|The Reproductive Organs of The Female Horse|
By Stephen Champion, BSc (Hons) BVM&S MRCVS,
It is important for anyone with an active interest in breeding horses, to understand the basic anatomy of the female reproductive tract. This will allow an owner to recognise any external abnormalities in their horse, to understand why some horses are a better bet for breeding than others, and to interpret a veterinary report of a breeding examination.
The pelvic cavity is the structure through which the caudal reproductive tract passes, together with the rectum and last section of the urinary system. It is roofed by the sacrum, or fused bones of the vertebral column, and the first 2 or 3 tail vertebrae. Other bones, tendons, ligaments, nerves and blood vessels form the walls of the pelvis, giving the cavity the shape of a truncated cone, through which a foal will pass (hopefully quickly!) during parturition. At the hindquarters end of the pelvis, the soft tissue areas are referred to as the perineum and comprise the areas around the anus and vulva. It is confined, in the mare, between the two large semi-membranosus muscles, giving the perineum a recessed appearance.
The female reproductive tract comprises the vulva and vestibule, the vagina, cervix and uterus, leading to the uterine tubes and the ovaries.
The two ovaries are the site at which follicles are produced containing the ova, or eggs, which are ultimately fertilised during conception. They lie deep in the abdomen close to the dorsal wall (nearest the back) and can be palpated by the vet per rectum during a fertility examination. By palpating the follicles on the ovary, the vet is able to tell if the mare is ready to be mated by the stallion. He can also assess the ovary for disease. The ovary is very long compared to other species and can measure up to 10cm across in draught mares. They are generally kidney-shaped, the indentation comprising the ovulation fossa, the site of rupture of the mature follicles.
The ovaries are connected to the uterus by two uterine tubes, which are convoluted and about 20cm in length. Both the ovaries and the uterine horns are held in place by modified ligaments called the mesovarium and mesosalphinx respectively. Fertilisation of the ova by the stallion’s sperm takes place in the uterine tubes and only fertilised ova are admitted to the uterus after this event.
The uterus has a large body and two divergent horns. The horns lie in the abdomen and the body lays part in the abdomen and part in the pelvis. This is the part of the reproductive tract where the new foal will attach at only a few days old, where the placenta will form to feed and oxygenate it and where the foal will develop until birth. The uterus can be palpated again per rectum by your vet who may be assessing the health of the uterus for breeding or the viability of a foal being carried within it.
The cervix is a 6cm tube connecting the uterus to the vagina. Its consistency varies enormously depending on the stage of the cycle the mare is at and varies from soft to very firm. It is another useful indicator that is used to decide whether the mare is ready to be bred. During oestrus or the birth process, called parturition, the cervix is open – otherwise it is firmly closed, preventing bacteria from entering.
The vagina is long in the mare (nearly as long as the body of the uterus) and is remarkably distensible in length and circumference, a feature that has obvious benefit during parturition! It is connected towards the hindquarters of the horse to the vestibule and vulva. Noteworthy features are the urethral opening (from where the horse’s urine is emitted) and the clitoris within the ventral (bottom) lips of the vulva. The clitoris is very prominent in mares in heat, when it is exposed by “winking”- movements of the labia. Also of interest is that three to five clitoral sinuses invade the clitoris. These harbour the bacteria responsible for contagious equine metritis (CEM) and are swabbed by your vet when testing for this disease before breeding has begun.
The relationship of the vulva to the pelvic skeleton varies significantly. Usually it is ventral to the floor of the pelvis and its opening is closed. However, sometimes in Thoroughbreds in particular the opening is more dorsal and the closure is less effective. This can cause “windsucking”, where air is drawn into the vulva, leading to the introduction of bacteria into the vagina and uterus causing infections. Endometritis (uterine infection and inflammation) and reduced reproductive performance can result.
This article represents a brief overview of the reproductive tract of the non-pregnant mare. It should be appreciated that significant anatomical and physiological changes occur to the reproductive tract during pregnancy, and to a lesser extent, during normal seasonal behaviour. These are beyond the scope of this article, but owners should familiarise themselves with these changes when caring for a pregnant mare.