Ultrastructural evidence of variation in the number of secretory granules within the granulosa cells of the sheep corpus luteum.

The granulosa luteal cells of the sheep corpus luteum secrete their hormonal products by exocytosis of granules. Electron micrographs of randomly selected granulosa cells from nine corpora lutea at day 10 of the oestrous cycle were examined to obtain the cellular density of these granules. From the area of the cell, calculated using an x-y digitiser, and the number of granules observed, the number of intracellular secretory granules per microns 3 of luteal cell cytoplasm was calculated. There was a large variation in the number of granules per cell within the same corpus luteum and between corpora lutea taken at the same stage of the cycle. The number of intracellular granules in nine corpora lutea varied from 2.12 +/- 1.05 granules per microns 3 (mean, SD, n = 30) cells to 0.36 +/- 0.18 granules per microns 3 (mean, SD, n = 26). These morphological data suggest that the variation in granule synthesis in individual cells may contribute to the variation in hormone content of corpora lutea at the same stage of the cycle, and the episodic release of hormone into the plasma.