The Mare’s Reproductive Anatomy

Mare reproductive anatomy includes four areas that play an essential part in pregnancy and foal delivery: the vulva, vagina, cervix and uterus. Each plays its own unique part.

The uterus is a bicornuate organ composed of a body and two divergent horns. During an estrous cycle, its ovary produces eggs as well as female sexual hormones such as estrogen and progesterone that stimulate its production by the uterus.


A horse’s vulva serves as the initial barrier between its uterus and the external environment, providing physical seal and producing small amounts of mucus for lubrication purposes. Sperm enters during fertilisation (occurring around ovulation) only, helping protect against abortion caused by infection.

A mare’s vulva resembles a cup with lips that meet to form a seal, so that air and environmental contaminants such as faeces do not infiltrate into her uterus and lead to bacteria or fungal infection which could potentially compromise both fertility and pregnancy in her. When properly sealed, airborne contaminants cannot aspirate into the uterus, leading to health risks that affect fertility as well as pregnancy in mares.

The uterus is a large, deep organ with two short horns that terminate in an oviduct. During natural breeding it serves as the main site for deposition of semen and most embryonic development occurs there. Furthermore, hormone production occurs here as well as serving as the repository for fertilised eggs to be placed upon fertilisation.


The vagina is a distensible fibromuscular canal connecting the outer vulva to the cervix of the uterus, located between its opening at anus and urine’s exit through urethra (urethra being where urine exits the body). The external opening of this femoral canal is guarded by labia minora; its walls covered by non-keratinized stratified squamous epithelium; between which is lamina propria which contains smooth muscle bundles of circular and longitudinal fibers with blood vessels and lymphatic ducts as well as thick envelope of mucosa folds called rugae.

The cervix is a narrow neck-like structure that separates the vagina from the uterus and relaxes when mares go into heat, closing when not. It contains an infundibulum for anchorage to anchor it to an oviduct; and has mucosal envelope producing fluid for lubrication as well as glands producing testosterone for stimulating sexual activity and producing sperm production; it even aids ejaculating.


The cervix serves as an entryway between the vagina and uterus. Its lower part, extending into the vagina, is known as the ectocervix; while its upper section, which opens onto the uterus, is known as endocervix; each part is lined by thin columnar epithelium shorter in height than what lines vulva.

Hormones play an essential role in fertility: Follicle-stimulating hormone, produced by the pituitary gland, starts ovulation; luteinizing hormone and testosterone boost sperm production; while progesterone promotes accessory sex organ development as well as reproductive fluid production.

The male copulatory organ, or penis, consists of three corpora cavernosa that are covered by tunica albuginea sheaths; and one Corpus Spongiosum with its single Corpora cavernosa covered by tunica albuginea sheath; as well as one Corpus Spongiosum with its single Corpus Spongiosum that houses the urethra in an angioelastic sheath known as glans penis and which encases it. When not erected, its muscleovascular nature allows it to lay flaccid; to achieve an erection, its sheath needs to be filled with blood; in contrast ruminants and boar require more blood in order for successful erection; the genital sac, commonly known as scrotum, houses the testes and epididymides within its walls.


The uterus is the organ in the reproductive tract responsible for implantation and pregnancy, producing mucous secretions to provide lubrication and protect from invading microorganisms. Cervixes do not block transport of sperm; entry only opens up during fertilisation (around ovulation). No barriers exist between transport of sperm to this point and entry at fertilisation (around ovulation). They produce mucous secretion to lubricate this area while helping keep microorganisms out!

In unmated females, the external os of the cervix protrudes into the cranial vagina and may feature a hymen (Figs. 1C and F). Furthermore, long cervices have firm folds of connective tissue without transverse cervical rings that make catheterisation challenging.

The uterus is bicornuate in shape with two regularly cylindrical and curved horns protruding towards its front end, suspended by a broad ligament which divides forward to support both an ovary and tube within it, creating an ovarian bursa.


Ovaries (also referred to as female gonads) produce eggs and female sexual hormones like estrogen and progesterone that prepare the uterus for conception. Ovaries are housed within shallow depressions called “ovarian fossae” on either side of the uterus in the pelvic cavity lateral wall.

Ovaries of domestic mammals possess an unusual tunica albiguinea connective tissue layer with dense, highly vascularised fibers known as tunica albiguinea that houses dense, richly vascularised connective tissues called tunica albiguinea that serves to house mature follicles that rupture during ovulation, rather than at various points across its surface like other species do. This allows mature follicles to rupture before rupture occurs as opposed to having multiple points on which ruptures occur as is seen elsewhere on ovary surfaces of other domestic species.

Follicles are present in an ovary from birth; however, only a minority will reach maturity and release an egg. This process is stimulated by puberty’s release of follicle stimulating hormone. As each follicle matures it gradually expands until eventually bulging out of the ovary in an act called ovulation – at which point it transforms into Corpus Luteum which secretes progesterone hormone into the system.

Ultrasound scanning of the ovaries is used to identify follicles at different stages of development, as shown below in an ovulation cycle using 3D-ISM to quickly detect all follicles with diameters greater than one millimeter.






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