Reproductive Anatomy of the Mare and Stallion



Ovarian Size, Shape and Dimensions

            Ovaries are bean-shaped in the mare but vary in size depending on the mare’s age and season of the year.

Maiden mares in winter have ovaries of 2-4 cm in length and 2-3 cm in width. With the onset of reproductive activity in spring, the ovaries will increase in size to 5-8 cm in length and 3-4 cm in width. The consistency of the ovaries also changes with season: In winter, the ovaries are hard whereas they become softer in spring

Ovaries in older mares are more variable, regardless of the season. During the period of sexual quiescence (anoestrous), the ovaries are usually between 3 and 6 cm in length. The ovaries will increase in size during the seasonal breeding period when the mare exhibits repeated cycles of reproductive activity (oestrous). At this time, the ovaries will usually be 6-8 cm in length and 3-5 cm in width. The shape of the ovaries in older mares also differs in that the ovaries lose their bean shaped appearance and become irregular in shape.

            The typical mare’s ovary has a medial and lateral surface and a convex border to which the broad ligament containing blood vessels and nerves is attached. The opposite or free border has a small depression called the ovulation fossa. It is at this site that the mature ova are shed during ovulation. The mare can ovulate only at this specific location because the ovary is covered by a tough fibrous capsule which encloses the ovary except at the ovulation fossa.

            The ovaries in the mare are situated in the sublumbar region, ventral to the fourth or fifth lumbar vertebrae. In a mare of average size, the ovaries are located 50-55 cm from the vulva.


Normal Non-Pregnant Uterus of the Mare. Note the position of the ovary, uterus relative to the pelvis.


The ovary is made up of a medulla (the inner portion) and the cortex (the outer portion).

The medulla contains blood vessel, nerves and connective tissue.

The cortex contains cells and tissues involved in ovum production and hormone production.


The outermost layer of the cortex is the surface epithelium (formerly called the germinal epithelium, since it was believed that immature female germ cells (oogonia) originated from that layer. However, it is now known that immature (primordial) germ cells arise from embryonic gut tissue, undergo a series of mitotic divisions and migrate to the cortex of the embryonic gonad. The oogonia populate the cortex of the ovary and undergo mitotis and start meiosis. They become suspended partway through meiosis until they are ovulated (in some animals like horses and humans, this maybe over 20 to 40 years later).

Just beneath the surface epithelium is the parenchyma, the functional layer of the cortex that includes the developing oocytes and surrounding follicle cells and the cells that produce hormones.

            Ovaries contain the female gametes (ova). Hundreds of thousands of these are present at birth and no more will develop during the mare’s lifetime. These gametes are present in the ovary in the form of primary follicles. A primary follicle consists of a central germ cell called the oogonium, enclosed by a single layer of follicular cells.

After puberty, ova mature and are released from the mare’s ovaries at intervals. These intervals are determined by secretion of hormones from the anterior pituitary. Of the many hundreds of thousands of primary follicles present in the mare’s ovary at birth, only a tiny fraction reach maturity and liberate ova. Most of the follicles are destined to die without developing or to undergo partial development and then die (regression).


The Genital Tract

The genital tract consists of fallopian tubes, uterus, cervix and vagina.

It extends from the vulval orifice to the ovaries.

The genital tract has a number of important functions:

·      receives the male organ,

·      forms a pathway up which the spermatozoa travel to meet the ovum,

·     'houses' the developing embryo and

·     provides a passage for expulsion of the fetus to the exterior.


The Oviduct

The oviducts or fallopian tubes are two tortuous tubes 20-30 cm long extending from the uterine horns to the ovaries.  They are 2-3 cm in diameter at their uterine end and 4-8 mm in diameter at their ovarian end.  The ovarian extremity of the oviduct is funnel-shaped and its edge is irregular with projections (fimbriae) and is attached to the ovulation fossa.  The tube communicates with the peritoneal cavity via a small opening through which the ovum can pass at ovulation.

1.    The tube consists of three layers: an outer fibrous and serous coat,

2.    a middle muscular coat of circular and longitudinal fibres and

3.    an inner mucous membrane composed of single-layer columnar ciliated and secretory cells.


