Uterus - Anatomy & Physiology

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Introduction

The Uterus is the organ of pregnancy as this is where implantation and development of the feotus occurs. The Uterus is the reproductive organ with the most species variations. These variations occur in both the anatomical types of uterus as well as the uterine horn appearance and endometrial linings.

Anatomical Types of Uteri

Duplex Uteri

• Completely separate uterine horns each with their own cervical canal. Duplex uteri are seen in rabbits and marsupials.
• As well as having 2 uterine horns and cervical canals Marsupials also have two vaginas.

Bicornuate Uteri

• (Previously Bipartite Uteri) Bicornuate uteri have a relatively small uterine body and two uterine horns of varying development.
• A releatively large uterine body and short, poorly developed uterine horns, as seen in the Mare, are due to a high degree of paramesonephric duct fusion.
• Moderately developed uterine horns as in the Cow, Sheep and Goat arise due to an intermediate degree of fusion.
• Long uterine horns and a small uterine body as seen in the sow, bich and queen arise due to a low degree of fusion of the paramesonephric ducts.
• This varying degree of paramesonephric duct fusion reflects the species differences in litter sizes, Monotocous species having short horns and polytocous animals having much longer uterine horns.

Simplex Uteri

• Complete fusion of the paramesonephric ducts forming a single uterine body with no uterine horns. This is seen in primates and especially humans.

Uterine Horn Appearance

Species	Mare	Cow	Ewe	Sow	Bitch	Queen
Uterine Horn Appearance	Straight and divergent	Round	Round	Intestinal loop appearance	Straight and divergent	Straight

Structure

Perimetrium

• Sorosal connective tissue layer continuous with the Broad Ligament.

Myometrium

• Muscularis layer made up from thick inner circular smooth muscle layer and thin outer longitudinal muscle layer.
• The circular and longitudinal layers are seperated by the Stratum Vasculare, a connective tissue layer containing the blood vessels and nerves of the uterine wall.

Endometrium

• Mucosa + Submucosa layer containing endometrial glands.
• The surface of the endometrium differs between species:
  • Ruminants - Numerous round button like elavations of the endometrium called Caruncles occur. These are the sites of attachment and maternal tissue contribution to the placenta. Maternal Caruncle + feotal cotyledon = A placentome.
  • Sow and Mare - uterine folds are present.
• Not shed in domestic species, only shed in primates.

Function

• Provides a suitable environment for embryo development and attachment. The secretions produced by the endometrial glands are important for maintaining the preimplantion embryo.
• In response to increasing amounts of oxytocin production by the corpus luteum during the luteal phase the endometrium produces luteolytic PGF2a to cause degeneration of the corpus luteum if the female is not pregnant.
• The uterus contributes varying amounts of maternal tissues towards the placenta.
• The myometrium is involved with sperm transport through the uterus towards the oviduct.
• Contractions of the myometrium during parturition is important for feotus and placenta expulsion.

Anatomical Location & Boundaries

• The Uterus body and Uterine horns are located within the abdominal cavity dorsal to the Intestinal mass.
• The uterus is attached to the dorsal body wall via the Mesometrium Broad Ligament.

Histology

The appearance of the uterus varies with the stages of the oestrus cycle.

Follicular phase

• Increasing numbers of uterine glands developing and elongating within the endometrium due to the influence of Oestrogens produced by the developing follicles.
• General increase in the thickness of the endometrium.
• The epithelium lining the glands is Simple Columna epithelium.

Luteal phase

• The endometrium is at its maximum thickness with a large number of highly developed glands.
• This is the secretory phase for the endometrium.

Anoestrus

• The endothelium is relatively thin with little endometrial proliferation or gland development.
• The glands are lined by Simple Cuboidal epithelium.

Innervation

The uterus is innervated by both Sypathetic and Parasympathetic fibres which play a part in the regulation of uterine activity. This is highlighted by Iatrogenic manipulation of parturition using β-Adrenoreceptor Agonists for delaying parturition and Antagonists for inducing parturition. However, uterine activity and normal parturition can be acheived when these nerves are severed.

Vasculature

• Uterine branch of the Ovarian artery supplies the cranial parts of the Uterine horns.
• Uterine artery supplies the rest of the uterine horns and the uterine body. This is a branch off the Internal Iliac artery in most domestic species, except the Mare where insted it is a branch off the External Iliac artery. The Uterine artery and the Ovarian artery anastomose within the Broad ligament.