Quantitative cell changes and vascularisation in the early corpus luteum of the pregnant rat

Early corpora lutea (CL) of the rat were histologically examined on Days 1, 2, 4, and 6 of gestation. Measurements were taken of total volume and of the number of luteal and endothelial cells in one CL of both ovaries of five rats at each stage examined. CL volume increased over the 6 days from -.076 to 1.39 microliter and peripheral plasma progesterone levels from 8.1 to 33.2 ng/ml. The number of luteal cells per CL (range 303,000 to 370,000) did not significantly change, and there was no evidence of mitosis or death amongst these cells. Luteal cell volume increased from 1.74 to 3.49 pl and nuclear volume from 0.25 to 0.38 pl, the former being the major cause of CL growth. The CL appeared to be richly vascularized, even on Day 1, and the number of endothelial cells per CL (range 289,000 to 354,000) remained relatively constant over the period examined. It was concluded that the number of luteal cells per CL is determined prior to or around ovulation in the rat and that subsequent growth of the CL is due to hypertrophy and not hyperplasia.