Ultrastructural Study of Secretory Granules in the Corpus Luteum of the Sheep During the Estrous Cycle

Abstract

The ultrastructure of the corpus luteum of the sheep has been examined throughout the estrous cycle. Small, densely staining granules 0.2 µm in diameter, are observed within the luteal cell at Day 2 of the cycle; they increase in number as the cycle progresses and at Day 6 the first signs of secretion are noticed. Maximum secretion of granules takes place at Days 10 and 11, and this is followed by a progressive decline in secretory activity from Day 12 to Day 15 of the cycle. These clear-cut changes in granule formation and secretion therefore reflect the established pattern of progesterone secretion by the ovary during the estrous cycle in the sheep. Fractions were obtained from corpora lutea of sheep by homogenization and differential centrifugation. A correlation was obtained between the presence of densely staining granules (approx 0.2 µm diam) and the progesterone content of each fraction. Many types of nonsteroidogenic endocrine cells are known to have a secretory pathway whereby hormones are packaged in the Golgi region and then discharged from the cells via granules; the present work provides the first indication of a steroid hormone, viz. progesterone, being secreted in a similar way.