Ultrastructural study of granules in the corpora letea of several mammalian species

The ultrastructure of the corpora lutea of the goat, cow, rabbit, pig and guinea pig have been examined at a stage when progesterone was being secreted by the luteal cells. Densely staining granules, 0.2micrometer in diameter, are present within the luteal cells of all the species examined. Granules were detected in the extra-cellular spaces of the corpora lutea of the goat, cow and pig, but not of the rabbit or guinea pig. The luteal cells of the various species have the same basic structure but differ in the amounts of lipid and forms of smooth endoplasmic reticulum present. A common mechanism of hormone secretion, in granule form, is postulated for all luteal cells.