Un aggiornamento sulla fisiologia riproduttiva della cavalla

Regulation of follicular development In mares - follicular deviation and dominance: Since the advent of transrectal ultrasonography for evaluation of the mare's reproductive tract, the dynamics of follicular development and ovulation has been studied extensively. Follicular development in mares occurs in waves of development that are characterized by the coordinated growth of a number of follicles (7 to 11). Over several days, a dominant follicle emerges and continues to grow whereas the remaining subordinate follicles undergo atresia and regress in size. During the estrous cycle of mares, waves of follicular development are characterized as primary if the dominant follicle becomes the ovulatory follicle during estrus. In some mares, a secondary wave of follicular development may occur earlier in diestrus. This secondary wave of follicular development may lead to a large follicle that either undergoes atresia or may ovulate during diestrus (diestrous ovulation). Therefore, mares may have a primary follicular wave during the estrous cycle which may or may not be preceded during diestrus by a secondary follicular wave. There appears to be some tendency for this pattern to be consistent within mares across estrous cycles although other factors such as season may also influence this effect. During follicular growth in mares, small follicles (6 mm) begin to grow in a coordinated fashion at the onset of the follicular wave until there is a deviation in the diameter of individual follicles with a change in the growth rates between the two largest follicles. This coordinated growth phase appears to last about 6 days in mares before the onset of follicular deviation and selection of a dominant, preovulatory follicle. In the primary (ovulatory) follicular wave in mares this deviation in the size of the largest follicle occurs at approximately 22.5 mm. Destruction or ablation of the largest follicle at this point results in the next-largest follicle assuming the role of the dominant follicle. During follicular growth, the follicle which first reaches 20 mm in diameter will become the dominant follicle approximately 93% of the time. For practical purposes, a follicle can be defined as a dominant follicle when it reaches 30 mm in the mare. Regulation of Seasonal Reproduction: Although the role of photoperiod in regulation of seasonal reproductive activity has been well established in the mare, research over the past few years has further defined the modulating effects of nutrition, body condition, climate as well as other factors in determining the onset of reproductive activity in mares. In addition, we have a better understanding of the neuroendocrine factors which regulate reproductive season in mares with potentially better means to manipulate reproductive season in mares. Although photoperiod is the predominant factor regulating seasonal reproduction in mares, environmental temperature and nutrition / body condition clearly modulate the response. Differences in the onset of ovulation in mares between years were correlated with environmental temperature. Likewise, mares in poor body condition (BCS < 5/9) had a longer interval to first ovulation than did mares in higher body condition. Pasture grazing of mares has also been associated with an early onset of reproductive seasonality, thus supporting a "green grass" effect in mares. Materal recognition of pregnancy in mares: The process whereby the equine conceptus prevents lysis of the corpus luteum with subsequent maintenance of elevated progesterone concentrations is termed maternal recognition of pregnancy. Although the precise nature of the embryonic signal responsible for pregnancy recognition in mares has yet to be determined, there is considerable new information about the process of luteolysis and about how the conceptus affects luteostasis. It is clear that conceptus mobility within the uterus up through approximately D16 after ovulation is critical to the process of pregnancy recognition. In particular, during the period between Day 10 to 14, there is extensive conceptus mobility within the uterus that appears to be driven by prostaglandin secretion (most likely by the conceptus). As noted above, the precise nature of the conceptus signal responsible for pregnancy recognition in mares has not been defined; however, it is clear that the presence of the conceptus in the uterus serves to markedly reduce the secretion of PGF _2α in pregnant compared to cyclic mares. A recent study has demonstrated that the expression of cyclooxygenase-2 (COX-2) by the equine endometrium is significantly lower in pregnant versus nonpregnant mares and that suppression of COX-2 by the conceptus may represent a key step in blocking PGF_2α secretion by the endometrium In pregnant mares. COX-2 is important in the enzymatic conversion of arachidonic acid to PGH₂, an unstable intermediate prostaglandin that is further metabolized to PGF _2α. Inhibition of COX-2 may represent a critical step in blocking endometrial release of PGF_2α and luteostasis during early pregnancy in mares.