Electron microscoppic studies of the corpus luteum in pregnant rats : effetcs of conceptus and pituitary

Ultrastructure of the corpora lutea (CL) of pregnant rats were studied at different stages of pregnancy. The CL cells of pregnant rats are distinguished from the non-pregnant animals by the increases in the large secretory granule (type-I granule) and well developed mitochondria (so-called disk-type), abundant endoplasmic reticulum (both smooth surfaced and rough surfaced endoplasmic reticulum), and enlarged Golgi complex. In addition, a new type of the secretory granule with small size (type-II granule) appeared after day 16 of pregnancy. These changes were less evident when the conceptus number was reduced to one on day 7 of pregnancy (one-conceptus rats). Interestingly, hypophysectomy on day 12 of pregnancy in the one-conceptuts rats resulted in the significant increase in type-II granuls, which, however, were completely suppressed by ACTH injections. It was concluded form these ultrastructual studies that the CL of the pregnant rat was stimulated by the conceptus-derived substances particularly during the second half of pregnancy, and that ACTH would exert inhibitory effect on the luteal function in pregnant rats.