Uterus

Structure/Morphology

The uterus is a muscular (smooth) organ with three main layers. It consists of an inner endometrial layer, a middle myometrial layer, and an outer serosal layer known as the perimetrium. The thickness of the endometrial layer fluctuates during the menstrual cycle as it prepares for potential implantation of an ovum.

Anatomical Relations

The uterus sits between the bladder anteriorly and the rectum posteriorly. The position of the uterus varies according to the levels of distension of the bladder and the rectum. The upper two thirds of the uterus is the muscular uterine body. The superior-most portion of the uterine body is called the fundus. The junction between the fundus and the uterine body is marked by the point where the uterine tubes open into the uterus.

The lower one third of the uterus consists of the fibrous cervix. In non-pregnant females, the cervix tilts anteriorly to the axis of the vagina (anteversion) and the uterine body tilts anteriorly relative to the position of the cervix (anteflexion). These anatomical relations give it its characteristic anteverted and anteflexed position. However, in up to 15% of females, the uterus is retroverted (Standring, 2016).

The uterus is enveloped and supported by the peritoneum. The peritoneum covers the fundus and extends inferiorly between the uterus body and the bladder anteriorly where it forms the vesicouterine pouch. Posteriorly, the peritoneum extends between the rectum and the uterus and forms the rectouterine pouch (pouch of Douglas). It is also enveloped in a portion of the broad ligament called the mesometrium.

Additional ligaments that support the uterus and the cervix in the pelvic cavity consist of the round ligament of the uterus, uterosacral, transverse cervical, and pubocervical ligaments.

Function

During the menstrual cycle, the endometrial lining of the uterus prepares for potential implantation of an ovum (approximately day 21 of a typically 28-day cycle). After ovulation, there are high levels of estrogen and progesterone, which aid in endometrial stromal proliferation and differentiation of cells.

If fertilization occurs, the early embryo travels to the uterus and implants within the endometrial lining. It is here where the placenta forms so that embryonic and fetal development can continue.

If fertilization does not occur, the endometrial lining will be maintained until the corpus luteum breaks down. This initiates menstruation, where the majority of the endometrial lining is shed and expelled from the uterus and, thus a new menstrual cycle begins.

Arterial Supply

The uterine artery arises from the anterior division of the internal iliac artery. It travels within the broad ligament and passes over the ureter. It reaches the lateral aspect of the uterus at the junction between the uterus and the cervix. Here, it bifurcates into ascending and descending branches. The ascending branch courses towards the uterine tubes as it travels within the broad ligament. The descending branch descends along the lateral aspect of the cervix and anastomoses with the vaginal artery.

Venous Drainage

Uterine veins travel within the broad ligament and drain into the internal iliac veins.

Innervation

The inferior hypogastric plexus is the primary source of innervation of the uterus. Autonomic (pelvic and sacral splanchnic nerves) and efferent fibers supplying the uterus pass through the inferior hypogastric plexus prior to innervating the uterus. Branches that travel with the uterine arteries, within the broad ligament, supply the uterine body and tubes. They connect with the ovarian plexus along the uterine tubes. The uterine nerves end within the myometrium and endometrium and accompany the vasculature.

Sympathetic fibers from T12—L1 induces uterine contraction and vasoconstriction while parasympathetic fibers S2—S4 facilitates vasodilation and uterine relaxation. The ability of the nerves to influence uterine function is affected by the influence of the endocrine system (Standring, 2016).

Lymphatic Drainage

The parauterine lymph nodes are situated around the uterus, beneath the peritoneum in the broad ligament. They drain lymph from the body of the uterus and cervix, sending it to the external and internal iliac nodes. Some lymph is also returned to the external iliac lymph nodes via the obturator lymph nodes. Some vessels may also drain into the sacral and superficial inguinal lymph nodes (Földi et al., 2012).

List of Clinical Correlates

—Uterine cancer

—Uterine prolapse

—Bicornate uterus

—Hysterectomy

References

Földi, M., Földi, E., Strößenreuther, R. and Kubik, S. (2012) Földi's Textbook of Lymphology: for Physicians and Lymphedema Therapists. Elsevier Health Sciences.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.