Reproductive Anatomy of the Mare

I. REPRODUCTIVE ANATOMY OF THE BROOD MARE

NEURO ENDOCRINE ORGANS
1. Pineal gland- small ovoid structure located between the cerebral hemispheres slightly dorsal to the pituitary. Neural pathway from eye to pineal gland controls seasonal reproduction in the mare. Perceives and interprets daylength. Shuts down pulsatile release of GnRH from hypothalamus during short days. Note that the pathway exits the brain, then returns to it.
  1. Eye
  2. Retinohypothalamic tract
  3. Hypothalamus (Suprachiasmatic nucleus)
  4. Median Forebrain Bundle
  5. Spinal cord
  6. Superior cervical ganglia
  7. Pineal gland- releases melatonin in the absence of light, preventing release of GnRH .
2. Hypothalamus- median eminence releases Gonadotropin Releasing Hormone (GnRH) and prolactin inhibiting factor (PIF) which are carried to the anterior pituitary via the hypothalamic-hypohyseal portal system. A neural stalk from the hypothalamus forms the posterior pituitary.
3. Pituitary- formed from a cup like dorsal out pouching of oral mucosa (anterior pituitary) cupped around a neural stalk (posterior pituitary). Anterior pituitary releases FSH and LH in response to GnRH, and releases prolactin in the absence of prolactin inhibiting factor (PIF); the posterior pituitary releases oxytocin in response to neural signals from the hypothalamus.
REPRODUCTIVE TRACT
1. Ovaries- are unique in the mare due to their "inside out" arrangement. Blood supply (ovarian artery) enters over the outside surface (medulla), and ovulation occurs from the interior (cortex) into a recess within the ovary (ovulation fossa). Follicles are surrounded by an outer layer of thecal cells and inner layer of granulosa cells. Thecal cells secrete androgens which are converted to estrogen by the granulosa cells during estrus. Following ovulation, the corpus luteum develops from granulosa and thecal cells and is the source of progesterone during diestrus and early pregnancy.
2. Oviduct- sometimes called the "uterine tube" it starts as a wide, flaccid funnel (infundibulum) that covers the ovulation fossa. It quickly narrows to form a convoluted sack-like tube (ampulla) traveling over the lateral surface of the ovary. This then narrows further to form a long, highly muscular tube which enters the uterus at the uterotubal junction (UTJ). The mare is unusual in that unfertilized ova are retained within the oviduct, apparently unable to pass through the UTJ. The oviductal epithelium is ciliated.
3. Uterus- A "T" shaped organ in the mare, consisting of a body and two horns.
  1. Blood supply- Middle uterine artery branches from the external iliac artery in the horse. It is a homologue of the cremaster artery in the male. Large branches enter the uterus over its dorsal surface. There is also a smaller caudal uterine artery (from the urogenital artery, itself from the internal pudendal artery) and cranial anastomotic branches with the ovarian artery. Veins draining the equine uterus (ovarian and uterine veins) do not share a counter current mechanism with the ovarian artery, as they do in the sheep and cow.
  2. Myometrium- outer longitudinal layer and an inner circular layer of smooth muscle. Myometrial contractions are important in uterine defense in the mare. Contracts in response to oxytocin and PGF2a.
  3. Endometrium- longitudinal folded in the mare. The endometrial surface secretes mucus and is ciliated. The oviductal papilla is a small muscular projection marking the entrance of the oviduct into the uterus. The endometrium is the site of PGF2a release during the estrous cycle.
4. Cervix- Thick walled but can be dilated manually. Folded longitudinally, as extension of endometrial folds. No transverse rings as in the cow. Epithelium is ciliated and secretes mucus. Forms an important barrier to ascending infection of the uterus.
5. Vagina- Cranial portion formed by fusion of the paramesonephric ducts cranial to the transverse fold. Caudal to the transverse fold formed from invagination of ectoderm. The urethra opens just caudal to the transverse fold.
6. Vagino-vestibular sphincter- formed by skeletal muscle in vestibular wall (constrictor vestibuli). Also an important barrier against ascending infection. May be damaged at foaling. Fails in mares with pneumovagina.
7. Vestibule- Immediately cranial to vulval opening. Four rows of papillae on the floor of the vestibule mark the openings of vestibular glands.
8. Vulva- external opening of the reproductive tract. Outermost mechanical barrier to contamination. Wall contains a constrictor vulvae muscle.
9. Clitoris- lies within a pouch at the ventral vaginal commissure. Homologous to the penis. The equine clitoris has a single median sinus about 1 cm deep and two lateral sinuses a few mm deep. The median sinus may serve as a reservoir for pathogenic bacteria, especially Taylorella equigenitalis.
LIGAMENTOUS ATTACHMENTS
1. Broad ligament- a ligamentous sheet extending from about L3 to S4 dorsally down to the surface of the ovaries and tubular genitalia. Portion of the broad ligament called the mesovarium attaches to ovaries; the portion of the broad ligament called the mesosalpinx attaches to the oviduct; and the portion of the broad ligament called the mesometrium attaches to the uterus. Within the broad ligament run the arteries and veins supplying the reproductive tract. The broad ligament contains smooth muscle continuous with the myometrium.
2. Proper ligament of the ovary- runs between the tip of the uterine horn to the caudal pole of the ovary. Helpful in locating the ovary during rectal palpation.
3. Ovarian bursa- a membranous pouch into which the ovary can fit, formed laterally by the mesosalpinx and medially by a fold of broad ligament attached to the proper ligament of the ovary.
4. Round ligament of the uterus- lies between the lateral side of the broad ligament and the lateral body wall, extending to the inguinal ring. Homologous with the gubernaculum (ligament of the tail of the epididymis) in the male.