Ovulatory Follicles Are Selected at the Onset of Luteolysis (Large Domestic Animals)
Until the advent of ultrasonography, it was difficult to identify growth patterns of follicles in domestic animals, especially those of follicles that develop during the luteal phase of the cycle.
The concept that follicles do develop during the luteal phase was emphasized by the earlier work of Rajakowski, who described the midcycle follicle in the cow. With ultrasonography, it has been possible to define follicular growth and regression during the luteal phase of the cycle in the cow and mare. In cattle the predominant pattern is for several dominant (large) antral follicles to develop sequentially during the cycle (Figure 36-1). The follicular cycles are distinct to the extent that follicle regression usually begins (as indicated by follicle size) before the onset of the growth of the next follicle. The first dominant follicle regresses about midluteal phase, with a second dominant follicle beginning growth immediately. Whether the second dominant follicle is the ovulatory follicle, or whether a third develops, depends on the stage of the follicle at the time of regression of the corpus Iutcum (CL). If the second dominant follicle has begun to regress at the time of
FIGURE 36-1 Mean (±standard error of the mean) profiles of diameters of dominant follicles and the largest subordinate follicle and the cross-sectional luteinized area of the corpus Iuteum (CL) for the interovulatory intervals with three and two follicular waves in cattle. Regression (Pdynamic once the rapid growth phase is achieved; the follicle(s) must be acted on through proper gonadotropin stimulation within a few days, or the result is death of the follicle. If the rapidly growing antral follicle is not exposed to the proper gonadotropin environment, atresia (regression) of the follicle begins almost immediately. Follicles that regress are invaded by inflammatory cells, and the area previously occupied by the antral follicle is eventually filled by connective tissue; that is, the follicle is replaced by an ovarian scar.