Morphometric Analysis of Cell Types in the Ovine Corpus Luteum throughout the Estrous Cycle1

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Abstract
The cellular composition of ovine corpora lutea obtained during the 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 steroidogenic and nonsteroidogenic cells. Luteal weight progressively increased throughout the estrous cycle (p < 0.05). 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 the corpus luteum occupied by small luteal cells (19 .+-. 2%) or large luteal cells (36 .+-. 1%) througout the estrous cycle. The total number of steroidogenic cells per corpus luteum increased from 21.8 .+-. 3.7 (.times. 106) on Day 4 to 61.7 .+-. 5.4 (.times. 106) on Day 8 (p < 0.05) and remained elevated thereafter. The number of small luteal cells was 10.0 .+-. 2.7 (.times. 106), 39.7 .+-. 1.4 (.times. 106), 46.1 .+-. 5.8 (.times. 106), 49.0 .+-. 13.7 (.times. 106), and 29.9 .+-. 8.6 (.times. 106) on Days 4, 8, 12, 16 (NR), and 16 (R), respectively (p < 0.05, Day 4 vs. Days 8, 12, 16 NR). In contrast, the number of large luteal cells was 11.8 .+-. 1.5 (.times. 106) on Day 4 and did not vary significantly during the remainder of the estrous cycle. The numbers of nonsteroidogenic cell types increased (p < 0.05) from Day 4 to Day 16 (NR) but were decreased in regressed corpora lutea (Day 16 R). Regression was characterized by a 505 decrease (p < 0.05) in the total number of cells per corpus luteum from 243 .+-. 57 (.times. 106) on Day 16 (NR) to 125 .+-. 14 (.times. 106) in the total number of cells per corpus luteum from 243 .+-. 57 (.times. 106) on Day 16 (NR) to 125 .+-. 14 (.times. 106) on Day 16 (R) (p < 0.05). Small luteal cells remained constant in volume throughout the entire estrous cycle (2520 .+-. 270 .mu.m3), whereas large luteal cells increased in size from 5300 .+-. 800 .mu.m3 on Day 4 to 16,900 .+-. 3300 .mu.m3 on Day 16 (NR) (p < 0.05). In summary, small luteal cells increased in number but not size throughout the estrous cycle, whereas large luteal cells increased in size but not number.