Relationship between the secretion of the corpus luteum and the length of the follicular phase of the ovarian cycle.

The variation in the length of the follicular phase in many mammals may be related to the cellular origin of oestradiol secreted during the luteal phase. In all species the time taken for a small developing follicle (4-5 layers of granulosa cells) to mature to a preovulatory follicle may be the same as that which has been found experimentally in the mouse (10-17 days). In animals such as the sheep, in which there is no source of oestradiol other than the Graafian follicle, follicular development proceeds unimpaired throughout the luteal phase, and the 'follicular phse' which involves only the final stages of maturation of the Graafian follicle is relatively short. In primates, however, in which there is an extrafollicular source of oestrogen from the CL, the secretion of gonadotrophins is suppressed during each luteal phase to a level too low to initiate and maintain follicular development. At the end of each luteal phase and the beginning of the subsequent follicular phase, therefore, it is necessary to initiate the growth of a new crop of small follicles. The time taken for these follicles to develop inot preovulatory follicles determines the length of the follicular phase.