Hay there! The birds, bees, and butterflies seem especially active this week, so I decided another blog on the birds & the bees was appropriate. Last week we introduced the reproductive tract of a mare, its anatomy and function, particularly during the transition into cyclicity. We discussed how the ovary parents the behavior of the uterus and cervix with the presence of large follicles. We introduced that when a large follicle is present, uterine edema (increased water or turgidity) makes thick, welcoming folds, both ready for an egg to come settle in, and an open pathway for sperm to enter through the cervix, and that all of these factors must be present for a mare to be ready for breeding.

The ovary parents the behavior of the uterus and cervix because the of the nature of the hormones being secreted by the various structures on the ovary, and the pattern of blood flow to these structures. These hormones largely change the pattern of blood flow to the uterus, making the layers and folds relatively more or less turgid. Estrogen, which women can thank for most of their complaints in life, increases blood flow to the reproductive tract and relaxes (opens) the cervix – the entrance to the uterus. Estrogen is produced in significant amounts by these magic 35-38mm (dominant) follicles. Whether or not we use deslorelin to help us time ovulation, the follicle ovulates when it is ready, releasing the egg to descend to the uterine horn.

Once the follicle ovulates, which can occur on one or both ovaries, the remaining tissue collapses on itself and begins to transform into another ovarian organ – the corpus luteum (CL). The CL produces the hormone progesterone, which changes the reproductive tract whether or not a breeding and/or pregnancy occurs. Progesterone prepares the uterus for and maintains pregnancy. It reduces uterine edema and closes the cervix tight, so nothing can get in (or out). This is the toned, ‘out of heat’ uterus.  We measure progesterone to determine if the mare has reached cyclicity during the spring transition because it will only be produced in significant amounts if the ovary has produced a follicle large enough to ovulate, and result in formation of a CL.

The CL is destroyed by Prostaglandin (I know, it sounds annoyingly similar to progesterone). Prostaglandin is produced by the uterus when no pregnancy is recognized after more than 2 weeks, and during times of inflammation (eg, infection in the uterus). Prostaglandin will only kill the CL once it is mature enough to recognize it – 5 days post-ovulation. This is significant when it comes to ‘short cycling’ a mare. If, for whatever reason, breeding cannot occur this cycle, we can give the horse a shot of prostaglandin (‘prostin’ or Lutalyse) in the muscle to ready the horse for breeding much more quickly. Another time prostaglandin is significant is if a mare double ovulates, and twins occur. If we diagnose twins and crush one for the sake of the other, the inflammation from the act of crushing can result in the loss of both pregnancies.

In review – there are two major ovarian structures, and corresponding hormones, that dictate how the uterus and cervix behave. An egg-containing follicle produces estrogen, which readies the tract for breeding. A corpus luteum (CL), which forms from the ovulated follicle, produces progesterone which readies the tract for pregnancy. Prostaglandin will result in lysis (rupture or death) of the CL, which can result in short-cycling of the mare, or termination of pregnancy. If this summary does not make sense, reread the above blog more slowly.

Don’t forget about our Vaccine Seminar THIS THURSDAY, April 26th at 6 P.M. at the clinic (PLEASE R.S.V.P.!)!  We look forward to seeing you there, and don’t forget to come give me some pets! May your litter boxes be clean and your food bowls filled!


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