The oviduct can be divided into 4 parts:


1.The fringe-like fimbriae, which surrounds the ovary and captures the egg after ovulation,

2.The infundibulum, the funnel-shaped opening near the ovary

3.The ampulla, a thin-walled portion with a large inside diameter and many ciliated cells, and

4.The isthmus, a thicker-walled tube that is thicker because of the smooth muscle surrounding the lumen.



The ampulla has thin walls that fold in on themselves, greatly increasing the surface area of the inner epithelium. The majority of the inner cells are ciliated but a few secretory (non-ciliated) cells are present. The cilia help move the egg down the oviduct after ovulation. Oestrogens regulate the beating of the cilia and cause them to beat toward the isthmus. Fertilization usually occurs in the ampulla or at the ampullary-isthmic junction. At this junction, the ampulla connects to the isthmus. This junction is also a constriction that regulates passage of sperm and the egg. The isthmus has a smaller inside diameter and outside diameter and the wall is much thicker, since the lumen is enveloped with smooth muscle. This smooth muscle can contract the isthmus to move sperm to the site of fertilization. The contractile activity is regulated by estrogens. The isthmus can act as a sperm reservoir to supply sperm to eggs at the site of fertilization. The isthmus also has more secretory cells and fewer ciliated cells than the ampulla. These secretory cells have many microvilli to increase the surface area for secretion.

The isthmus is connected to the uterus through the uterotubal junction.


The Uterus

The uterus provides a place for development of the fetus, if fertilization has occurred.  The uterus consists of a corpus (body), a cervix (neck), and two comua (horns). 

The mucous membrane lining the interior of the uterus (endometrium) is highly glandular. The endometrium varies in thickness and vascularity with hormonal changes in the ovary and with pregnancy.


The uterus is a hollow muscular organ continuous with the fallopian tubes anteriorly and opening posteriorly through the cervix into the vagina.  It is attached to the sublumbar region by two folds of peritoneum (the broad ligaments) and consists of

·two horns,

·a body

·and a neck (cervix). 

The horns are about 25 cm long and the body 18-20 cm in length and 10 cm in diameter.

The wall of the uterus consists of three coats: outer serous layer continuous with the broad ligaments;

 2. a muscular layer (myometrium) containing an external layer of longitudinal fibres and an internal layer of circular fibres. Estrogens increase the "tone" of the myometrium making it a firmer while progesterone makes the uterus more flaccid.

 3. a mucous membrane (endometrium) comprising luminal epithelium, a stroma of connective tissue, glands and their ducts. Estrogens cause a thickening of the endometrium and stimulate growth of the endometrial glands.

Progesterone causes the endometrial glands to coil, branch, and to secrete "uterine milk", a secretion that aids in embryo growth. These hormones help prepare the uterus for pregnancy. If the animal does not become pregnant, the endometrial glands regress.


The uterus is supplied blood by the uterine artery and the uterine branch of the utero-ovarian artery and its nerves are derived from the uterine and pelvic sympathetic plexuses.

The uterus consists of two uterine horns and a uterine body. The proportion of the uterus composed of horns and body varies greatly in different species. Many litter-bearing species have long uterine horns and a fairly small uterine body (bicornuate type of uterus). Sows have uterine horns that account for about 90% of the uterine capacity and may be as long as 4-5 feet. Contrastingly, cows and sheep have shorter uterine horns.

The mare has a large uterine body and small horns and the body accounts for about 50% of the capacity. This uterine type is referred to as bipartite.

Animals with a duplex uterus have a separate cervical canal for each uterine horn and examples include rats, rabbits, and guinea pigs. Humans and primates have a simple uterus. Their uteri are pear-shaped with no uterine horns. The opossum and some other marsupials have two separate vaginas, two cervices, two uteri, oviducts, and ovaries. The penis in these species is forked to match the morphology of the two vaginas.


The Cervix

The cervix is a hard-walled inelastic organ. One end is continuous with the body of the uterus and the other end protrudes into the vagina.

   In the mare, the cervix is the constricted posterior part of the uterus.  It is about 7 cm long and 4 cm in diameter in the sexually inactive phase and projects into the anterior vagina.

