Infertility of the mare has a great impact to the equine industry through increased breeding cost and the loss of produce of genetically valuable mares. A large group of mares are affected with chronic endometritis and reproductive failure. Degeneration of the uterine vasculature called elastosis was found in these mares. The associated factors to elastosis and the potential compromise in uterine blood flow and perfusion were the objectives of this research project.

In the first chapter, a large pool of uterine biopsies was collected and their degree of uterine vascular elastosis was evaluated. In addition to the vascular status, mare age, parity status, number of foals and endometrial biopsy grade were studied in order to determine which of these factors had a stronger association with vascular degeneration. The results revealed that the severity of these lesions was strongly associated with increased number of foals produce by each mare, followed by a weaker association with poor endometrial grade and no association with mare age.

In the second chapter, the uterine blood flow and perfusion of mares with different degrees of vascular degeneration were evaluated. Fluorescent microspheres, which are considered a gold standard technique for blood perfusion measurements were used for the first time in the reproductive tract of the mare. Mares were grouped according to their degree of vascular degeneration (no degeneration or mild changes versus moderate to severe degeneration) and their stage of cycle (estrus or diestrus). The results obtained indicate that in mares with normal uterine vasculature there are cyclic changes in uterine perfusion with higher levels during estrus when compared to diestrus. Secondly, in mares affected with uterine vascular elastosis the difference between stages of cycle was absent and they had significantly lower levels of blood perfusion when compared to reproductively healthy mares.

Finally in chapter 3, the response of uterine perfusion to 17β estradiol, a specific vasodilator of the reproductive vascular bed, was evaluated in control mares and mares affected with elastosis of the uterine vasculature throughout the different phases of the estrous cycle. An increase in uterine perfusion was measured only in control mares. During estrus, an overall increase in uterine perfusion of about 50% and during diestrus the increase was limited to the uterine horns and was of a lower magnitude, 25%. Estradiol failed to increase blood perfusion in mares with degenerated vasculature.

In conclusion, degeneration of the uterine vasculature seems to be strongly associated with increasing number of foals and it is associated with decreased uterine perfusion below physiological levels found in reproductively normal mares. Additionally, there is a compromise in the response to vasodilator agents in affected mares which might be related to impairment in the vasodilatory mechanisms or in the vascular compliance of the degenerated vessels.

Decreased uterine blood perfusion might compromise preparation of the endometrium, myometrial contractility, hormonal signaling between embryo and mare, fetal nourishment and the overall fertility of the mare.