Uterus

ANATOMY

The uterus is a single muscular organ found on the midline of the body. It is regionalized into three primary portions. The superior portion of the uterus is the fundus, which typically leans anteriorly. This orientation is not seen from a frontal section where the fundus is shown upright. The larger portion of the organ inferior to the fundus is the uterine body, which contains the hollow uterine cavity. The uterus constricts at the isthmus, giving rise to the inferior region of the organ, the cervix. The cervix is the muscular neck of the uterus that partially protrudes into the vagina. The uterine cavity continues through the cervix as the cervical canal. The border between the uterine cavity and the cervical canal is the internal os. The border between the cervical canal and the vagina is the external os.

The wall of the uterus has 3 layers:

1. The perimetrium is the visceral peritoneum, a connective tissue coat on the outer periphery of the organ.

2. The myometrium is the middle, smooth muscle layer. Contraction of the uterus is responsible for expelling the fetus during childbirth.

3. The endometrium is an epithelial layer on a bed of loose connective tissue.

PHYSIOLOGY

The endometrium changes due to hormonal signals, preparing for implantation and supporting development of the offspring. When the oocyte isn’t fertilized, a large portion of the endometrium known as the stratum functionalis dies and is shed as menstrual flow. When the oocyte is fertilized, the developing offspring along with the embryonic membranes and placenta are housed in the uterus. Contraction of the uterus is the primary force enabling childbirth.

Figure 1: Illustration of the uterus, including the positioning of different anatomical parts of the organ.