Rate of blood flow and growth of the corpora lutea of pregnancy and of previous cycles throughout pregnancy in the rat.

The rate and distribution of ovarian blood flow was measured with radioactive microspheres in nulliparous rats at Days 4, 7, 10, 13, 16 and 22 of gestation; term is Day 23. Ovaries were dissected into large corpora lutea (or CL of pregnancy), small CL, assumed to be those of previous cycles, and the ovarian stroma which included follicles and additional small CL. Total ovarian blood flow was similar over Days 4-10, but then increased progressively to reach 916 +/- 69 microliter/min at Day 22. The main cause of this increase was a 786% rise in blood flow to the large CL over Days 10-22. Weight of the CL of previous cycles increased by 289% over Days 10-16 and their rate of blood flow and histological appearance were similar to those of the large CL. These results indicate that neither reduced blood flow nor vascular degeneration, as determined by light microscopic examination, initiates the reported fall in progesterone secretion near term. At least some of the CL of previous cycles are maintained and indeed grow during gestation and have a blood circulation and histological appearance characteristic of steroidogenic tissue.