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Uterus

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(Redirected fromWomb)
"Womb" redirects here. For other uses, seeWomb (disambiguation)."In utero" redirects here. For the album by Nirvana, seeIn Utero."Uterine" redirects here. For maternal half-siblings, seeUterine siblings."Intrauterine" redirects here. For the contraceptive devices, seeIntrauterine device."Hystera" redirects here. Not to be confused withHysteria.
Uterus
Diagram of human uterus and surrounding structures
Details
PrecursorParamesonephric ducts
SystemReproductive system
ArteryOvarian arteryanduterine artery
VeinUterine veins
LymphBody and cervix tointernal iliac lymph nodes,fundus topara-aortic lymph nodes,lumbar and superficial inguinal lymph nodes
Identifiers
Latinuterus
Greekὑστέρα (hystéra)
MeSHD014599
TA98A09.1.03.001
TA23500
FMA17558
Anatomical terminology

Theuterus(fromLatinuterus,pl.:uteri) orwomb(/wuːm/) is theorganin thereproductive systemof mostfemalemammals,including humans, that accommodates theembryonicandfetal developmentof one or morefertilized eggsuntilbirth.[1]The uterus is ahormone-responsivesex organthat containsglandsin itsliningthat secreteuterine milkfor embryonic nourishment. (The termuterusis also applied to analogous structures in some non-mammalian animals.)

In the human, the lower end of the uterus is a narrow part known as theisthmusthat connects to thecervix,the anterior gateway leading to thevagina.The upper end, the body of the uterus, is connected to thefallopian tubesat theuterine horns;the rounded part, the fundus, is above the openings to the fallopian tubes. The connection of theuterine cavitywith a fallopian tube is called theuterotubal junction.Thefertilized eggis carried to the uterus along the fallopian tube. It will havedividedon its journey to form ablastocystthat willimplantitself into the lining of the uterus – theendometrium,where it will receive nutrients and develop into theembryoproper, and laterfetus,for the duration of thepregnancy.

In thehuman embryo,the uterus develops from theparamesonephric ducts,which fuse into the single organ known as a simplex uterus. The uterus has different forms in many other animals and in some it exists as two separate uteri known as a duplex uterus.

Inmedicineand related professions, the termuterusis consistently used, while theGermanic-derived termwombis commonly used in everyday contexts. Events occurring within the uterus are described with the termin utero.

Structure

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Different regions of the uterus displayed and labelled using a 3D medical animation still shot
Different regions of the uterus, and thevagina,displayed and labelled using a 3D medical illustration

In humans, the uterus is located within thepelvic regionimmediately behind and almost overlying thebladder,and in front of thesigmoid colon.The human uterus is pear-shaped and about 7.6 cm (3.0 in) long, 4.5 cm (1.8 in) broad (side to side), and 3.0 cm (1.2 in) thick.[2][3]A typical adult uterus weighs about 60 grams. The uterus can be divided anatomically into four regions: thefundus– the uppermost rounded portion of the uterus above the openings of thefallopian tubes,[4]thebody,thecervix,and thecervical canal.The cervix protrudes into thevagina.The uterus is held in position within the pelvis byligaments,which are part of theendopelvic fascia.These ligaments include thepubocervical ligaments,thecardinal ligaments,and theuterosacral ligaments.It is covered by a sheet-like fold of peritoneum, thebroad ligament.[5]

Layers

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Uterine wall thickness (cm)[6]
Location Mean (mm) Range (mm)
Anterior wall 23 17 - 25
Posterior wall 21 15 - 25
Fundus 20 15 - 22
Isthmus 10 8 - 22
Vertical section of mucous membrane of human uterus

The uterus has three layers, which together form theuterine wall.From innermost to outermost, these layers are theendometrium,myometrium,andperimetrium.[7]

The endometrium is the innerepithelial layer,along with itsmucous membrane,of themammalianuterus. It has a basal layer and a functional layer; the functional layer thickens and then is shed during themenstrual cycleorestrous cycle.Duringpregnancy,theuterine glandsandblood vesselsin the endometrium further increase in size and number and form thedecidua.Vascular spaces fuse and become interconnected, forming theplacenta,which suppliesoxygenand nutrition to theembryoandfetus.[8][9]

The myometrium of the uterus mostly consists ofsmooth muscle.The innermost layer of myometrium is known as thejunctional zone,which becomes thickened inadenomyosis.[10]

The perimetrium is aserouslayer of visceralperitoneum.It covers the outer surface of the uterus.[11]

Surrounding the uterus is a layer or band of fibrous and fatty connective tissue called theparametriumthat connects the uterus to other tissues of the pelvis.

