Effects of Prostaglandin F2α-lnduced Luteolysis on the Populations of Cells in the Ovine Corpus Luteum1

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Abstract
Receptors for prostaglandin (PG) F2.alpha. in the vine corpus luteum are localized on large steroidogenic luteal cells. Therefore, it was hypothesized that during luteolysis, the first demonstrable effects of PGF2.alpha. would occur in thepopulation of large luteal cells. To test this hypothesis the numbers and sizes of large and small luteal cells, fibroblasts, capillary endothelial cells, and pericytes were determined in corpora lutea collected 12, 24, or 36 h (6 animals/group) following administration of PGF2.alpha. on Day 10 postestrus and from untreated ewes on Days 10 and 12 postestrus. The numbers and sizes of luteal cells were determined after enzymatic dissociation of the luteal tissue into single cell suspensions and by morphometric analysis of luteal slices. Serum levels of progesterone decreased (p < 0.05) within 12 h of treatment, indicating that luteolysis was induced. Recovery of the two types of steroidogenic luteal cells following enzymatic dissociation was different (p < 0.05). Recovery of both steroidogenic cell types decreased with time after PGD2.alpha. treatment, suggesting that they had become more fragile. As determined by morphometry, the number of large luteal cells was not different at any time point examined; however, by 36 h after treatment, the average diameter of large luteal cells had decreased (p < 0.05). In contrast, by 24 h after treatment, there was a decrease in the number of small luteal cells (p < 0.05) but no change in their diameter. In summary, our original hypothesis that the number of large luteal cells would decrease prior to a decrease in the number of small luteal cells during PGF2.alpha.-induced luteolysis was incorect. Luteolysis was associated with decrease in the number of small luteal cells, capillary endothelial cells, and pericytes but no large luteal cells. Additionally, fragility increased and diameter of large luteal cells decreased during luteolysis. However, all changes in the population of luteal cells occurred seveal hours after serum levels of progesterone had decreased.