Morphometric analysis of cell types in the ovine corpus luteum throughout the estrous cycle

The cellularcomposition of ovine corpora lutea obtained during tbe early (Day 4), mid (Days 8 and 12), and late (Day 16) stages of the estrous cycle was determined by morphometric analysis. Individual corpora lutea were collected via midventral laparotomy from a total of 19 ewes. A center slice from each corpus luteum was processed for electron microscopy and subsequent morphometric analysis of the numbers and sizes of steroido-genic and nonsteroidogenic cells. Luteal weight progressively increased throughout the estrous cycle (p<O.05). Corpora lutea collected on Day 16 were assigned to one of two subgroups on the basis of gross appearance and weight: nonregressed (NR, 542 ± 25 mg) or regressed (R, 260 ± 2 mg). There were no significant changes in the proportion of tbe corpus luteum occupied by small luteal cells (19 ± 2%) or large luteal cells (36 ± 1%) throughout the estrous cycle. The total number of steroidogenic cells per corpus luteum increased from 21.8 ± 3.7 (x 106) on Day 4 to 61.7 ± 5.4 (x 106) on Day 8 (p<O.05) and remained elevated thereafter. The number of small luteal cells was 10.0 ± 2.7 (x 106), 39.7 ± 1.4 (x 106), 46.1 ± 5.8 (x 106), 49.0 ± 13.7 (x 106), and 29.9 ±8.6 (x 106) on Days 4,8, 12, 16 (NR), and 16 (R), respectively (p<O.05, Day 4 vs. Days 8, 12, 16 NR). In contrast, the number of large luteal cells was 11.8 ± 1.5 (x 106) on Day 4 and did not vary significantly during the remainder of the estrou