The myometrium is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle cells (also called uterine myocytes Its main function is to induce uterine contractions.
Structure
The myometrium is located between the endometrium (the inner layer of the uterine wall) and the serosa or perimetrium (the outer uterine layer).
The myometrium can be divided into three layers:
- The inner one-third thickness is termed the junctional or sub-endometrial layer. In most mammals it is characterized by fibers oriented in a circular way. Humans, having a single uterus from the fusion of two Müllerian ducts, have the fibers forming two cones. Mice have an unfused uterus, so the arrangement is simply circular.
- The middle layer occurs in both mice and humans. In mice it is a hard-to-spot mesh-like structure that probably helps coordinate the forces from the inner and outer layers.
- The thick outer layer in most mammals is in a longitudinal direction. It is unknown what the other layers develop from.
Muscular structure
The molecular structure of the smooth muscle of myometrium is very similar to that of smooth muscle in other sites of the body, with myosin and actin being the predominant proteins.) during pregnancy to allow for the uterus to become several times its non-gravid size, and contracts in a coordinated fashion, via a positive feedback effect on the "Ferguson reflex", during the process of labor. After delivery, the myometrium contracts to expel the placenta, and crisscrossing fibres of middle layer compress the blood vessels to minimize blood loss. A positive benefit to early breastfeeding is a stimulation of this reflex to reduce further blood loss and facilitate a swift return to prepregnancy uterine and abdominal muscle tone.
Uterine smooth muscle has a phasic pattern, shifting between a contractile pattern and maintenance of a resting tone with discrete, intermittent contractions of varying frequency, amplitude and duration.