The uterus is a secondary sex organ. Secondary sex organs are components of the reproductive tract that mature during puberty under the influence of sex hormones produced from primary sex organs (the ovaries in females and the testes in males).
Anatomical Structure
The uterus is a thick-walled muscular organ capable of expansion to accommodate a growing fetus. It is connected distally to the vagina, and laterally to the uterine tubes.
The uterus has three parts;
- Fundus – top of the uterus, above the entry point of the uterine tubes.
- Body – usual site for implantation of the blastocyst.
- Cervix – lower part of uterus linking it with the vagina. This part is structurally and functionally different to the rest of the uterus.
The Cervix
The cervix is the lower portion of the uterus, an organ of the female reproductive tract. It connects the vagina with the main body of the uterus, acting as a gateway between them.
The cervix is composed of two regions; the ectocervix and the endocervical canal. The ectocervix is the portion of the cervix that projects into the vagina. It is lined by stratified squamous non-keratinized epithelium. The opening in the ectocervix, the external os, marks the transition from the ectocervix to the endocervical canal.
The endocervical canal (or endocervix) is the more proximal, and ‘inner’ part of the cervix. It is lined by a mucus-secreting simple columnar epithelium. The endocervical canal ends, and the uterine cavity begins, at a narrowing called the internal os.
Functions of the cervix
The cervix performs two main functions:
- It facilitates the passage of sperm into the uterine cavity. This is achieved via dilation of the external and internal os.
- Maintains sterility of the upper female reproductive tract. The cervix, and all structures superior to it, are sterile. This ultimately protects the uterine cavity and the upper genital tract by preventing bacterial invasion. This environment is maintained by the frequent shedding of the endometrium, thick cervical mucus and a narrow external os.
Histological Structure
The fundus and body of the uterus are composed of three tissue layers;
- Peritoneum – a double layered membrane, continuous with the abdominal peritoneum. Also known as the perimetrium.
- Myometrium – thick smooth muscle layer. Cells of this layer undergo hypertrophy and hyperplasia during pregnancy in preparation to expel the fetus at birth.
- Endometrium – inner mucous membrane lining the uterus. It can be further subdivided into 2 parts:
- Deep stratum basalis: Changes little throughout the menstrual cycle and is not shed at menstruation.
- Superficial stratum functionalis: Proliferates in response to oestrogens, and becomes secretory in response to progesterone. It is shed during menstruation and regenerates from cells in the stratum basalis layer.
Ligaments
The tone of the pelvic floor provides the primary support for the uterus. Some ligaments provide further support, securing the uterus in place.
They are:
- Broad Ligament: This is a double layer of peritoneum attaching the sides of the uterus to the pelvis. It acts as a mesentery for the uterus and contributes to maintaining it in position.
- Round Ligament: A remnant of the gubernaculum extending from the uterine horns to the labia majora via the inguinal canal. It functions to maintain the anteverted position of the uterus.
- Ovarian Ligament: Joins the ovaries to the uterus.
- Cardinal Ligament: Located at the base of the broad ligament, the cardinal ligament extends from the cervix to the lateral pelvic walls. It contains the uterine artery and vein in addition to providing support to the uterus.
- Uterosacral Ligament: Extends from the cervix to the sacrum. It provides support to the uterus.
Vascular Supply and Lymphatics
The blood supply to the uterus is via the uterine artery. Venous drainage is via a plexus in the broad ligament that drains into the uterine veins.
Lymphatic drainage of the uterus is via the iliac, sacral, aortic and inguinal lymph nodes.