123 Physiological regulation of ovarian follicle growth and maturation in the pig

Abstract The understanding of the regulation of ovarian follicular development in the post-pubertal female pig stems from the published data of many researchers including our own. Luteinizing hormone (LH) induces the final maturation and ovulation of roughly 15–30 large antral (6–10 mm diameter) follicles in the pig. These follicles undergo their final maturation during the follicular phase 3–7 days before ovulation; however, the ovulated follicles have grown for more than 3 months prior to reaching the preovulatory stage. An activated primordial follicle grows to the early antral stage in slightly over 80 days, and advances further to the 2–4 mm antral stage during the preceding luteal phase, prior to selection. Preantral growth can proceed without gonadotropins. Antral follicles approximately 1 mm in diameter express Follicle stimulating hormone (FSH) receptors in their granulosa layer and depend on FSH for growth. LH receptors are initially restricted to the theca cells and LH is required for growth beyond the 2 mm stage. Granulosa cells subsequently express LH receptors while FSH receptor mRNA wanes in larger follicles, and there is a shift from FSH to LH dependence at approximately 5 mm in follicle diameter. FSH drives estradiol production through expression of CYP19A1 and it protein aromatase, with highest production in large preovulatory follicles before the LH surge. The LH surge decreases aromatase mRNA, protein and estradiol production. In the hours immediately following the LH surge many mRNAs are transiently decreased as the follicle reorganizes into an early corpus luteum. These restructuring events involve breakdown of the basement membrane between the theca and granulosa, hypertrophy and migration of cells, neovascularization and a new commitment of the residual follicle cells to manufacture large quantities of progesterone by the induction of genes involved in de novo steroidogenesis.