Reproductive Anatomy and Physiology of the Mare

LESSON TWO

Anatomical Structure Function

Ovary (paired)	Follicle growth (secretion of estrogen) Corpus Luteum (CL) Secretion of progesterone Ovary produces ovum (egg)
Oviduct (paired), also called fallopian tubes or uterine tubules	Where fertilization occurs in the mare. Connects ovary with the uterus.
Uterus (consists of two horns and a body)	Secretion of hormones. Site of embryonic and fetal growth and maturization.
Cervix	Connects uterus to vagina. Closed during pregnancy, diestrus and anestrous. Open during estrus to allow for sperm introduction and parturition to allow for foal delivery.
Vagina	Connects external genitalia with internal genitalia. Accepts penis during copulation.
Vulva {consists of vulvar lips (labia) and clitoris}	Entrance to the vaginal tract.

Understanding the anatomy and physiology of the mare reproductive system is the key to the operation of a successful Equine breeding operation. It is not only mandatory for the Veterinarian, but the breeding farm manager as well, to be well versed in the details and peculiarities of the mare’s estrous cycle.

Reading assignment: Chapter 1 and 2 in required text (Manual of Equine Reproduction).

Understanding and being able to visualize the anatomy of the mare’s reproductive system is mandatory for a successful breeding farm operation.

A veterinarian using a technique called rectal palpation often examines mares at a breeding farm daily or at least every other day.

This procedure is performed in the following manner:

With the mare in a chute or restrained in a way to prevent excessive movement, the veterinarian will take a plastic sleeve covered with a lubricating material (K-Y jelly) and enter the rectum of the mare with this sleeved hand and arm. Manure is removed until there is no fecal matter interfering with the veterinarian’s ability to actually feel through the wall of the rectum and grasp the left or right ovary. The veterinarian can actually determine follicular structures, their size and consistency, during this process. He/she will continue to examine the mare’s entire reproductive tract in the same manner and make notes as to what was found.

An ultrasound machine can be used in the same manner by taking the ultrasound probe into the palpating hand of the examiner. As you can see form the diagram, the rectum is located above the reproductive tract, making the palpation a fairly simple procedure. If you wish to participate in the equine breeding industry, knowledge of these intricate examinations should be able to be understood and this can be done by the study of the anatomy and physiology of the mare’s reproductive tract. It will be up to you to have a working knowledge of the anatomy of the mare’s reproductive tract seen in the diagrams above and the pictures found in the text.

In reading of Chapter 1 and 2 you will learn the major hormones that control the mare’s estrous cycle. The key features to consider in these chapters will be detailed below. Use this as a guide for study and review purposes.

The Mare's Estrous Cycle

Terminology

"Estrous refers to the entire reproductive cycle of the mare (usually considered to be 21 days).
"Estrus" refers to the "heat" stage of that cycle when the mare is receptive to the stallion, shows interest in being bred by the stallion. It is considered to be 5 days of the 21-day estrous cycle on average.
"Diestrus" refers to the period in the 21-day estrous cycle when the mare is not receptive to the stallion, an average of 16 days, normally.
"Anestrus” refers to the compete absence of estrus; winter anestrous refers to the short daylight days of the winter months when the mare will often not go through any estrous cycles.
The mare is “seasonally polyestrus”, meaning that she undergoes regular estrous cycles during a portion of the year (late spring, summer and early fall), and usually during the winter months she is in winter anestrous. This is nature's way of preventing the arrival of a foal during bad weather.

These cycles are controlled by the mare's endocrine system (hormones produced by endocrine glands that control the cycle), which in turn respond to an increase or decrease in daylight duration with the onset of spring or fall, which affects the pineal gland. This study of hormonal activity is known as endocrinology.

It is important to understand there is a closely linked feedback system between many of the reproductive hormones present in the mare which will alter the level or presence of some hormones as levels of other different hormones increase or decrease. This means that artificially altering a single hormone will be likely to have an effect on one or more of the other hormones. The same can be said of natural hormonal changes - whether they are happening in a correct manner or not. Many of these hormonal changes do occur naturally, but when something becomes unbalanced either naturally or artificially, we can see estrous cycle problems develop in the mare.

A mare “breaking down” during the estrus (heat) period.

Hormones Active during the Estrous Cycle of the Mare

• Gonadotropin Releasing Hormone (GnRH)
• Secreted by the hypothalamus of the brain, GnRH has a controlling influence over other reproductive hormones.
• Early in the year stimulation of the pineal gland by light - either natural or artificial - causes a reduction of melatonin secretion, which in turn allows GnRH to be secreted by the hypothalamus, thereby stimulating the production of other hormones (FSH, LH). The exact connection between decreased melatonin production and increased GnRH levels is not understood.
• The use of injectable GnRH to facilitate the onset of early estrus by shortening the transitional phase, has not been shown to be very successful, and therefore is not generally recommended or attempted in the breeding industry.

