Female Reproductive Tract - Horse Anatomy

Ovaries

The ovary is the female Gonad homologous to the male Testes.The structures found within the ovary are undergoing constant changes throughout the oestrus cycle from the Follicles containing Oocytes, to the formation of Corpus Haemorrhagicum, Corpus Luteum, and finally Corpus Albicans. The ovaries of the mare are two kidney-shaped structures situated ventral to the 4th and 5th lumbar vertebrae, supported by the mesovarium. During the non-breeding season they measure 2-4cm in length and 2-3cm in width. During the sexually active stage, they are 6-8cm in length and 3-4cm in width.

The Ovaries of a Mare showing the Ovulation Fossa- Courtesy of A.Crook, Copyright RVC 2008

In the mare, the structure of the ovary is reversed compared to other species. The cortex is in the centre and is enclosed by a dense, richly vascularised connective tissue layer, which is analagous to the medulla of other domestic mammals.

Tunica Albiguinea

The connective tissue layer covering the ovarian cortex. The whole ovary is contained within the tunica albiguinea except the ovulation fossa. Overlying this structure is a single layered germinal epithelium.

Cortex

Contains Follicles in various stages of development as well as the Corpus Luteum (includes Corpus Haemorrhagicum and Corpus Albicans). The cortex reaches the surface of the ovary at the ovulation fossa, a deep indentation at the free margin. This is where mature follicles rupture in ovulation, as opposed to at various points on the surface in other domestic mammals. Follicles can be identified by transrectal palpation, but Corpora Lutea cannot. Identification of Corpora Lutea requires ultrasonography.

Medulla

The Medulla is made up of dense connective tissue. This is where all of the lymphatics, nerves and vasculature of the ovary are found.

Histology

Stroma

The body of the ovary (ovarian stroma) consists of:

• Spindle-shaped cells
• Fine collagen fibres
• Ground substance

Stromal cells resemble fibroblasts, but some contain lipid droplets. Bundles of smooth muscle cells are scattered throughout the stroma.

Cortex

Contains:

• Follicles containing oocytes in various stages of development
• Atretic Follicles
• Corpora lutea
• Corpora albicans

The superficial cortex is more fibrotic than the deep, and is called the tunica albuginea. On the surface of the ovary is the germinal epithelium. This is a continuation of the peritoneum.

Medulla

The medulla is highly vascular. It contains hilus cells, which are similar to the Leydig cells of the testes.

Vasculature

Arterial Supply

• The ovarian artery (a branch of the Aorta) and ovarian branches of the uterine artery form anastomoses in the mesovarium and the broad ligament. From this arterial plexus ~10 coiled helicine arteries enter the hilus of the ovary. Smaller branches form a plexus at the corticomedullary junction, giving rise to straight cortical arterioles, which radiate into the cortex. Here they branch and anastomose to form vascular arcades, which give rise to a rich capillary network around follicles.

Venous Drainage

Venous drainage follows the course of the arterial system. Medullary veins are large and tortuous. The ovarian artery is closely associated with the uterine vein. This is important for the transfer of luteolytic PGF2α from the uterus to the ovary.

Lymphatics

Arise in the perifollicular stroma and drain into larger vessels, which coil around the medullary veins.

Innervation

Sympathetic fibres of the autonomic nervous system supply blood vessels and terminate on smooth muscle cells in the stroma around follicles. They may play a role in follicular maturation and ovulation, but the main control is via the endocrine system.

Function

The ovary has two main functions:

• Producing the female gametes oocytes via Gametogenesis.
• Producing the reproductive hormones Oestrogens and Progesterone, an endocrine function.

Processes Taking Place In The Ovary

• Oogenesis
• Folliculogenesis
• Ovulation
• Corpus Luteum Formation
• Corpus Luteum Regression

Additional Learning Resources

Mare ovary and uterus dissection

Bisected ovary from the mare potcast

Oviducts

The Oviduct is the tube that links the ovary to the uterus and which the ovulated oocyte travels down to become fertilised by sperm present in the female tract. It is also refered to as the Fallopian tube, Uterine tube or Ovarian tube. The Oviduct is suspended from the abdominal wall by the mesosalpnix of the broad ligament. It functions to provide regulation check points for unfertilised oocytes. The mucosal glands produce oviduct secretions integral for:

• Supporting the unfertilised oocyte
• Supporting spermatozoa in the oviduct
• Increasing the fertilising capabilities of the spermatozoa
• Development of the early embryo

The oviduct is devided into 3 anatomical regions:

Infundibulum

The cranial ovarian end of the oviduct. It comprises of numerous fimbrae and the opening into the oviduct tube, the ostium.

Ampulla

The longest region of the oviduct, occupying more than half of its total length, and also has the largest diameter. This is the site of fertilisation. It is distinguished by its many mucosal folds. The ampulla is joined to the isthmus via the Ampullary-Isthmus junction. This junction is important in the mare, as it acts as a regulatory checkpoint allowing only fertilised ova to pass any further along the oviduct and into the uterus.

Isthmus

The caudal end of the oviduct joined to the uterus. The Isthmus is thicker walled than the ampulla and smaller in diameter. Its folded mucosa forms a functional reservoir for sperm in the female tract. Sperm in the female tract reach the isthmus of the oviduct and bind to the mucosal epithelial cells, forming a functional reservoir. The sperm are only released from the isthmus mucosa by the action of paracrine signals from an ova travelling down the oviduct. The formation of a functional reservoir is a possible supporting mechanism of block to polyspermy, as only a few sperm are released from the isthmus mucosa at any one time. This results in only a few sperm being in the vicinity of the ova at a time and so in a position of fertilising the ova.

Uterotubal Junction

The oviducts open into the uterine horn through the uterine ostium. This marks the site of the uterotubal junction. This junction is gradual in ruminants and pigs, but abrupt in the horse and carnivores. In the horse, the uterine ostium is located on top of a papilla, which forms a barrier against ascending infections.

Histology

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  Histological Section of the Oviduct to show Fimbriae of the Infundibulum- Courtesy of J.Bredl, Copyright RVC 2008

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  Histological Cross-Section of the Oviduct to show the Ampulla- Courtesy of J.Bredl, Copyright RVC 2008

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  Histological Cross-Section of the Oviduct to show the Isthmus- Courtesy of J.Bredl, Copyright RVC 2008

Infundibulum

Fimbriae are finger-like projections that aid the infundibulum in gliding over the surface of the ovary.

Ampulla

• Ciliated Columna Epithelium
• Thin muscularis layer
• Fern-like mucosal folds

Isthmus

• Simple columnar epithelium
• Thick muscularis layer divided into inner circular layer and outer longditudinal layer
• Few mucosal folds

Vasculature

• Tubal branch of the ovarian artery.