Growth and cellular proliferation of pig corpora lutea throughout the oestrous cycle

Corpora lutea were obtained from gilts on days 2, 4, 8, 12, 15 or 18 after oestrus. Luteal fresh masses and DNA contents increased linearly (P < 0.01) from day 2 to day 12 and day 2 to day 15, respectively. Changes in the ratio of protein:DNA were greatest between days 2 and 4 and days 15 and 18, whereas changes in DNA content were relatively small during the same intervals. Thus, a major component of changes in the size of the corpus luteum during the early and late periods of the luteal phase was cellular hypertrophy. Proliferation of luteal cells in vivo (nuclear incorporation of 5-bromo-2-deoxyuridine, a thymidine analogue) was greatest on day 2 and decreased exponentially (P < 0.01) throughout the oestrous cycle. Results from co-localization of 5-bromo-2-deoxyuridine and factor VIII (von Willebrand factor), a marker of endothelial cells, or 5-bromo-2-deoxyuridine and 3 beta-hydroxysteroid dehydrogenase, a marker of steroidogenic cells, indicated that some of the luteal steroidogenic cells proliferated early in luteal development. However, during early and mid-cycle, most of the luteal cell proliferation occurred in the endothelial cells. Thus, during growth of the pig corpus luteum, which is extremely rapid, most of the proliferating luteal cells are vascular endothelial cells. This observation is consistent with the high vascularity and blood flow of the mature corpus luteum and implies a critical role for angiogenesis in luteal development in the pig, as has been proposed for several other mammalian species.