Owners of mares should know correct anatomical terminology, have a working knowledge of the normal reproductive cycle, and be acquainted with common problems that could occur. The following article is designed to help mare owners understand the normal reproductive tract of the mare, and the words that veterinarians use to describe normal and abnormal aspects of that anatomy.
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We'll begin this discussion at the outside of the mare's reproductive tract and work our way inward. First, of course, is the perineum--a loosely defined area that includes the vulva, anus, and surrounding area.
In the mare, the conformation of this area is of clinical importance due to its role in protecting the genital tract from the entrance of air and bacteria. Poor conformation, where the vulval lips gape or the anus is sunken, compromises the vulval seal that normally maintains a barrier between the inner reproductive structures and the environment. When this seal fails due to poor conformation, the mare is predisposed to pneumovagina, or vaginal windsucking, in which air is sucked into the vagina through the open vulva.
Continued exposure of the reproductive tract to air results in chronic inflammation, which can lead to endometritis (inflammation of the lining of the uterus). Passage of bacteria into the more susceptible parts of the mare's reproductive tract also might result in infections.
The vulva is the external area of the mare's reproductive system; it protects the entrance to the vagina as discussed earlier (see Figure 2 on page 80). The outer area is covered with normal skin, while the inner area is lined by mucous membrane and opens into the vagina. The upper limit (dorsal commissure) is situated approximately 2.8 inches (7 cm) below the anus.
Within the lower part of the vulva (ventral commissure) lies the clitoris; it's important in mare health because of its potential to harbor disease-causing bacteria such as Taylorella equigenitalis (the causal agent for contagious equine metritis), Pseudomonas spp., and Klebsiella. This area is sometimes swabbed for testing in mares prior to covering.
Within the walls of the vulva lies the vulva constrictor muscle, which is responsible for maintaining a close seal between the vulval lips. It also controls the inversion and exposure of the clitoral area during estrus known as winking.
Through the vulva, we proceed inward to the vagina, which on average is 7.1-9.1 inches (18-23 cm) long and 3.9-5.9 inches (10-15 cm) wide. In the properly conformed mare, the floor of the vagina should rest upon the pelvis, and the walls should be collapsed and apposed (touching each other). The hymen membrane, if present, divides the vagina into anterior (cranial, or nearest the head) and posterior (caudal, or nearest the tail) sections.
In some texts, the posterior vagina is referred to as the vestibule. The urethra (the tube leading from the bladder) opens just behind the hymen (toward the tail) in the vestibule. This area contains some glands, while the anterior vagina does not.
The walls of the vagina are muscular with a mucous lining. The elasticity conferred by this muscle layer allows the considerable stretching required during delivery of a foal (parturition).
The cervix lies between the vagina and the uterus. It is a tight, thick-walled sphincter muscle. The cervix' appearance is quite different during estrus than during diestrus (when the mare is not cycling). The muscle tone and cervix size (plus its secretions) are governed by cyclic hormonal changes.
When the mare is sexually inactive (during diestrus), the cervix is white, tightly contracted, and measures about 2.4-3.1 inches (6-8 cm) long by 1.6-2.0 inches (4-5 cm) wide. Cervical secretion during this time is minimal and thick in consistency. In contrast, during estrus the cervix appears pink and muscle tone is more relaxed (the cervix appears to "flower" into the vagina). This, plus an increase in secretions, eases the passage of the penis into the entrance of the cervix (see Figure 3 at right).
The lining of the cervix consists of a series of crypts or folds, which continue inward to become folds in the uterine endometrium. These folds enable the cervix to expand greatly during foaling.
Next comes the uterus, which is a hollow, muscular organ joining the cervix and the oviducts (see Figure 3 on page 82). The upper part of the uterus is attached to the lumbar region by two broad ligaments.
The uterus is divided into two areas--the body and the horns (which are closer to the mare's head than the body of the uterus). The body of the non-pregnant mare's uterus normally measures 7.1-7.9 inches (18-20 cm) long and 3.1-4.7 inches (8-12 cm) wide, and divides into two uterine horns. The horns are approximately 9.8 inches (25 cm) long, and they narrow from 1.6-2.4 inches (4-6 cm) wide to 0.4-0.8 inches (1-2 cm) wide as they approach the oviducts.
The mare's uterus has a relatively large body compared to the horns (60:40 split) as the mare normally carries a single fetus within the uterine body. This differs from other farm livestock (for example, the pig), where the horns are more predominant and allow multiple fetuses to be carried. Normally, the uterine walls are flaccid (relaxed and loose) and intermingle with the intestine.
The uterine wall consists of three layers:
- An outer, serous layer (thin connective tissue called the perimetrium) that merges with the broad ligaments;
- A central, muscular layer (myometrium); and
- An inner, mucous membrane lining (endometrium) (see Figure 4 on page 82).
