The uterus is a muscular pear-shaped organ of reproduction in the female. It is specialized for containing and nourishing a developing embryo from implantation to paturition (birth). The epithelial lining of the uterus undergoes cyclic changes that make it hospitable for the early embryo if fertilization has occurred. The muscular and elastic elements are specialized for expansion with the growing baby and for the expulsion of the baby at birth.
The never pregnant (nulliparous) uterus is a 7-8 cm long to 4-5 cm wide, muscular pear-shaped organ lying in the pelvic cavity on the superior surface of the bladder. The uterus weighs under 50 grams and is divided into the broad-ended fundus, body and thin isthmus that ends in the uterine cervix. The cervix is made mostly of dense connective tissue, about 2.5 cm in length and is covered interiorly by a mucous secreting ciliated epithelium at the upper regions and by stratified squamous epithelium at the vaginal end. The opening of the cervix into the vagina is almost at a right angle to the long axis of the vagina. The uterine cavity has a triangular shape that is widest at the fundus and flattened in sagittal section (sliced through, front-to-back). Uterine (Fallopian) tubes enter the uterus as the fundus and are supported by the broad ligaments as they span the distance from the ovary to the uterus.
When the bladder is empty the uterus angles forward over the bladder. As the bladder fills the uterus is lifted dorsally and may become retroflexed pressing against the rectum. Uterine blood supply is via the uterine and ovarian arteries with venous return traveling via the uterine veins. The hypogastric and ovarian nerve plexuses supply sympathetic and parasympathetic fibers as well as carry uterine afferent sensory fibers on their way to the spinal cord (T11 & 12).
Histologically, the uterus is composed of three layers: 1) an outer perimetrium composed of connective tissue, 2) a thick smooth muscle and elastic tissue, myometrium, and 3) a mucosal epithelial lining called the endometrium. The myometrium consists of roughly four layers of smooth muscle. The tubular glands of the endothelium can be seen near the myometrium near their beginning in the lamina propria. The epithelium contains simple columnar and ciliated cells. The lamina propria is made of extracellular matrix, many fibroblasts and mostly reticular fibers. Blood supply to the uterus is carried by the arcuate arteries that branch to supply the endometrium in the form of straight arteries to endometrial epithelium and coiled arteries to the menstrual epithelium.