Female Reproductive System
Uterus and Cervix
The uterus is a pear-shaped organ. Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections:
- Fundus – the portion of the uterus superior to the opening of the uterine tubes.
- Body – the middle section of the uterus (aka corpus).
- Cervix – the narrow inferior portion of the uterus that projects into the vagina
The body of the uterus narrows to a waist termed the isthmus, continuing into the cervix which is embraced about its middle by the vagina; this attachment delimits a supravaginal and vaginal part of the cervix.
The isthmus is 1.5 mm wide. The anatomical internal os marks its junc- tion with the uterine body but its mucosa is histologically similar to the endometrium. The isthmus is that part of the uterus which becomes the lower segment in pregnancy. At its upper end, the cervix widens to form the lower uterine segment – the isthmus; the lower uterine segment in turn widens into the uterine fundus.
The position of the uterus and cervix is variable. The position of the cervix decides the ‘version’ and the position of the uterus decides the ‘flexion’ of the uterus. The most common variations are:
- An anteverted, anteflexed uterus.
- A retroverted, retroflexed uterus.
- An anteverted, retroflexed uterus.
Uterine and cervical appearances change with the phase of menstruation. The myometrial and cervical stromal signal increases in the second half of the cycle. When performing a hysterectomy, good knowledge of the blood supply and adjacent anatomy is critical.
Vascular and Lympathic Supply
The uterine artery (from the internal iliac) runs in the base of the broad ligament and crosses above and at right angles to the ureter to reach the uterus at the level of the internal os. The artery then ascends in a tortuous manner alongside the uterus, supplying the corpus, and then anastomoses with the ovarian artery.
The uterine artery also gives off a descending branch to the cervix and branches to the upper vagina.
The veins accompany the arteries and drain into the internal iliac veins, but they also communicate via the pelvic plexus with the veins of the vagina and bladder.
Lymphatic drainage of the uterus is via the iliac, sacral, aortic and inguinal lymph nodes. Specifically:
- The fundus (together with the ovary and Fallopian tube) drains along the ovarian vessels to the aortic nodes, apart from some lymphatics which pass along the round ligament to the inguinal nodes.
- The body drains via the broad ligament to nodes lying alongside the external iliac vessels.
- The cervix drains in three directions—laterally, in the broad ligament, to the external iliac nodes; posterolaterally along the uterine vessels to the internal iliac nodes; and posteriorly along the recto-uterine folds to the sacral nodes.
Always examine the inguinal nodes in a suspected carcinoma of the corpus uteri—they may be involved by lymphatic spread along the round ligament.
The wall of the uterus is made up of three layers:
- The most superficial layer is the serous membrane, or perimetrium, which consists of epithelial tissue that covers the exterior portion of the uterus.
- The middle layer, or myometrium, is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, and the muscle fibers run horizontally, vertically, and diagonally, allowing the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman’s period. Anteriorly directed myometrial contractions also occur near the time of ovulation, and are thought to possibly facilitate the transport of sperm through the female reproductive tract.
- The innermost layer of the uterus is called the endometrium. The endometrium contains a connective tissue lining, the lamina propria, which is covered by epithelial tissue that lines the lumen. Structurally, the endometrium consists of two layers: the stratum basalis and the stratum functionalis (the basal and functional layers). The stratum basalis layer is part of the lamina propria and is adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogen and progesterone. In the luteal phase of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized egg, and—should fertilization not occur—it is only the stratum functionalis layer of the endometrium that sheds during menstruation.
Connective Tissue of the Female Reproductive System
Several ligaments maintain the position of the uterus within the abdominopelvic cavity.
The broad ligament is a fold of peritoneum that serves as a primary support for the uterus, extending laterally from both sides of the uterus and attaching it to the pelvic wall.
The round ligament attaches to the uterus near the uterine tubes, and extends to the labia majora. Finally, the uterosacral ligament stabilizes the uterus posteriorly by its connection from the cervix to the pelvic wall.