Secretion of granules by the luteal cells of the sheep and the goat during the estrous cycle and pregnancy

The fine structure of luteal tissue from sheep and goats at the mid-stage of the estrous cycle and during pregnancy was examined. At the mid-stage of the estrous cycle, when the corpus luteum of both species is secreting progesterone, the ultrastructure of the luteal cells is similar in that granule secretion is prevalent. In late pregnancy, the corpus luteum of the sheep produces very little progesterone and there is an absence of granule secretion. In the pregnant goat, the production of progesterone from the corpus luteum is high and there is plentiful evidence of granule formation and secretion.