FOLLICULAR DYNAMICS IN SHEEP AND CATTLE

In sheep and cattle, recruitment (i.e. entrance in the acutely gonadotrophin dependent stage), selection (i.e. adjustment of ovulations) followed by dominance occur at regular intervals whatever the reproductive stage (prepuberal, luteal phase, anoestrus). Endocrine, paracrine (between follicles) and autocrine (within follicle) signals are likely to be involved. In sheep in which FSH and LH secretions are inhibited by GnRH agonist administration, recruitment does not proceed. Using this model, FSH and basal levels of LH have been shown to be key signals for recruitment. However, in cycling sheep and cattle, the exact recruitment signal is not clear. Furthermore, this signal may be modulated locally via growth factors, mainly through an autocrine action of epidermal growth factor (EGF). Selection of the ovulatory follicle in sheep is a passive process where the largest follicle of the cohort of recruited follicles inhibits the FSH support to the other follicles via negative feed back action of its estradiol and inhibin. In cattle, this passive process is probably supported by an active process whereby the selected follicle directly suppresses growth of other follicles via the secretion of an unknown paracrine substance. In both species, maintenance of the dominant follicle while the others regress could be related to an autocrine stimulation of the dominant follicle by Insulin Like growth factor 1 (IGF1) together with the differentiation of LH receptors.