The length of the cervix that protrudes into the vagina (cervical os) varies in different species. Its lumen is usually small, although it can relax sufficiently to allow a fetus to pass through at birth (parturition). The connective tissue is broken down by degradative enzymes just prior to parturition. The cervix acts as a block to prevent bacterial contamination of the uterus. In the sow, the cervical canal is funnel-shaped with a corkscrew configuration that matches the shape of the penis.

The outer layer of the cervix is a serosa layer.

The middle layer is made up of fibrous connective tissue interspersed with smooth muscle fibers. This gives the cervix its hard and inelastic properties. The inner layer is composed of mucosa with secretory cells. The cervix secretes large amounts of a sticky mucus during estrus. Estrogens during heat also cause the cervix to dilate. After mating a plug forms in the cervix from cervical mucus and male accessory gland secretions. This plug protects the uterus during pregnancy. Prior to birth, the cervix undergoes an amazing sudden relaxation, allowing the fetus to pass through and be born.


The Vagina


The vagina extends horizontally through the pelvic cavity from the cervix to the vulva. 

It is about 20 cm long and up to 12 cm in diameter but its walls are normally in apposition.  Most of the vagina is uncovered by peritoneum and it is surrounded by loose connective tissue, veins and fat. 

Its wall is composed of muscular and mucous coats.  The mucous coat is very elastic and covered with stratified epithelium. That means this epithelium has several layers of cells on top of one another rather than just one layer.

The vagina is divided into anterior and posterior parts by a transverse fold (hymen) which is usually patent. 

The vagina is really a tubular duct that connects the cervix with the external genitalia. In many species (human, cow, and ewe), semen is deposited here during normal mating. Semen is deposited in the vagina near the cervix. In the mare, semen is deposited in the uterus at mating. The environment of the vagina is hostile to sperm (acidic pH). If sperm are not transported into the cervix fairly quickly, they usually die in the vagina.


The Vulva

The vulva or external genitalia consist of the vestibule and the labia.

            The vestibule starts at the duct from the external urethral orifice, the point at which the bladder dumps into the female tract and runs to the labia, or lips of the vulva. The labia majora is homologous to the scrotum in males and is visible externally. The labia minora is not well-developed in most farm animals.

   The labia includes the inner and outer folds of the vulva.

The vulva is covered by mucous membrane and is continuous in front with the vagina and opens externally about 7 cm below the anus. 

          It consists of two lips (the labia majora) connected by a dorsal and ventral comissure. 

          Immediately inside the ventral commissure is the glans clitoridis (or clitoris). The clitoris is visible inside the labia, is homologous to the glans penis, and contains erectile tissue. It is not prominent enough to be used in estrous detection in most species but in the mare, frequent contractions of the labia expose the erect clitoris (clitoral "winking").

   In the mare. the clitoris lies within a pouch or fossa which may contain a variable quantity of smegma-like inspissated material.  The clitoris itself contains three sinuses or cavities, which may also contain inspissated material and may be a reservoir of 'infection' for some venereal-disease-producing organisms. The sinuses are situated towards the top or dorsal aspect of the clitoris.  The central sinus is the largest and the lateral ones may not be present in all cases. Routine bacteriological swabbing of the clitoris and clitoral sinuses  is now a routine part of venereal disease management on stud farms.




The Testes


The two testicles (or testes)of a stallion correspond functionally to the ovaries in a mare. In the mare, the ovaries remain inside the body and cannot be seen. However, while the testicles develop inside the body of a male horse in the womb, they move down during the last month before birth into a special sac just under the skin called the scrotum.

Spermatogenesis (the production of sperm) can only take place at temperatures slightly below the core body temperature and for this reason most mammals have the testes located outside the abdomen in the scrotum.


The scrotum is a double sac, located between the hind legs, at the root of the penis and each side of the sac contains one testicle. When descended[1], the testicles can be seen and palpated within the scrotum. When one or both testicles do not descend properly into the scrotum, the horse is called a rig (or cryptorchid).  Testicles that do not descend into the scrotum do not produce sperm. However, an undescended testicle will produce hormones so this horse will have the behavioural characteristics of a stallion.


