The inhibition of growth of the smaller follicles does not
depend on follicle-to-follicle inhibitory mechanisms, but
follicle deviation involves important changes in the largest
follicle. These are characterized by an increased sensitivity
to circulating concentrations of FSH. Dramatic changes in
the insulin-like growth factor (IGF) system (IGF-I and -II,
IGF binding protein, IGF binding protein proteases) in the
largest follicle before the beginning of size deviation play
a crucial role (Beg and Ginther, 2006). Simultaneously, the
dominant follicle suppresses circulating concentrations of
FSH, most probably due to follicular synthesis and release of
oestrogens and inhibin. This hypothesis is supported by the
finding of an increased oestradiol production in the future
dominant follicle one day before follicle deviation (Gastal
et al., 1999) together with an inverse relationship between
circulating concentrations of FSH and inhibin in the cyclic
mare (Bergfelt et al., 1991). It is feasible that systemic LH
is not involved in follicle deviation but is required for further
growth of the dominant follicle after the beginning of
deviation (Ginther, 2000).