Ovarian follicular growth and development in ruminants is characterised by two or three consecutive follicular waves per oestrous cycle. Each wave involves the recruitment of a cohort of follicles and the selection of a dominant follicle. The dominant follicle continues to grow and mature to the preovulatory stage while others undergo atresia.
The growth of follicular waves is initiated by a rise in circulating FSH.
All follicles growing as a cohort contain specific receptors for FSH and depend on this gonadotrophin in their growth. At this stage the growing follicles do not have a sufficient population of LH receptors to respond to a LH-like stimulation. For this reason this stage of follicular growth is sometimes called FSH-dependent.
Figure 1: Follicular dynamics in a 3-wave cycle
For reasons not yet properly understood only one dominant follicle is selected from among the cohort recruited by the small rise in FSH. A defining characteristic of the dominant follicle appears to be its greater capacity for estradiol production.
Selection of the dominant follicle is associated with a decrease in FSH, FSH is maintained at basal levels. The selected future dominant follicle acquires LH receptors which allow it to continue growth in the environment of low FSH and increasing LH levels.
It seems that by decreasing the FSH support, the selected follicle depletes subordinate follicles from their vital growth stimulant while it benefits from both the low FSH and growing LH stimulation.
Following its selection, DF growth, oestrogen activity and lifespan are controlled by the LH pulse pattern. In response to maximum concentrations of oestrogens produced by the dominant follicle a massive LH release takes place. It is called a pre-ovulatory LH surge or peak and acts as the direct stimulation of ovulation.