Uterus

Embryology

The uterus develops from the paranemesophrenic (Müllerian) ducts, which are the primordial fetal structures of the female reprodutive tract. More specifically, the uterovaginal primordium is formed by the fusion of the caudal, vertical parts of the paranemesophrenic ducts. This primordium will give rise to the lower part of the uterus that will slowly enlarge during development, ultimately resulting in the fundus and body of the uterus in the adult female. One can begin to see clear signs of the uterine body at a gestational age of 9 weeks. The endometrium and the muscular wall of the uterus develop from the mesenchyne of the genital cord. 

Anatomy of the uterus

Histology of the uterus

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.

Do you want to find out how much you've learned about the histology of the uterus? Take a sneak peek below!

Function

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. A total blood loss during this phase is about 80ml; more than that is considered abnomal (menorrhagia). 
• 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.