Subcellular location of progesterone in the bovine corpus luteum: a biochemical, morphological and cytochemical investigation

The subcellular location of progesterone in the bovine corpus luteum has been investigated by biochemical, morphological and cytochemical techniques. Differential centrifugation of tissue homogenates showed that up to one-third of the total progesterone could be sedimented. Most of this particulate progesterone banded discretely at low density on sucrose gradients. Marker enzyme studies showed that it was not associated with either the microperoxisomes, lysosomes or mitochondria nor appreciably with the vesiculated subcellular components such as plasma membrane and microsomes. Morphological examination of tissue sections showed that the luteinized cells of the bovine corpus luteum contain electron dense granules. Three types of granules were present. Some of these granules were identified as microperoxisomes and lysosomes by cytochemical techniques, but a third type of granules was also present. Morphological and cytochemical studies of different regions of the sucrose density gradient confirmed the distribution of subcellular organelles as judged by marker enzyme techniques. Electron dense granules were observed in material fixed from the progesterone peak, while microperoxisomes and lysosomes were found in denser regions of the gradient and separate from the progesterone band. These results support the hypothesis that some of the progesterone in the bovine corpus luteum is sequestered within electron dense granules.