Doctor of Philosophy (PhD)


Department of Veterinary Clinical Sciences

Document Type



Progestagen is crucial for regulating early pregnancy in most mammalian species, including the horse. This dissertation research sought to elucidate the effect of endogenous and exogenous progestagens on the early development of equine embryos and to characterize reproductive parameters in the subsequent estrus using a prostaglandin F-induced hypoluteal environment, namely the aluteal pregnancy model. The first chapter reviewed the life cycle of the corpus luteum and its progesterone secretory function in mares. The existing literature supports the notion that the early developing corpus luteum is responsive to prostaglandin F, and complete luteolysis or prevention of luteal formation can be achieved by using an appropriate treatment regimen with prostaglandin F in mares. The second chapter investigated the effect of progestagen deficiency or supplementation on early embryonic development using the aluteal pregnancy model in mares. We found that developmentally retarded embryos with adequate quality could be collected on day 7 after ovulation in aluteal cycles. Furthermore, a long-acting injectable formulation of progestagen, altrenogest, could induce a favorable endometrial environment to support adequate embryonic development until day 7, without the progestational support from the corpus luteum. Nonetheless, compromised pregnancy was noted at a later stage on day 24 when pregnancy was solely supported by the long-acting injectable altrenogest. The third chapter described the profile of reproductive hormones in aluteal cycles with or without altrenogest supplementation and delineated the reproductive parameters of the subsequent estrus. Mares in aluteal cycles developed a characteristic hormonal profile of low plasma progesterone and persistently elevated plasma luteinizing hormone. In the subsequent estrus developed under aluteal environment, mares had an increased number of follicles and occurrence of multiple ovulations. The fourth chapter investigated the effect of constant-rate infusion of prostaglandin F during the first 24 hours after ovulation on luteal function. This treatment regimen did not reduce the interovulatory interval and concentrations of plasma progesterone in diestrus. Together, our findings characterized the early embryonic development and reproductive parameters of pregnant mares during luteal and aluteal cycles. Furthermore, effects on embryonic development and dam’s reproductive endocrine profiles induced by the aluteal environment could be partially reverted by altrenogest supplementation.

Committee Chair

Pinto, Carlos R.