A gelding is a horse that has been castrated by having its testicles cut off. Castration of the male horse has been carried out by humans since the horse was domesticated and produces an animal which is easier to manage and less aggressive. Traditionally, all male horses were normally castrated unless they were to be kept as stallions specifically for breeding.



The testes lie horizontally in the scrotum. In an adult horse, each testis is 8 to 12 cm long, 6 to 8 cm high, 5 cm wide and weighs between 200 and 300 g. Each testis consists of a network of very thin tubes, called the seminiferous tubules. These tubules are lined by cells that grow, divide and differentiate into mature spermatozoa. It takes between 60 and 90 days for the complete development of a typical sperm. When the spermatozoa are fully formed, they are released into the tubule and are passed along to a large collection area attached to the testis called the epididymis. Here, they undergo final maturation prior to being ejaculated.

The testicles produce literally billions of spermatozoa (individual sperms) at each ejaculation. Sperm production in the male horse usually starts at about one year of age (puberty) but full reproductive capacity is not reached until the horse is about 4 years old. The sperm are produced in the testes by the division of the specialized cells lining the seminiferous tubules. After development, the spermatozoa are stored in a structure that is continuous with the testicle called the epididymis. Spermatozoa can be stored in the epididymis for a number of days. Thereafter, if they are not ejaculated they are broken down and reabsorbed.


The accessory sex glands in the stallion


The typical ejaculate volume in the horse is between 50 and 75 ml, although volumes as high as 150 ml have been reported. The volume of semen and the amount of spermatozoa in the ejaculate is higher during the breeding season.



You should be aware that most of the volume in the stallion’s ejaculate actually comes from the accessory sex glands. These are a series of glands that are located further up the stallion’s reproductive tract. You will learn about these later. Considering that only one spermatazoa is required to fertilize the female ovum, the stallion produces a vast excess at each ejaculation. Typically, a stallion will ejaculate between 4 and 12 billion spermatozoa. Not all of these will have normal shape and motion.


The duct which carries the sperm from the testicle to the urethra and from there to the exterior is called the vas deferens.[2] It is a continuation of the epididymis and links the testes with the urethra. It is  accompanied by blood vessels, nerves and muscle fibres, in a structure  known as the spermatic cord.[3]


Inside the body, the vas deferens joins the urethra at the head of the bladder. The urethra is the tube that carries urine from the bladder to the exterior, via the penis. In the stallion, spermatozoa also pass through a series of tubes through the urethra to the exterior. Along the way, accessory sex glands discharge their secretions into the ejaculate.[4] It is the combination of the spermatozoa from the testes and the secretions from the accessory gland that make up the semen. It is the watery secretions from these accessory sex glands that contribute most of the volume in the stallion’s ejaculate. 


The Penis


The male organ of copulation is the penis. The penis is attached at its root to the pelvis, and runs forward, horizontal to the ground, between the hind legs in a special sheath called the prepuce or sheath. The prepuce covers most of the penis when it is not erect so that the penis is not normally visible. In an average horse, the relaxed penis is about 46 cms (1½ ft). long. In erection, it doubles[5] in length and thickness and projects forward from the prepuce. The tip of the penis is also expanded during erection, resembling the rose of a watering can, and is called the glans penis. The increase in size of the penis during erection is due to an increase in blood supply. The tissues of the penis are spongy and expand and become firmer when the blood supply is increased. The canal that carries urine and semen, the urethra, runs within the penis and opens at the tip in the centre of the glans. Occasionally, this area can harbour infections in the stallion.

            The penis and prepuce of the stallion are routinely swabbed as part of a good venereal disease management programme. The timing and frequency of these swabs is dictated to Thoroughbred breeders in the Horse Racing Levy Board’s Code of Practice.


[1] The testicles usually descend at birth.

[2] In humans, cutting the vas deferens, called vasectomy, is commonly used as a method of birth control since it prevents the spermatozoa leaving the testes.

[3] The spermatic cord is cut at castration.

[4] The accessory sex glands are called the ampulla, the bulbourethral, the vesicular and prostate glands and discharge their secretions directly into the urethra.