Equine Reproductive Anatomy
43-69

Embryology

• see Pathways to Pregnancy and Parturition

• You should also review the bovine notes and Pathways to Pregnancy and Parturition.
• The pineal gland is between the cerebral hemispheres and is connected by a stalk to the brain.
• The pineal gland secretes melatonin.  Melatonin is secreted in higher amounts in times of increasing darkness, so less melatonin is released during the cycling season than occurs during the times of longer daylight.
• Melatonin appears to inhibit  cyclicity by decreasing the amount of GnRH released.
• Some mares with high melatonin, however, continue to cycle. This may be due to an insensitivity to melatonin or some other factors may be involved.

• The ovaries are kidney bean shaped and their size varies with season.

An ovary cut in two showing the hilus at the bottom.

• The ovaries are suspended by the mesovarium portion of the broad ligament.
• There are three surfaces used to identify structures on the ovary . 
  • The cranial pole is attached to the fimbria, 
  • The caudal pole is attached to the uterus by the proper ligament of the ovary
  • The lateral and medial surfaces. 
• The dorsal (attached surface) is mesenteric and the ventral (free surface) is anti-mesenteric.
• The ovulation fossa is at the 'hilus' of the ovary.
• The entire ovary is covered by peritoneum, which means that some mares will show pain at ovulation because of the stretching of the peritoneum.

Location

• The ovarian location is somewhat variable, but is usually at 10 and 2 O'clock positions by the shaft of ileum. They are always at the end of the uterus, however.

• The ovaries may be behind the broad ligament and may need to be 'flipped out' from behind the ileum in order to thoroughly palpate them.

• The germinal epithelium is on the inside, rather the outside of the ovary (inside out compared to a cow). This prevents external ovulation of the oocyte and ovulation is only through the ovulation fossa at the hilus of the ovary. This internal location of the germinal epithelium prevents the CL protrusion and palpation as in the cow.

• Only follicles on the equine ovary are readily palpable.
• The CL cannot be palpated.

• The oviduct has three main segments, the fimbria (infundibulum), ampulla and isthmus.
• The fimbria catches the ova for transport to the remaining segments of the oviduct. There may be cysts present by the oviducts, which are remnants of the Müllerian (tubo-ovarian cysts) or Wolffian (epoophoron or paroopheron cysts) ducts. These cysts are usually not clinically significant, but occasionally may be confusing during palpation or large enough to physically interfere with ovulation.
• The uterotubal junction (UTJ) acts as a mechanical barrier as is identified as the ovulation papilla in the uterus.

Endoscopic view of the UTJ in a mare.

• The uterus is 'T' shaped and suspended by the mesometrial aspect of the broad ligament. It is usually half abdominal and half pelvic.

• It is dorsal to the bladder and anterior to the pelvic brim. The location may vary due to intestinal displacement, however.
• The broad ligament attaches dorsally to the uterus (the ventral uterus is free) and then it attaches dorsally to the sublumbar region from the 3rd-4th lumbar to the 4th sacral vertebra. The broad ligament may contain large amounts of fat. The long attachment allows exteriorization surgically.
• The endometrial lining consists of hundreds of folds.

Folds in the endometrium.

• The knowledge of endometrial histology is important for assessing fertility. 
• The layers include:
  • The epithelium, 
  • The stratum compactum which lies under the epithelium and goes down to the glands
  • The stratum spongiosum which includes the glands and goes down to the myometrium.

• The cervix in the mare has an internal os and an external os.
• It has no cartilage and the tone changes during cycle.
• There are longitudinal folds, of which the exterior outgrowth is the frenulum. This is seen in the anterior vagina as closed flower bud in diestrus and a 'wilted rose' during estrus.

• The cervix forms the final barrier to uterine contamination.

• The vagina is a potential space that is normally dorsally-ventrally flattened.
• Loss of support by ligaments and poor condition may cause anterior-ventral sloping of the pelvic floor and allow urine to pool by the cervical os.
• The vagina ends at the vaginal vestibular junction.
• Vaginal-vestibular fold
  • The vaginal-vestbular fold is the second barrier to contamination of the uterus.
  • It is formed by the end of Müllerian duct.
  • This is where you may see a persistent hymen.

• The urethra exits posterior to the transverse fold.

• The vestibule extends from the vagina to the labia.
• It is of ectoderm (cloacal) origin.
• This is an area for potential tears at foaling, which are classified as first, second, or third degree tears.Vulvar labia
• The labia present the first line of defense to contamination of the reproductive tract.

• They are composed of skin and mucus membranes.
• The desirable conformation is vertical and in the same plane as the rectum. At least 80 % of the lips should be below the pelvic brim.
• Poor conformation leads to a weak seal and allows contamination of the vestibule and vagina.
• Tears, a sunken rectum, the vulva above the pelvis, and tilted labia all contribute to contamination.
• The causes for poor conformation include tears, age, and weight loss.
• The 'wind sucker' tests for air entry to the vagina when the vulvar lips are pulled apart.

Clitoris/clitoral fossa

Examination of the clitoris and clitoral fossa.

• The clitoris is inside the lower vulva.
• The clitoris is homologous to the penis and can be seen everted as 'winking' during estrual teasing. This is because of the erectile tissue it contains.
• The fossa may harbor contaminants, which include CEM (Contagious Equine Metritis) and other bacteria.
• In the fossa are the clitoral sinuses, which are the preferred site to sample for CEM.

• The bladder is ventral to the uterus and great care should be taken not to confuse it with the uterus during examination.
• The lateral ligament of bladder is posterior to the broad ligament and contains the round ligament of the bladder (which is umbilical in origin).
• The ventral ligament of the bladder is from the urachus.