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The uterus is a pear-shaped organ located in the pelvis, between the bladder and rectum. It is a vital part of the female reproductive system, playing a critical role in menstruation, fertility, pregnancy, and labour. The uterus is supported by the pelvic floor muscles, which are a group of muscles that form the base of the core muscles. These muscles help stabilise the core and assist with essential bodily functions, such as urination, defecation, and sexual intercourse. Pelvic floor muscles also help protect the spine and organs by absorbing outside pressure. Ligaments in the pelvis, lower back, and hips also help hold the uterus in place.
| Characteristics | Values |
|---|---|
| Location | Between the bladder and rectum in the pelvis |
| Shape | Hollow, pear-shaped |
| Layers | Three: perimetrium (outer), myometrium (middle), endometrium (inner) |
| Myometrium | Thick, smooth muscle layer; expands during pregnancy and contracts to push out the baby |
| Endometrium | Mucous membrane lining; thickens during ovulation and is shed during menstruation |
| Perimetrium | Thin, outer layer composed of epithelial cells |
| Support | Pelvic floor muscles, ligaments, perineal body |
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Pelvic floor muscles
The pelvic floor is composed of several layers of muscles and other tissues. These layers stretch like a hammock from the pubic bone at the front to the tailbone at the back, and from one sitting bone to the other. The pelvic floor muscles are normally firm and thick. Two main muscles intertwine to form the pelvic floor muscles: the levator ani, which wraps around the entirety of the pelvis, and the coccygeus. The levator ani is composed of three separate muscle components: pubococcygeus, puborectalis, and iliococcygeus.
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Myometrium
The myometrium is the highly muscular middle layer of the uterus. It is the major muscular portion of the uterus, formed by an intricate network of smooth muscle fibres dispersed throughout an extracellular matrix of connective tissue. The myometrium is composed of bundles of smooth muscle fibres surrounded by irregular arrays of collagen fibrils and blood vessels.
The myometrium is made up of myometrial smooth muscle cells (myocytes) that form functional contractile bundles. These bundles are connected by an abundance of connective tissue, which is interspersed with arterial and venous vasculature. The myometrium contains a "side population" of stem cells, which are believed to enable the myometrium to grow during pregnancy. The molecular structure of the smooth muscle of the myometrium is similar to that of smooth muscle elsewhere in the body, with myosin and actin being the predominant proteins. However, there is approximately six times more actin than myosin in the myometrium.
During pregnancy, the myometrium stretches and expands in size and number, allowing the uterus to become several times larger than its non-gravid size. The myometrium also contracts in a coordinated manner during labour, helping to push the baby out. After delivery, the myometrium contracts again to expel the placenta, and crisscrossing fibres of the middle layer compress the blood vessels to minimise blood loss.
The myometrium is topographically separated into the fundal, corpus (upper), and isthmal (lower) segments. During active labour, the fundal region contracts while the lower segment relaxes towards the cervix and thins. This process is believed to be an important evolutionary adaptation to bipedal life.
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Perimetrium/Peritoneum
The uterus is a pear-shaped organ located in the pelvis, between the bladder and the rectum. It is responsible for menstruation, fertility and pregnancy. The uterus is surrounded by the circumjacent connective tissue called the parametrium. The perimetrium is the outermost layer of the uterus, forming its external skin. It is also referred to as the peritoneum.
The peritoneum is a double-layered membrane that covers the uterus almost entirely, except for the ventral part of the cervix. It forms two recesses: the vesicouterine pouch and the rectouterine pouch (pouch of Douglas). The peritoneum is continuous with the abdominal peritoneum and is composed of epithelial cells. It is a thin, protective layer that shields the uterus from friction by creating a smooth surface and secreting a lubricating fluid.
Deep to the perimetrium is the myometrium, the middle layer of the uterus. This layer is composed of visceral muscle tissue and is responsible for the uterus's expansion during pregnancy and its contraction during childbirth. The myometrium is highly muscular and can undergo hypertrophy and hyperplasia during pregnancy to prepare for the expulsion of the fetus at birth.