• Follicle stimulating hormone (FSH) and Luteinizing hormone (LH)
• Both FSH and LH are produced in the pituitary gland and are regulated in a similar manner by GnRH..
• LH concentrations are lowest during the diestrus phase of the cycle, rising only a few days before the onset of estrus to a peak usually on the day of, or shortly after ovulation, to then drop to previous levels over the next few days. LH then is responsible for the maturation and ultimate ovulation of the ovum containing follicle (Primary Follicle)
• The low levels of LH during the diestrus phase (nonreceptive phase) are necessary to maintain the corpus luteum (CL). The CL is the structure formed by the tissue remaining from the ovulated follicle, and this CL produces progestins which are responsible for the behavioral characteristics of the mare in the non-receptive stage.

o FSH levels are found to increase just before the beginning of estrus and continue in an elevated level as the primary follicle (ovulatory follicle) gets closer to maturation. As this primary follicle grows on the ovary it produces estrogen which is responsible for the behavioral characteristics shown by the mare in heat (heat period or estrus). High levels of estrogen feedback on the pituitary to cause the greater secretion of LH and a slowing down of the production of FSH. In this way the cycle repeats itself unless pregnancy is encountered.

• More on Estrogens
• Estrogens also causes the relaxation of the cervix during estrus as well as production of vaginal secretions found during estrus (heat) in the mare.
• Estrogen presence has a negative impact on FSH, but a positive impact on LH as mentioned above in the feedback phenomenon of the estrous cycle.

• More on progestins (Progesterone)
• Progesterone is the hormone that, among other things, in non-pregnant mares prevents the display of estrus as mentioned above. It is also, in part, responsible for increasing uterine and cervical tone in the diestrus mare;
• Progesterone is secreted by the Corpus Luteum (CL) that is formed in the evacuated follicle left behind after the mare has ovulated.
• Luteal Progesterone has a positive impact on FSH and a negative impact on LH. In other words, it has the same feedback control on the pituitary gland as high levels of estrogen has, except the high levels of progestins slow the production of LH by the pituitary and stimulate the production of FSH by the pituitary if there is no pregnancy.

• Prostaglandin
• Secretion of Prostaglandin F2α by the lining of the uterus commences if pregnancy is not detected at about day 14 post-ovulation.
• The presence of PGF2α (prostaglandin) causes the "lysing" (death or destruction ) of the CL and an almost immediate drop in circulating levels of progesterone, which permits the mare to start displaying estrus.
• If pregnancy occurs during the previous estrus period of the mare, the prostaglandin is not secreted form the uterus, but instead progesterone is secreted to continue to maintain pregnancy.

A Brief Sequential Overview of the Regular Estrous Cycle

• Key Features:
• The entire estrous cycle from ovulation to ovulation takes approximately 21 days.
• Ovulation of a follicle occurs, releasing the ova for potential fertilization.
• The cavity left behind on the ovary by the evacuated follicle fills rapidly with blood and in some cases forms a Corpus Hemorrhagicum prior to creation of luteal tissue to become a Corpus Luteum (CL) as a result of the presence of LH.;
• By about 5 days after ovulation, the CL is fully functional and secreting progesterone. This is also known as the luteal phase (diestrus).
• FSH action early in diestrus may produce a mid-cycle follicle that will sometimes ovulate, but most often regresses.
• Around day 13 post-ovulation the uterus secretes PGF2α (prostaglandin) which causes the destruction ("lysing") of the CL, which will then permit the onset of estrus behavior (heat or receptivity of the stallion by the mare).
• FSH activity in late diestrus will have caused the selection of a dominant follicle (Primary follicle) which under the influence of increasing estrogen levels early in estrus will develop to reach ovulatory status approximately 21 days after the previous ovulation occurs. In some instances there are two dominant follicles, which is why twins are occasionally seen in the light breed mare).
• The mare will display receptive behavior (estrus) towards the stallion for 5-7 days and will ovulate in the last 24-48 hours of that display period.

The following will help you understand the use of hormones in a breeding farm operation.

Hormones Associated with Reproduction

Hormone	Primary Production site	Target organ	Effect	Therapeutic use
Estrogen - Heat hormone	Ovary (ovarian follicle)	Uterus, cervix, and external genetalia of mare	Expression of estrus. Female sex characteristics, genital tract changes	Enhances estrus (heat) but may interfere with normal cycling.
Progesterone -Pregnancy Hormone	Corpus luteum of ovary (CL) also the placenta during pregnancy	Uterus Mammary gland	Maintains pregnancy	Prevent abortion. Synchronize or suppress estrus (regumate)
Prostaglandin	Uterus	Corpus Luteum (CL)	Luteolysis.(CL regression)	Synchronize estrus. Treat persistent CL.
Gonadotropin releasing hormone (GnRH)	Hypothalamus	Pituitary gland (master gland)	LH and FSH release
Follicle Stimulating Hormone(FSH)	Pituitary gland	Ovary-mares Seminiferous tubules-male	Growth of follicle-mare spermatogenesis-male	Same as effect
Luteinizing Hormone (LH)	Pituitary gland	Mature follicle–mare Leydig cells –male	Follicle maturation and ovulation Stimulates testosterone production	Same as effect. (gonadotropin(HCG)
Oxytocin	Pituitary gland	Uterus , mammary gland	Uterine contraction, milk let down	Retained placenta elimination, induce abortion or foaling
Prolactin	Pituitary gland	Mammary gland	Milk production
Relaxin	Ovary, placenta	Pelvic tissues	Relaxation of pelvic area for foaling