The uterus, also known as the womb, is an about 8 cm long hollow muscular organ in the female pelvis and lies dorsocranially on the bladder. It consists of several anatomical parts, such as the cervix, isthmus, and body. While its anatomy sounds simple, its histology is more complicated. It consists of three major layers, but the exact histological structure depends upon the state - if it is in the or proliferative phase.
Also, as an organ of the female reproductive system it is responsible for the development of the embryo and fetus during pregnancy. This article will describe the anatomy, histology, and functions of the uterus, as well as shed some light on the basics of the menstrual cycle.
|Parts||Cervix, isthmus, corpus, fundus (base)|
|Position & orientation||Anteflexion and anteversion|
|Peritoneal recesses||Vesicouterine and rectouterine pouches|
|Blood supply||Uterine arteries|
|Venous drainage||Uterine venous plexus that flows into the internal iliac vein|
|Innervation||Inferior hypogastric plexus|
|Lymphatics||iliac (external and internal), obturator, and para-aortic lymph nodes|
Phases: proliferative and secretory
Layers: endometrium (functional and basal layers), myometrium (subvascular, vascular, supravascular layers), perimetrium
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
Parts & Position
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 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).
It is supplied by branches of the uterine arteries (branch of the internal iliac artery). One branch travels within the broad ligament of the uterus until the region close to the ovarian hilum, where it forms an anastomosis with the uterine branches of the ovarian artery (branch of the abdominal aorta). The second branch supplies the cervix and anastomoses with several branches of the vaginal artery. The uterine artery also gives several perforating branches within the uterine wall that form two surrounding systems around the uterus called the posterior and anterior arcuate arteries. The venous blood drains through the uterine venous plexus into the internal iliac vein.
The nerves derive from the inferior hypogastric plexus. Sympathetic innervation originates from the T12 and L1 spinal segments, while the parasympathetic nervous supply is provided by the S2 to S4 spinal segments. 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.
Lymphatic vessels drain lymph from the body and cervix of the uterus to the iliac lymph nodes (external and internal), as well as the obturator lymph nodes. In turn, the fundus is drained to the para-aortic lymph nodes.
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.
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.
- lies dorsally onto the bladder
- divided into cervix, isthmus and corpus
- forward bent angle between cervix and isthmus = anteflexion
- axis of uterus towards vagina = anteversion
- microscopically: endometrium (muscous membrane), myometrium (muscle layer), perimetrium (peritoneum)
- function: menstrual cycle, pregnancy, childbirth