Biology of Reproduction, Vol 1, 83-106, Copyright © 1969 by Society for the Study of Reproduction

Fine Structure and Progesterone Levels in the Corpus Luteum of the Pig during the Estrous Cycle

¹ Department of Anatomy, Tufts University School of Medicine, Boston, Massachusetts 02111; and Department of Animal Science, Iowa State University, Ames, Iowa 50010

The cells of the corpus luteum of 28 gilts have been characterized by electron microscopy and progesterone levels in adjacent luteal tissue during several stages of the estrous cycle. The progesterone content was determined in the corpora lutea of 19 of these animals. There was a close correlation between morphological changes during the cycle and the level of progesterone in luteal tissue. During luteinization (Day 1) granulosa cells at the periphery of the ruptured follicle were cuboidal to columnar and separated by irregular extracellular spaces which contained precipitated liquor folliculi. Deeper within the corpus luteum the cells were hypertrophied and the nucleus was eccentrically located. In the peripheral cells, the cytoplasm contained long cisternae of granular endoplasmic reticulum and many free polysomes; in the center the cisternae were short and granular endoplasmic reticulum was present. By Day 4, in the early part of the secretory phase (Days 4, 8, and 12) luteinization was essentially complete. The cells had hypertrophied and contained masses of agranular endoplasmic reticulum. Polysomes were restricted to subplasmalemmal and perinuclear locations. Small coated vesicles were abundant in association with the Golgi complex; large coated vesicles were numerous in relation to the plasmalemma. The presence of coated vesicles may be related to cellular transport. At Day 4, nucleolar size was maximum with the nucleolonema in a large, loose coil. Nuclear structure showed little change until cell death. Between Days 4 and 12 there was an increase in collagen fibers in and microvilli projecting into the intercellular spaces. During regression (Days 14, 16, and 18) there was an increase in cellular droplet lipid, cytoplasmic disorganization and vacuolation of agranular endoplasmic reticulum. Luteal tissue progesterone levels were maximal between Days 8 and 12 of the cycle. As the level of progesterone declined in the terminal phase of the cycle, there was an increase in the number of lysosomes, a marked vacuolation of the agranular endoplasmic reticulum and invasion of connective tissue. These events resulted in formation of the corpus albicans. Once lysosomal activity was initiated, particularly at Days 16 and 18, the process was irreversible.