Ultrastructural and cytochemical study of the bovine corpus luteum.

The large luteal cells comprise the bulk of the bovine corpus luteum during the most active secretory period of the oestrous cycle. At least 3 types of granules were present in mid-luteal cells: microperoxisomes, primary lysosomes and secretory granules. Only the last appear to be exocytosed. The concentration of these secretory granules in the luteal cytoplasm correlated with the reported profile of progesterone secretion by these cells throughout the cycle. Changes in the cytoplasmic proportions of the other subcellular organelles were consistent with the mobilization of steroid precursors stored in lipid droplets, synthesis of increased amounts of progesterone and protein, and the packaging of these products into discrete secretory granules.