The myometrium is the middle layer of the

uterine The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic and fetal development of one or more fertilized eggs until bir ...

wall, consisting mainly of uterine smooth muscle cells (also called uterine myocytes) but also of supporting stromal and vascular tissue. Its main function is to induce

uterine contraction Uterine contractions are muscle contractions of the uterine smooth muscle that can occur at various intensities in both the non-pregnant and pregnant uterine state. The non-pregnant uterus undergoes small, spontaneous contractions in addition to ...

Structure

The myometrium is located between the

endometrium The endometrium is the inner epithelium, epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The funct ...

(the inner layer of the uterine wall) and the serosa or

perimetrium The perimetrium (or serous coat of uterus) is the outer serosal layer of the uterus, derived from the peritoneum overlying the uterine fundus, and can be considered a visceral peritoneum. It consists of a superficial layer of mesothelium, and a ...

(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 form the fusion of two

Müllerian duct The paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo in the reproductive system of humans and other mammals that run down the lateral sides of the genital ridge and terminate at the sinus tubercle in the primitive uroge ...

s, 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. In humans there is also a mesh-like layer, except it is slightly more prominent and has many blood vessels. * The thick outer layer in most mammals is in a longitudinal direction. Primates including humans also have them in a largely longitudinal direction, though the boundary with the middle layer can be very blurry. It is the major contractile tissue during parturition and abortion. The junctional layer is probably capable of

peristaltic Peristalsis ( , ) is a type of intestinal motility, characterized by symmetry in biology#Radial symmetry, radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an wikt:anterograde, anterograde dir ...

and anti-peristaltic activity. The "circle and longitudinal" layout is similar to the

muscular layer The muscular layer (muscular coat, muscular fibers, muscularis propria, muscularis externa) is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa. It is responsible for gut movement such as peristalsis. The Latin ...

of the intestines. It is generally believed that the junctional layer derives from the

, or more specifically the

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

surrounding the duct. In 2022, it was found that the Woffian duct's mesenchyme also migrates to contribute. There is an obvious partitioning: the mesometrial side, derived from the Woffian, has more glands. Accordingly, implantation occurs on the antimesometrial side, while placentation happens on the mesometrial side. It is unknown what the other layers develop from. The myometrium contains a "side population" of stem cells. They are presumably what allows the myometrium to grow during pregnancy.

Muscular structure

The molecular structure of the

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

of myometrium is very similar to that of smooth muscle in other sites of the body, with

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

and

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

being the predominant proteins. In uterine smooth muscle, there is approximately 6-fold more actin than myosin. A shift in the myosin expression of the uterine smooth muscle may be responsible for changes in the directions of uterine contractions during the menstrual cycle.

Function

Contraction

The myometrium stretches (the smooth muscle cells expand in both size and numberSteven's and Lowe Histology p352) during

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

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 The Ferguson reflex (also called the fetal ejection reflex) is the neuroendocrine reflex comprising the self-sustaining cycle of uterine contractions initiated by pressure at the cervix, more precisely, the internal end of cervix, or vaginal wal ...

", during the process of

labor Labour or labor may refer to: * Childbirth, the delivery of a baby * Labour (human activity), or work ** Manual labour, physical work ** Wage labour, a socioeconomic relationship between a worker and an employer ** Organized labour and the labour ...

. After

delivery Delivery may refer to: Biology and medicine *Childbirth *Drug delivery *Gene delivery Business and law *Delivery (commerce), of goods, e.g.: **Pizza delivery ** Milk delivery ** Food delivery ** Online grocer *Deed ("delivery" in contract law), a ...

, the myometrium contracts to expel the

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

, 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. As noted for the macrostructure of uterine smooth muscle, the junctional layer appears to be capable of both

and anti-peristaltic activity.

Resting state

The

resting membrane potential The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential. The re ...

(V_rest) of uterine smooth muscle has been recorded to be between -35 and -80 mV. As with the resting membrane potential of other cell types, it is maintained by a Na⁺/K⁺ pump that causes a higher concentration of Na⁺ ions in the extracellular space than in the intracellular space, and a higher concentration of K⁺ ions in the

intracellular space Intracellular space is the interior space of the plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and prote ...

than in the extracellular space. Subsequently, having K⁺ channels open to a higher degree than Na⁺ channels results in an overall efflux of positive ions, resulting in a negative potential. This resting potential undergoes rhythmic oscillations, which have been termed ''slow waves'', and reflect intrinsic activity of

slow wave potential A slow-wave potential is a rhythmic electrophysiological event in the gastrointestinal tract. The normal conduction of slow waves is one of the key regulators of gastrointestinal motility. Slow waves are generated and propagated by a class of pac ...

s. These slow waves are caused by changes in the distribution of Ca²⁺, Na⁺, K⁺ and Cl⁻ ions between the intracellular and extracellular spaces, which, in turn, reflects the permeability of the plasma membrane to each of those ions. K⁺ is the major ion responsible for such changes in

ion flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport pheno ...

, reflecting changes in various K⁺ channels.

Excitation-contraction

The

excitation-contraction coupling Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

of uterine smooth muscle is also very similar to that of other

in general, with intracellular increase in

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(Ca²⁺) leading to contraction.

Restoration to resting state

Removal of Ca²⁺ after contraction induces relaxation of the smooth muscle, and restores the molecular structure of the

sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion levels are kep ...

for the next contractile stimulus.

References

{{Authority control Pelvis Mammal female reproductive system