The innermost layer of the uterus is the endometrium, which is also known as the stratum functionalis or the mucous membrane. It is composed of simple columnar epithelial tissue and is supported by a highly vascular connective tissue that provides nourishment to the developing embryo and fetus during pregnancy. The endometrial lining changes throughout the menstrual cycle, thickening and becoming rich in blood as ovulation approaches. If an egg is fertilized, it implants into the endometrium, and pregnancy begins. If not, the endometrium is shed during menstruation.
The pelvic floor muscles provide primary support to the uterus, working alongside ligaments to hold it in place. These muscles stretch from the pubic bone to the tailbone and play a crucial role in stabilizing the core and assisting with essential bodily functions. They can become weakened over time due to various factors, such as injury, menopause, or natural aging, leading to conditions like incontinence or pelvic organ prolapse.
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Endometrium
The endometrium is the innermost lining layer of the uterus, consisting of the functional (superficial) and basal layers. The basal layer contains stem cells that regenerate the functional layer. The functional layer is dynamic, changing in response to the monthly flux of hormones that guide the menstrual cycle. It is also the layer where a fertilised egg (or blastocyst) implants if conception takes place.
In preparation for possible pregnancy, the endometrium thickens and becomes enriched with blood. If a fertilised egg is implanted, the lining stays in place. If not, the functional layer of the endometrium is shed as menstrual bleeding. This process repeats every menstrual cycle unless pregnancy occurs.
The endometrium is made up mostly of mucosal tissue. The stroma is a layer of connective tissue that varies in thickness according to hormonal influences. In the uterus, tubular glands reach from the endometrial surface through to the base of the stroma, which also carries a rich blood supply.
During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus.
Abnormalities of the endometrium can result in concerns such as endometriosis (tissue grows outside the uterus), hyperplasia (irregular thickening), and cancer. Endometrial hyperplasia is caused by excess estrogen stimulation, which can occur during perimenopause when ovulation becomes irregular or after menopause when the menstrual cycle stops altogether. It can also be caused by medications that act like estrogen or high doses of estrogen after menopause.
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Cardinal ligament
The cardinal ligament is a major ligament of the uterus. It is a paired structure on the lateral side of the uterus, providing lateral stability to the cervix and upper vagina. It is formed as a thickening of connective tissue at the base of the broad ligament of the uterus. The ligament extends laterally on both sides from the cervix and vaginal fornix to attach to the lateral wall of the pelvis.
The cardinal ligament is composed of loose areolar connective tissue surrounded by blood vessels, nerves, and lymphatics. It contains a network of collagen fibres and a few isolated elastic fibres with numerous cellular elements, particularly fibroblasts. The female ureter, uterine artery, and inferior hypogastric (nervous) plexus course within the cardinal ligament.
The cardinal ligament is involved in radical hysterectomy procedures for cervical cancer due to the lymphatic drainage of the cervix. It is routinely cut during some uterine operations, although this can have side effects. During a vaginal hysterectomy, the traction and cutting of the cardinal ligaments move the ureter out of the operative field, guarding it against possible iatrogenic injury.
The ligament is also important in providing support and stabilisation for the pelvic organs, including the uterus. It attaches the lateral side of the vagina and cervix to the lateral pelvic wall, providing support to the vagina and cervix.
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Frequently asked questions
The uterus is supported by the pelvic floor muscles and perineal body. The pelvic floor muscles help stabilise the core and assist with essential bodily functions.
The uterus is a pear-shaped organ that plays a critical role in menstruation, fertility and pregnancy. It is hollow and muscular and sits between the rectum and bladder in the pelvis.
The uterus has three main functions: pregnancy, fertility and the menstrual cycle. The uterus can stretch to grow a baby during pregnancy and contract to help push the baby out of the vagina.
