The Uterus – Anatomy, Nerve Supply, Functions

The uterus is a hollow muscular organ located in the female pelvis between the bladder and rectum. The ovaries produce the eggs that travel through the fallopian tubes. Once the egg has left the ovary it can be fertilized and implant itself in the lining of the uterus. The main function of the uterus is to nourish the developing fetus prior to birth.

The uterus is a major female hormone-responsive secondary sex organ of the reproductive system in humans and most other mammals. In the human, the lower end of the uterus, the cervix, opens into the vagina, while the upper end, the fundus, is connected to the fallopian tubes. It is within the uterus that the fetus develops during gestation. In the human embryo, the uterus develops from the paramesonephric ducts which fuse into the single organ known as a simplex uterus. The uterus has different forms in many other animals and in some it exists as two separate uteri known as a duplex uterus.

Anatomy of The Uterus

The uterus is a thick-walled muscular structure that lies in the midline of the abdominal pelvic cavity. It contains three layers: the endometrium (innermost layer), myometrium, and the perimetrium (outermost layer). The endometrium’s thickness and structure vary based on hormonal stimulation

The uterus has four parts: the fundus, corpus, isthmus, and cervix. The corpus is the largest segment and connects to the cervix via the isthmus. The cervix connects the uterine body to the vaginal lumen. The uterus sits posterior to the bladder and anterior to the rectum.[rx][rx]

Uterus Support Structure

The round ligament? connects the uterus to the abdominal wall and includes the artery of Sampson. The broad ligament connects the lateral portion of the uterus with the fallopian tube and ovary. The uterine artery, cardinal arteries, and ureter travel within the broad ligament. The ovarian ligament connects the ovary to the lateral surface of the uterus. The infundibulopelvic (IP) ligament connects the ovary to the abdominal wall. Within the IP ligament are the ovarian artery and vein.

Uterine Vasculature

The uterine artery is the main blood supply to the uterus, with some collateral supply from the ovarian artery.

Uterine Innervation

The uterus is innervated sympathetically and parasympathetically through the hypogastric nerve and pelvic splanchnic nerves, respectively.[rx][rx][rx]

Ligament? of Uterus

The uterus is a pear-shaped muscular organ of the female reproductive system. The uterus is divided into the fundus, lower uterine segment, and cervix uteri. The uterus hosts and nourishes the embryo and fetus until delivery.

• The fallopian tubes are 2 tubular structures attached to the uterus on each side. The fimbriated ends of the fallopian tubes are freely floating next to ovaries, on each side in the pelvis.
• The round ligament? of the uterus originates at the uterine cornu of the uterus and blend with the tissue of the mons pubis and labia majora.
• The cardinal ligament? is the inferior demarcation at the base of the broad ligament. The 2 cardinal ligaments support for the uterus and the cervix uteri.
• The broad ligament? is the peritoneal fold around the round ligament, parametrial connective tissue, arteries, veins, lymphatics, and nerves.
• The uterosacral ligaments are anteriorly attached to the cervix uteri. The uterosacral ligaments are posteriorly attached to sacral vertebrae.
• The ovaries rest in ovarian fossa in the lateral part of the pelvic cavity next to the iliac vessels. The ovaries contain germ cells.
• The ovarian ligaments are attached to the posterolateral aspect of the uterus.
• The infundibulopelvic ligaments are peritoneal reflections of the broad ligaments.
• The bladder is located anterior to the uterus.  The ureters are inserted into the trigon.
• The rectum is located between the vagina and the sacrum.
• The pelvic diaphragm supports all the viscera.
• The round ligament? helps maintain the anteversion position of the uterus during pregnancy. The cardinal ligaments support the uterus.

Blood Supply of The Uterus

The round ligament? receives blood from Sampson’s artery or the artery of the round ligament. The common iliac artery divides into the external and internal iliac arteries. The Internal iliac artery branches into the uterine artery, vaginal artery, superior, vesical artery, obturator artery, inferior gluteal artery, internal pudendal artery and obliterated umbilical artery. Simpson’s artery, a branch of the uterine artery, runs along the length of the round ligament. Ovarian arteries originate directly from the aorta. The left ovarian vein drains into the left renal? vein. The right ovarian vein drains directly into the inferior vena cava. The uterine vein drains to the pelvic nodes and para-aortic. The cervical? vein drains to the parametrial nodes, obturator nodes, pelvic nodes and para-aortic nodes. The ovarian vein drains to the pelvic nodes and para-aortic nodes.

Nerve Supply

• Afferent nerves supplying the uterus are T11 and T12. The sympathetic supply is from the hypogastric plexus and the ovarian plexus. Parasympathetic supply is from the S2, S3 and S4 nerves.

