Evidence for luteal cell hyperplasia during pregnancy.

The pronounced increase in luteal size and weight that occurs during rat pregnancy has been largely attributed to luteal cell hypertrophy. It is generally believed that hyperplasia does not play a role in luteal growth, since it is thought that luteal cell division in vivo does not occur. Recent data suggest that this may not be the case. Thus, to determine whether luteal cell hyperplasia occurs during rat pregnancy, osmotic minipumps filled with 3H-thymidine were implanted in timed-pregnant rats on Day 6 or Day 11 of pregnancy. These pumps provided a continuous infusion of 10 microCi 3H-thymidine per hour for up to 7 days. Seven days later (Day 13 and Day 18 respectively), rats were killed, and the ovaries were removed and prepared for autoradiography. Labeled cells, which have the morphological characteristics of luteal cells, were clearly observed in autoradiographs of ovaries exposed to 3H-thymidine. The labeling index of these cells from ovaries exposed to 3H-thymidine on Days 6-13 of pregnancy was 6.0% and from ovaries exposed to 3H-thymidine on Days 11-18 of pregnancy was 1.2%. Whether the presence of labeled cells signifies proliferation of luteal cells or whether these cells are derived from another cell type that develops into cells morphologically similar to luteal cells is not known at present. However, regardless of origin, these data clearly demonstrate that the number of parenchymal cells in the corpus luteum does increase during pregnancy.