A MORPHOMETRIC ANALYSIS OF THE CORPUS-LUTEUM OF THE COW DURING THE ESTROUS-CYCLE AND EARLY-PREGNANCY

Corpora lutea were collected from cows on Days 6, 8, 10, 12, 14, 16, 1 8 and 19 of the estrous cycle and early pregnancy (n=2/d) and were exa mined by light microscopy. Mean lutein cell diameter was significantly (P<0.05) greater in pregnant than in cyclic on Days 6, 8, 10, 12, 16, 18 and 19 (cyclic versus pregnant: Day 6: 13.9 +/- 0.22 vs 14.9 +/- 0 .24; Day 8: 13.8 +/- 0.20 vs 15.4 +/- 0.2; Day 10: 14.8 +/- 0.24 vs 17 .4 +/- 0.24; Day 12: 13.2 +/-0.25 vs 17.9 +/- 0.3 1; Day 16: 13.9 +/- 0.28 vs 16.5 +/- 0.31; Day 18: 13.0 +/- 0.22 vs 16.5 +/- 09.36, and Da y 19: 15.0 +/- 0.23 vs 17.6 +/- 0.33 mum, respectively). The distribut ion of cell sizes was leptokurtotic throughout the estrous cycle and t he first 10 d of pregnancy, but tended towards bimodality after Day 14 of pregnancy. The proportion of lutein cell cytoplasm occupied by vac uoles was lower in pregnant than in cyclic cows from the 12th day post estrus, but there was a marked (P<0.05) increase in vacuolation of ce lls from cows undergoing luteolysis. Stainable intercellular collagen was also less abundant in pregnant than cyclic cows from the 12th day post estrus. The higher rate of progesterone secretion of pregnant, co mpared with cyclic cows may be attributed to the greater numbers and g reater contribution to luteal mass of large lutein cells in the corpus luteum of pregnancy.