Summary. 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.
Although exogenous gonadotrophins are effective in promoting follicular development, other factors of extra- or intra-ovarian origin may limit follicular responsiveness to gonadotrophins.
Finally, it is suggested that the interfollicular relationships in polytocous animals may differ from those of monotocous animals and in the pig the dominant follicles may promote the maturation of the smaller follicles, in contrast to their inhibitory effect in other species. This may be achieved by oestrogens secreted from the dominant follicles passing to the ovarian artery via a sub-ovarian countercurrent exchange mechanism.
© 1985 Journals of Reproduction & Fertility Ltd