Myometrium
Encyclopedia
The myometrium is the middle layer of the uterine
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 contractions.
(the inner layer of the uterine wall), and the serosa or perimetrium
(the outer uterine layer).
The inner one-third of the myometrium (termed the junctional or sub-endometrial layer) appears to be derived from the Müllerian duct
, while the outer, more predominant layer myometrium appears to originate from non-Mullerian tissue, and is the major contractile tissue during parturition and abortion. Also, the junctional layer appears to function like a circular muscle layer, capable of peristaltic and anti-peristaltic
activity, equivalent to the muscular layer of the intestines.
of the myometrium is basically very similar in molecular structure to smooth muscle in other sites of the body, with myosin
and actin
being the predominant proteins expressed. In uterine smooth muscle, there is approximately 6-fold more actin than myosin. A shift in the myosin expression of the uterine smooth muscle has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle.
of a resting tone with discrete, intermittent contractions of varying frequency, amplitude and duration.
Also, as noted for the macrostructure of uterine smooth muscle, the junctional layer appears be capable of both peristaltic and anti-peristaltic activity.
recorded to be between -35 and -80 mV. As with the resting membrane potential of other cell types, it is basically 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 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
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, reflecting changes in various K+ channels.
in general, with intracellular increase in calcium
(Ca2+) leading to contraction. However, the stimulating factors for uterine smooth muscle differs from other types of smooth muscle, availing for separate coordination of uterine smooth muscle.
to allow for the harboring of the pregnancy, 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 reduce blood loss. Thus a positive benefit to early breast feeding is a natural stimulation of this reflex to reduce blood loss and facilitate a swift return to prepregnancy uterine and abdominal muscle tone.
. After pregnancy the uterus returns to its nonpregnant size by a process of myometrial involution.
Neoplasms of the myometrium are very common, termed uterine leiomyoma
ta or fibroids. Their malignant version, leiomyosarcoma
, is rare.
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
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 contractions.
Macrostructure
The myometrium is located between the endometriumEndometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...
(the inner layer of the uterine wall), and the serosa or perimetrium
Perimetrium
The perimetrium is the outer serosa layer of the uterus, equivalent to peritoneum.-External links: - "The Female Pelvis: The uterus" - "Mammal, uterus "...
(the outer uterine layer).
The inner one-third of the myometrium (termed the junctional or sub-endometrial layer) appears to be derived from the Müllerian duct
Müllerian duct
Müllerian ducts are paired ducts of the embryo that run down the lateral sides of the urogenital ridge and terminate at the Müllerian eminence in the primitive urogenital sinus. In the female, they will develop to form the Fallopian tubes, uterus, cervix, and the upper two-third of the vagina; in...
, while the outer, more predominant layer myometrium appears to originate from non-Mullerian tissue, and is the major contractile tissue during parturition and abortion. Also, the junctional layer appears to function like a circular muscle layer, capable of peristaltic and anti-peristaltic
activity, equivalent to the muscular layer of the intestines.
Molecular muscular structure
The smooth muscleSmooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
of the myometrium is basically very similar in molecular structure to smooth muscle in other sites of the body, with myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
and actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
being the predominant proteins expressed. In uterine smooth muscle, there is approximately 6-fold more actin than myosin. A shift in the myosin expression of the uterine smooth muscle has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle.
Contractile patterns
Uterine smooth muscle has a phasic pattern, shifting between a contractile pattern and maintenanceof a resting tone with discrete, intermittent contractions of varying frequency, amplitude and duration.
Also, as noted for the macrostructure of uterine smooth muscle, the junctional layer appears be capable of both peristaltic and anti-peristaltic activity.
Resting state
The resting membrane potential (Vrest) of uterine smooth muscle has beenrecorded to be between -35 and -80 mV. As with the resting membrane potential of other cell types, it is basically maintained by a Na+/K+ pump
Na+/K+-ATPase
Na+/K+-ATPase is an enzyme located in the plasma membrane in all animals.- Sodium-potassium pumps :Active transport is responsible for cells containing relatively high...
that causes a higher concentration of Na+
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ions in the extracellular space than in the intracellular space, and a higher concentration of K+
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
ions in the intracellular space than in the extracellular space. Subsequently, having K+ channels
Potassium channel
In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
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
Slow wave potential
In physiology, a slow-wave potential is a membrane potential that cycles between depolarizations and repolarizations. Slow wave potentials are generated by myocytes. Due to temporal summation, a slow-wave potential will periodically reach threshold and generate an action potential. This in turn...
s. These slow waves are caused by changes in the distribution of Ca2+
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, Na+, K+ and Cl-
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
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, reflecting changes in various K+ channels.
Excitation-contraction
The excitation-contraction coupling of uterine smooth muscle is also basically very similar to smooth muscleSmooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
in general, with intracellular increase in calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca2+) leading to contraction. However, the stimulating factors for uterine smooth muscle differs from other types of smooth muscle, availing for separate coordination of uterine smooth muscle.
Restoration to resting state
Removal of Ca2+ after contraction induces relaxation of the smooth muscle, and restores the molecular structure of the sarcoplasmic reticulum to avail for the next contractile stimulus.Functions
The myometrium stretches (the smooth muscle cells expand in both size and number) during pregnancyPregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
to allow for the harboring of the pregnancy, and contracts in a coordinated fashion, via a positive feedback effect on the "Ferguson reflex
Ferguson reflex
The Ferguson reflex is an example of positive feedback and the female body's response to pressure application in the cervix or vaginal walls.Upon application of pressure, oxytocin is released and uterine contractions are stimulated , until the baby is delivered...
"), during the process of labor. After delivery
Childbirth
Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...
the myometrium contracts to expel the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
and reduce blood loss. Thus a positive benefit to early breast feeding is a natural stimulation of this reflex to reduce blood loss and facilitate a swift return to prepregnancy uterine and abdominal muscle tone.
Pathology
Lack of contraction at this stage is termed uterine atonyUterine atony
Uterine atony is a loss of tone in the uterine musculature. Normally, contraction of the uterine muscle compresses the vessels and reduces flow. This increases the likelihood of coagulation and prevents bleeds. Thus, lack of uterine muscle contraction can cause an acute hemorrhage...
. After pregnancy the uterus returns to its nonpregnant size by a process of myometrial involution.
Neoplasms of the myometrium are very common, termed uterine leiomyoma
Leiomyoma
A leiomyoma is a benign smooth muscle neoplasm that is not premalignant. They can occur in any organ, but the most common forms occur in the uterus, small bowel and the esophagus.- Etymology:* Greek:** λεῖος leios "smooth"...
ta or fibroids. Their malignant version, leiomyosarcoma
Leiomyosarcoma
Leiomyosarcoma , aka LMS, is a malignant cancer of smooth muscle....
, is rare.