Nuclear Inclusions in Lutein Cells of Pregnant Rats

Corpora lutea of pregnancy in rats at various stages of gestation were observed with an electron microscope. Peculiar inclusions were observed in the nuclei of some lutein cells of late pregnancy. They are complexes of many different bodies, such as vacuoles, vesicles, lipid droplets and dense bodies. They are situated at about the center of the nucleus, and no limiting membrane is observed between the inclusions and the karyoplasm. As the rat lutein cells contain no strongly indented nuclei, the formation of the inclusions may not be ascribed to the invagination of the nuclear envelope. The possibility that a portion of the cytoplasm containing lipid droplets, smooth endoplasmic reticulum and lysosomes may be enclosed within the reforming nucleus at the last stage of mitosis seems most likely.