Current concepts of folliculogenesis in monovular and polyovular farm species

Follicular growth in sheep and pigs is under both endocrine and paracrine control, and the local factors involved include steroids, growth factors and other regulatory peptides such as the putative follicle regulatory protein. These factors can act either directly on ovarian cells, or by modulating the response to circulating gonadotrophins. It is suggested that during the early stages of development in sheep and pigs, follicles pass through a stage at which they are modulated by growth factors, after which endocrine control mechanisms become more important. There is evidence that interactions between follicle-stimulating hormone, insulin-like growth factor-1 and oestradiol further enhance development and steroidogenesis. Interfollicular relationships in polyovular pigs may differ from those in sheep, in that the more mature follicles may act to enhance development of the less mature in the selected population. Although it is unclear whether similar mechanisms operate in both hyperprolific pigs and prolific breeds of sheep, Meishan pigs and Booroola ewes do exhibit similar follicular characteristics compared with those of nonprolific breeds. The significance of this in relation to prolificacy requires further investigation.