Morphological Changes in the Corpus Luteum Correlated with Progestin Levels in the Rhesus Monkey during Early Pregnancy1

Corpora lutea (CL) and blood were obtained from rhesus monkeys at Days 13, 19, and 25 of pregnancy. Morphological changes were determined by light and electron microscopy examination and functional activity was monitored by the levels of progestins and progesterone in the ovarian vein and peripheral circulation. Corpora lutea were also taken from monkeys at Days 4 and 10 postovulation and Day 2 of the next cycle to determine structural changes during development, maturation, and degeneration. The Day 13 granulosa luteal cells resemble those of the mature CL seen in the nonfertile cycle. These cells are large and have an abundance of lipid droplets approximately 1 μm in diameter which appear lucent since the content from the majority of the droplets has been extracted. This is in contrast to the smaller lipid droplets seen in CL development (Day 4) in which the content is retained. A decline in the large lucent lipid droplets begins at about Day 13 and is accompanied by an increase in the number of dense bodies and a decrease in progesterone in both peripheral and ovarian veins. At Days 19 and 25, the absence of large lucent lipid droplets and an increase in dense bodies are the most striking feature of the granulosa luteal cells. However, the microvillus-lined intercellular channels and cytoplasmic organelles are similar to those seen at Day 13. Although progesterone levels have decreased in the ovarian vein by Day 25, a substantial amount of hormone can be detected. The role of theca luteal cells, which are easily identified during the same period since they have large lipid droplets and large mitochondria, is not clear. Both the morphological and hormonal data obtained in this study suggest that the CL changes from a highly functional structure at Day 13 to one of less activity by Day 25.