The ultrastructure of huma corpora luntea obtained approximately 2, 3, 5, 11, and 15 days after ovulation is reported. All specimens were fixed in Karnovsky's formaldehyde-glutaral-dehyde solution. The 5-day corpus luteum is presumed to represent, in terms of fine structure, the ultrastructural aspects of high progesterone production and is compared to younger differentiating and older regressing specimens. A distinct topographic relationship of cytoplasmic organelles is noted in the mature 5-day luteal cell. It consists of a peripheral cupshaped mass of tubular ER (endoplasmic reticulum), associated with granular ER, lipid droplets, and mitochondria, which merges with a large paranuclear Golgi area extending to a cell surface bordering a perivascular space. The plasma membrane of active luteal cells is described and its variations suggest areas of specialized surface activities. The prevalence and ultrastructure of more electron-opaque stellate cells, of phagocytes, and of thecal cells are reported.

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