The uterus (womb) is an about 8 cm long hollow organ in the female pelvis and lies dorsocranially on the bladder. It is surrounded by the circumjacent connective tissue (parametrium). The peritoneum covers the uterus almost completely (except the ventral part of the cervix) forming two recesses: ventrally the vesicouterine pouch and dorsally the rectouterine pouch (pouch of Douglas). The uterus is divided into the cervix, isthmus and corpus.
- The cervix (neck) lies subperitoneally and consists of a part projecting into the vagina (vaginal portion or portio) and a part fixed in the parametrium (supravaginal portion). The cervical canal has two orifices: the internal orifice to the isthmus and the external orifice to the vagina.
- The isthmus is an about 1 cm long narrow passage connecting the cervix and corpus.
- The corpus (body) lies intraperitoneally and has a “triangle” lumen through its connection to the isthmus and both fallopian tubes. The base of the uterus is called fundus.
In most women the corpus is bent forward against the cervix at the isthmus (anteflexion) and the long axis of the uterus is inclined towards the vagina (anteversion). The shape and size however may vary depending on age, number of pregnancies and hormonal status.
The uterus is supplied by the uterine artery (branch of the internal iliac artery) and uterine branches of the ovarian artery (branch of the abdominal aorta). The venous blood drains through the uterine venous plexus into the internal iliac vein. The nerves derive from the inferior hypogastric plexus. All vessels and nerves run through the lateral ligaments (ligamentum latum uteri), a broad duplication of the peritoneum connecting the lateral wall of the uterus with the pelvic wall.
The uterus has three layers: mucosa (endometrium), muscularis (myometrium) and serosa/adventitia (perimetrium).
- The endometrium (uterine mucous membrane) is lined with simple columnar epithelium (lamina epithelialis) and contains numerous tubular glands. It is followed by a cell-rich connective tissue layer (lamina propria). There is a transition to squamous non-keratinized epithelium at the portio (squamocolumnar junction). Physiologically the endometrium is divided into the functional layer (stratum functionale) and basal layer (stratum basale).
- The myometrium (uterine musculature) comprises a complex of three smooth muscle layers which are microscopically difficult to separate: (from the inside to the outside)
- The subvascular layer is rather thin and mainly participates in the sealing of the tubes and the separation of the endometrium during the menstrual cycle.
- The vascular layer is quite strong and well-perfused running around the uterus like a net. It plays a major role during labor.
- The supravascular layer is again a thin sheet of crossing muscle fibers stabilizing the uterine wall.
- The perimetrium equals the peritoneum and is surrounded by a thin connective tissue layer (Tela subserosa). In peritoneal free areas there is no serosa but adventitia.
The uterus plays an important role in human reproduction. It is the organ where the implantation and nourishment of the fertilized ovum takes place. Furthermore it helps pushing out the baby during birth through muscle contraction. The uteruses of women in childbearing age underlie an about 28-day hormone-controlled rhythm (menstrual cycle) which goes through three phases:
- During the menstrual phase (day 1 to 4) the spiral arteries in the endometrium contract leading to an ischemia and sloughing of the functional layer (desquamation). At the same time coagulation factors in the menstrual blood are decomposed in order to avoid blood clotting. This phase is predominately controlled by estrogen causing painful contractions of the uterine musculature.
- During the proliferative phase (day 5 to 14) the cells of the basal layer divide rapidly leading to a fast regeneration of the epithelium, functional layer and spiral arteries.
- The secretory phase (day 15 to 28) begins with the ovulation stopping the proliferation and preparing an implantation of the ovum. It comes to an intracellular accumulation of glycogen and a dilatation of the uterine lumen. The glands have a saw-tooth-like shape and increasingly secrete nutrients. The endometrial stromal cells resemble the decidua of the placenta (predecidual cells). The spiral arteries are triggered to supply a possible placenta. If no fertilization occurs during the secretory phase the spiral arteries contract and the cycle begins again.