Functions of The Uterus

The uterus carries out many functions

• Implantation site of the blastocyst
• Provides protection and support for the fetus to grow
• Site of menstruation

The primary function of the uterus is reproductive. The principal elements of uterine physiology are the endometrium and myometrium. The uterus accepts the ovum after fertilization, holds and provides nutrients and oxygen for the fetus and during birth, and it contracts to cause delivery. The uterus is a hormone-sensitive organ: differentiation, proliferation, exfoliation of the endometrium, and contraction during childbirth gets regulated by the interaction between itself and the hypothalamus, pituitary gland, and ovaries.[rx]

The reproductive function of the uterus is to accept a fertilized ovum which passes through the utero-tubal junction from the fallopian tube. The fertilized ovum divides to become a blastocyst, which implants into the endometrium and derives nourishment from blood vessels which develop exclusively for this purpose. The fertilized ovum becomes an embryo, attaches to a wall of the uterus, creates a placenta, and develops into a fetus (gestates) until childbirth. Due to anatomical barriers such as the pelvis, the uterus is pushed partially into the abdomen due to its expansion during pregnancy. Even during pregnancy, the mass of a human uterus amounts to only about a kilogram (2.2 pounds).

The uterus also plays a role in sexual response, by directing blood flow to the pelvis and ovaries, and to the external genitals, including the vagina, labia, and clitoris.

• During the menstrual phase (day 1 to 4) the spiral arteries in the endometrium contract leading to 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 (days 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 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 (decidual 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.

Mechanism

Reproductive Cycle 

In the female reproductive cycle, there are two concurrent cycles, the ovarian cycle, and the uterine (menstrual) cycle. The ovarian cycle consists of a series of events that occur during and following oocyte maturation. The uterine cycle consists of a series of changes within the endometrium in preparation for the arrival of a fertilized ovum that will develop within the endometrium until birth.

The reproductive cycle can subdivide into the menstrual phase, preovulatory phase, ovulation, and postovulatory phase. The function of the uterus in each of these phases follows:

In the menstrual phase, a decline in estrogen and progesterone levels stimulates the release of prostaglandins, which results in vasoconstriction of arterioles within the uterus. The vasoconstriction eventually leads to hypoperfusion of these cells, which results in cell death. This process initiates the sloughing off of blood, fluid, and epithelial cells from the endometrial walls into the cervix and out through the vagina.

In the preovulatory phase, estrogen is released into the blood, which repairs the endometrium. The endometrium undergoes other changes and doubles in thickness.

During ovulation, the follicle ruptures and releases an oocyte which enters the uterine tube.

In the postovulatory phase, progesterone and estrogen stimulate further growth of endometrial glands and thickening of the endometrium in preparation for the implantation of a fertilized ovum. If fertilization does not occur, progesterone end estrogen levels decline, and the menstruation stage occurs.

If the egg becomes fertilized, the zygote is propelled down the fallopian tubes into the uterus. The zygote cells divide rapidly during this descent. The cells of the zygote continue to divide until it becomes a blastocyst. This blastocyst implants by invading the wall of the endometrium.[rx]

Pregnancy

When the blastocyst successfully implants into the endometrial lining, it develops over many weeks into an embryo and then a fetus. During the development of an embryo into a fetus, changes occur within the endometrium that leads to placental formation. The placenta provides a crossing point between the developing fetus and maternal circulation. In pregnancy, the uterus hypertrophies to accommodate for the growing fetus. It typically reaches the height of the umbilicus at 20 weeks and the xiphoid process at 38 weeks.

Labor

The uterus undergoes many changes during labor due to the release of various hormones. Nearing the seventh month of pregnancy, progesterone levels begin to decline while estrogen levels steadily rise. The increasing ratio of estrogen to progesterone causes the myometrium to become more sensitive to stimuli that promote contractions. Additionally, fetal cortisol rises in the eighth month of pregnancy, which further reduces progesterone’s effects.[rx][rx]

As true labor approaches, oxytocin and prostaglandins both play a role in further stimulating myometrial contractions and increasing contractile strength. The fetus causes the myometrium and cervix to stretch, which also stimulates uterine contractions.[rx]

During true labor, the release of oxytocin and prostaglandins stimulate a positive feedback loop which continues to increase uterine contractile strength. These uterine contractions dilate and efface the cervix. Again a positive feedback loop begins where the effacement and dilation of the cervix further promote uterine contractions. Contractions become more frequent and longer in duration as labor progresses. The next stage of labor begins when the fetal head enters the birth canal and completes when the infant is born. The myometrium continues to contract after birth, causing the placenta to shear from the back of the uterine wall for delivery through the birth canal.[rx]