Vascular changes during regression of the corpus luteum of the guinea pig

Quantitative and qualitative changes in the microvasculature of the corpus luteum of the guinea pig during cyclical luteal regression were studied by light and electron microscopy. During luteal regression, between day 12 of the oestrous cycle and day 1 of the next cycle, there was a reduction in the extent of the luteal capillary bed as measured both by 'hit' counts and by differential counts of all nucleated cells. Luteal cells made up a constant proportion of congruent to 60% of the luteal tissue area up to day 1 of the next cycle, but had increased as a proportion of all nucleated cells by that stage. Both fibroblasts and macrophages increased in number and in area occupied during luteal regression. Ultrastructural changes in capillaries during luteal regression included an increased incidence and degree of protrusion of endothelial cells into the lumina of small blood vessels, and degeneration and death of many endothelial cells. In spite of the widespread presence of endothelial cell degeneration, most small vessels retained a continuous lining of apparently viable endothelium. It is concluded that in this species there is a rapid and substantial reduction in the luteal capillary bed, which commences during the early stages of luteal regression, and a mechanism is proposed by which the capillary bed may be reduced while retaining its integrity.