Fine structure of the human menstrual corpus luteum at its stage of maximum secretory activity,

Human corpora lutea from the period of maximum progesterone secretion during the menstrual cycle were examined with the electron microscope. Both the granulosa lutein and theca lutein cells contain abundant tubular smooth endoplasmic reticulum, mitochondria with tubular cristae, lipid droplets and numerous free ribosomes. Cisternae of rough endoplasmic reticulum occur in stacks and as scattered profiles in the lutein cells. Golgi elements are seen throughout the granulosa lutein cell cyoplasm and are associated with multivesicular bodies, lysosome-like granules, coated vesicles and microtubules. The coated vesicles apparently arise from the Golgi cisternae. Unlike the granulosa lutein cells, theca lutein cells are generally smaller and darker, exhibit a wider range of cell densities, have a Golgi complex clustered at one nuclear pole, and their lipid droplets are extracted by tissue preparation. Patches of microvilli that protrude into the perivascular spaces and a peripheral, fibrillar terminal web are unique to the granulosa lutein cells. The perivascular macrophages correspond to the “K” cells of light microscopy. No cells in the theca externa or the central coagulum exhibit the fine structure of steroid-secreting cells. The correlation of fine structure with function in human lutein cells is probably similar to that established for other steroid-secreting cells. The role of the theca lutein cells in steroidogenesis by the human corpus luteum remains unresolved.