membrane integrity and cytoplasmic density, together with disruption of cell organelles. Some degenerate endothelial cells were engulfed by viable endothelial cells. Macrophages were not seen to be involved in removal of dead endothelial cells, and integrity of the walls of capillaries was maintained while individual endothelial cells were deleted. Experimental findings were consistent with the hypothesis that cessation or reduction of flow of blood along capillaries plays an etiologic rule in endothelial cell deletion.