The Myometrium of Uterus
The myometrium of the uterus is the middle muscle layer of the uterus and it is the thickest of the three layers of the uterus. The cervical part of myometrium is made up of plain muscle tissue with large amount of fibrous tissue that gives the hard consistency of the cervix. But the muscle fibers and fibrous tissues are mixed in with out any orderly fashion.
The myometrium is about 10-12 mm thickness in the body of the uterus. In the body of the uterus three layers of myometrium can be easily distinguished in pregnant uterus. The outermost layer is just below the peritoneum or perimetrium and it is longitudinal, the fibers passing from the cervix anteriorly over fundus and reaches backside of cervix. The layer can not be distinguished in non pregnant because it is thin. The main function of this layer is expulsion of fetus.
The middle layer is the thickest among three layers of myometrium and it consists of bundles of muscle fibers separated by connective tissue. The amount of connective tissue varies with age. The muscle content is much abundant during child bearing age (more during pregnancy) and much less before puberty and after menopause. The blood vessels which supply the uterus are distributed in the connective tissue of the middle layer. The blood vessel calibers depend on the contraction of the muscle of this layer. The function of this layer is expulsion of the fetus and hemostasis (control of bleeding). This layer is also called living ligature of the uterus and is responsible control of bleeding in the third stage of labour. Inefficient contraction of this layer is and retraction of this layer is responsible for postpartum hemorrhage (PPH)
The innermost layer of the myometrium consists of circular muscle fibers. The layer is not well marked, but is best represented in the opening of fallopian tubes and the internal os. The main function of the inner layer is to act like sphincter.