Estradiol-induced changes in progesterone secretion by rabbit corpora lutea are associated with quantitative ultrastructural changes in luteal cells

In this study, we examined changes in luteal cell structure that accompany estradiol-altered progesterone production by the rabbit corpus luteum. To stimulate progesterone production, polydimethylsiloxane (Silastic) capsules containing 17β-estradiol were inserted sc into superovulated New Zealand White rabbits. Luteal progesterone production, assessed by measurement of progesterone in peripheral serum, was high after the estradiol-filled capsules were inserted, declined within 24 h after the capsules were removed, and increased within 32 h after reinsertion of the capsules. Stereological analyses at the light microscopic level revealed that the number of luteal cells and the volume of an average luteal cell did not differ significantly between estradiol-stimulated and estradiol-deprived rabbits over the time period employed. In contrast, stereological analyses at the electron microscopic level demonstrated that the surface areas of smooth endoplasmic reticulum and inner mitochondrial membrane declined with estradiol deprivation, but were restored by reimposition of estradiol stimulation. These changes in the surface area per cell of smooth endoplasmic reticulum and inner mitochondrial membrane were strongly correlated (r = 0.94 and r = 0.88, respectively) with changes in progesterone concentrations in peripheral serum. Changes in the surface area of lipid droplets per luteal cell also occurred, but were inversely correlated (r = -0.87) with progesterone levels. No significant changes were seen in the surface areas per cell of outer mitochondrial membrane or rough endoplasmic reticulum. These results demonstrate that estradiol stimulation and deprivation cause reversible quantitative changes in the rabbit luteal cell organelles known to be directly involved in progesterone biosynthesis. This leads to the conclusion that the steroidogenic activity of the luteal cell is tightly coupled to its subcellular structure.