The central myometrial layer consists of external longitudinal muscle fibers, a central vascular (supplied with blood vessels) layer, and internal circular muscle fibers. This muscular myometrial layer allows the necessary uterine expansion during pregnancy, and also provides the force for delivering a foal.
The inner endometrium is arranged in 12-15 longitudinal (lengthwise) folds with associated epithelial glands and ducts. The activity, and therefore the appearance, of these glands are dependent on cyclical hormonal changes. This endometrial layer is responsible for placental attachment and normally becomes inflamed by the foreign material deposited during natural cover or artificial insemination. Once the material is cleared by mechanical and cellular activity, the inflammation normally subsides. Persistent inflammation of the endometrium (endometritis) is a fairly common cause of decreased fertility in mares.
The Utero-Tubular Junction
Moving out of the uterus on our way to the ovaries, we come to the utero-tubular junction. This constriction or sphincter is seen as a papilla (small projection) at the end of each uterine horn that separates the horns from the oviducts.
Fertilization takes place in the oviducts, and only fertilized ova can pass through this junction and on to the uterus for further development. Fertilized ova appear to actively control their own passage, leaving the unfertilized ova behind to gradually degenerate.
The Oviducts (Fallopian Tubes)
The mare has two oviducts (fallopian tubes) extending from the uterine horns (see Figure 1 at left); they average 9.8-11.8 inches (25-30 cm) long. The oviducts are highly convoluted (having many overlapping folds) and lie within peritoneal folds (folds of the membrane that lines the body cavity). Their walls are similar in structure to those of the uterus, but they are thinner.
The diameter of the oviducts varies somewhat, starting at 0.08-0.20 inches (2-5 mm) wide nearest the uterine horn (the isthmus) and widening to 0.20-0.39 inches (5-10 mm) nearest the ovary (the ampulla). The lengths of the isthmus and the ampulla segments are approximately equal. Fertilization takes place in the ampulla, a region lined with fimbriae (hair-like projections) that move unfertilized ova into the ampulla to await the sperm, then move fertilized ova out of the ampulla and on toward the utero-tubular junction. The ampulla of each oviduct ends in the infundibulum, a funnel-like opening close to the ovary.
The infundibulum in the mare is next to a specific area of the ovary--the ovulation fossa--which is unique to the mare and is the only site of ova release. In other mammals, ovulation can occur over the whole surface of the ovary. The infundibulum is, therefore, relatively hard to distinguish in the mare, as it is smaller than those in other species in which the funnel-shaped structure surrounds the whole ovary. The infundibulum is also lined with fimbriae, which attract and catch the ova, guiding them toward the entrance of the oviducts.
Finally we arrive at the ovaries, which produce eggs (called ova or gametes) and hormones. These are two bean-shaped structures situated below the fourth and fifth lumbar vertebrae (see figures 1 and 5 on pages 80 and 84, respectively). They make the total length of the reproductive tract 19.7-23.6 inches (50-60 cm).
When the mare is sexually inactive, i.e., during the non-breeding season, her ovaries are 0.8-1.6 inches (2-4 cm) long and 0.8-1.2 inches (2-3 cm) wide, and they are hard to the touch. When the mare is sexually active (during the breeding season), they expand to 2.4-3.1 inches (6-8 cm) long and 1.2-1.6 inches (3-4 cm) wide, and feel softer due to the development of fluid-filled follicles. These follicles grow and ovulate in synchrony with the mare's estrous cycle.
Once a follicle has ovulated, a blood clot or corpus hemorrhagicum (CH) is formed in the vacated space, which then becomes the corpus luteum (CL) that secretes progesterone. If the mare is not pregnant, the CL degenerates before the next ovulation.
The convex (curved outward) outer surface of the ovary is attached to the broad ligament and is the entry point for the ovary's blood and nerve supply. The concave (curved inward) inner surface is free from attachment and is the location of the ovulation fossa where ovulation takes place. The whole ovary, except for the ovulation fossa, is contained within a thick protective layer.
The mare's ovary is made up of the inner cortex layer (of active gamete-producing tissue) and the outer medulla layer (of supporting tissue). All follicular, CH, and CL development occurs in the inner ovarian cortex.
The reproductive tract of the mare is a remarkable, complex system that is designed not only to maximize the chance of fertilization and maintain the resulting conceptus in a sterile environment, but also to expel that conceptus successfully at term. By understanding this complicated system in the mare, you should be better able to successfully discuss reproductive problems with your veterinarian.
About the Author
Mina Davies Morel, PhD, is head of the equine group at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University in the United Kingdom. She has particular interest in equine reproductive physiology and its application to stud management, and she is the author of a number of scientific papers and text books on the subject. She is a leisure rider and owner of Welsh Cob Section Ds.
POLL: University Equine Hospitals