Basic physiology of follicular maturation in the pig

The pig is an excellent animal in which to study the control of folliculogenesis in a polytocous species, and particularly to examine the inter-relationships between follicles from the same animal. Follicle recruitment occurs from the proliferating pool, and various studies suggest that this recruitment occurs between Days 14 and 16 of the oestrous cycle. The growth of follicles selected for ovulation is associated with rapid atresia of smaller follicles and a block to their replacement in the proliferating pool. However, there is a considerable range in the morphological and biochemical development of the dominant follicles in the early follicular phase, suggesting that follicles are recruited at markedly different stages of development, or that recruitment continues into the follicular phase. A significant and predictable relationship has been established between follicular diameter and follicular fluid volume, and a comparison of these two characteristics demonstrates a gradual increase in follicular tissue volume as a proportion of total volume. Growth of follicles from 2 to 4 mm is associated with a proportional increase in granulosa cell numbers, but above 4 mm the relationship is very variable even in selected follicles that are steroidogenically active. Therefore, the number of granulosa cells cannot be used as an indicator of atresia in pig follicles. LH receptors are present in thecal tissue throughout development, reaching maximal levels on Day 20 of the oestrous cycle and declining on Day 21. Granulosa cells possess receptors for LH only in the later stages of maturation, and again these are maximal on Day 20. The pattern of steroidogenesis in pig follicles is consistent with the two-cell theory of steroidogenesis in that androgen produced by the theca is aromatized to oestrogen by the granulosa cells, However, in contrast to that of many other species, the theca of the pig also produces oestradiol in quantities comparable to those secreted by the granulosa. As with morphological development, the selected population of preovulatory follicles shows a considerable range of biochemical development and follicles of identical size may show great dissimilarity in follicular fluid steroid concentrations and LH binding. Androgen availability rather than aromatase activity appears to be the limiting factor for steroidogenesis. There are also several nonsteroidal factors which have been isolated from porcine tissue and play some role in follicular maturation. Although exogenous gonadotrophins are effective in promoting follicular development, other factors of extra- or intra-ovarian origin may limit follicular responsiveness to gonadotrophins.(ABSTRACT TRUNCATED AT 400 WORDS)