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J. D. O'Shea and K. McCoy

Summary. Changes in luteal weight from about Day 20 to near term, and in quantitative histology as assessed by ultrastructural morphometry and light microscopic counts of mitosis and cell death on Days 30, 60, 100 and 142, were studied in 168 pregnant ewes.

Luteal weight (mean ± s.d.) remained constant at 0·56 ± 0·11 g until Day 120, and fell thereafter to reach 0·31 ± 0·11 g after Day 140 (P < 0·01). Up to Day 100, quantitative aspects of the composition of the luteal tissue showed no significant change, and values for volume density, cytoplasmic:nuclear ratio, cell number/mm3 and cell volume were comparable to values previously obtained for corpora lutea (CL) of the cycle. By Day 142 structural evidence of luteal regression was present, but regressive changes were much more marked in some CL than others. Mitosis was seen in a few cells (0·02–0·04%) on all of the days studied, but never in large luteal cells. Cell death was rarely seen up to Day 100, but had increased in incidence by Day 142 (P < 0·01). Luteal progesterone content, 55·2 ± 15·9 nmol/g on Day 30, was not significantly changed on Days 60, 100 or 142.

It is concluded that (1) structural regression of the CL of pregnancy does not begin until much later than the time (about Day 50) when pregnancy ceases to depend on the CL; (2) structural luteal regression begins before parturition, but its time of onset and/or rate of progression vary widely between animals; and (3) large and small luteal cells remain as distinctive populations throughout pregnancy, and their numbers at all stages can be accounted for by survival of the cells which differentiate during the genesis of the CL.

Keywords: corpus luteum; pregnancy; sheep; morphometry; luteal weight