Commensalandmutualisticorganisms are present in the uterus and form theuterine microbiome.[12][13][14][15]

Support

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Uterus covered by the broad ligament

The uterus is primarily supported by thepelvic diaphragm,perineal body,andurogenital diaphragm.Secondarily, it is supported by ligaments, including theperitoneal ligamentand thebroad ligament of uterus.[16]

Major ligaments

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The uterus is held in place by several peritoneal ligaments, of which the following are the most important (there are two of each):

Name From To
Uterosacral ligaments Posterior cervix Anterior face ofsacrum
Cardinal ligaments Side of the cervix Ischial spines
Pubocervical ligaments[16] Side of the cervix Pubic symphysis

Axis

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Normally, the human uterus lies in anteversion and anteflexion. In most women, the long axis of the uterus is bent forward on the long axis of the vagina, against the urinary bladder. This position is referred to as anteversion of the uterus. Furthermore, the long axis of the body of the uterus is bent forward at the level of the internal os with the long axis of the cervix. This position is termed anteflexion of the uterus.[17]The uterus assumes an anteverted position in 50% of women, a retroverted position in 25% of women, and a midposed position in the remaining 25% of women.[2]

Position

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Uterus shown in position in the body

The uterus is located in the middle of the pelvic cavity, in thefrontal plane(due to thebroad ligament of the uterus). The fundus does not extend above thelinea terminalis,while the vaginal part of the cervix does not extend below theinterspinal line.The uterus is mobile and moves posteriorly under the pressure of a full bladder, or anteriorly under the pressure of a full rectum. If both are full, it moves upwards. Increased intra-abdominal pressure pushes it downwards. The mobility is conferred to it by a musculo-fibrous apparatus that consists of suspensory and sustentacular parts. Under normal circumstances, the suspensory part keeps the uterus in anteflexion and anteversion (in 90% of women) and keeps it "floating" in the pelvis. The meanings of these terms are described below:

1. Anteversion with slight anteflexion
2. Anteversion with marked anteflexion
3. Anteversion with retrocession
4. Retroversion
5. Retroversion with retroflexion
Distinction More common Less common
Position tipped "Anteverted": Tipped forward "Retroverted":Tipped backwards
Position of fundus "Anteflexed": Fundus is pointing forward relative to the cervix "Retroflexed": Fundus is pointing backward

The sustentacular part supports the pelvic organs and comprises the largerpelvic diaphragmin the back and the smallerurogenital diaphragmin the front.

The pathological changes of the position of the uterus are:

  • retroversion/retroflexion, if it is fixed
  • hyperanteflexion – tipped too forward; most commonly congenital, but may be caused by tumors
  • anteposition, retroposition, lateroposition – the whole uterus is moved; caused byparametritisor tumors
  • elevation, descensus,prolapse
  • rotation (the whole uterus rotates around its longitudinal axis), torsion (only the body of the uterus rotates around)
  • inversion

In cases where the uterus is "tipped", also known asretroverted uterus,the woman may have symptoms of pain during sexual intercourse, pelvic pain during menstruation, minor incontinence, urinary tract infections, fertility difficulties,[18]and difficulty using tampons. A pelvic examination by a doctor can determine if a uterus is tipped.[19]

Blood, lymph, and nerve supply

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Diagram of uterine blood supply

The human uterus is supplied by arterial blood both from theuterine arteryand theovarian artery.Another anastomotic branch may also supply the uterus from anastomosis of these two arteries.

Afferent nerves supplying the uterus areT11 and T12.Sympathetic supply is from thehypogastric plexusand theovarian plexus.Parasympathetic supply is from the S2, S3 and S4 nerves.

Development

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Main article:Development of the reproductive system
Differentiation of uterus

BilateralMüllerian ductsform during early human fetal life. In males,anti-Müllerian hormone(AMH) secreted from the testes leads to the ducts' regression. In females, these ducts give rise to thefallopian tubesand the uterus. In humans, the lower segments of the two ducts fuse to form a single uterus; in cases ofuterine malformationsthis fusion may be disturbed. The different uterine morphologies among the mammals are due to varying degrees of fusion of the Müllerian ducts.

Various congenital conditions of the uterus can developin utero.Though uncommon, some of these aredidelphic uterus,bicornate uterusand others.[20]

See alsoList of related male and female reproductive organs.

Function

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Maximum penetration depth of the penis

The primary reproductive function of the human uterus is to prepare for the implantation of azygote,a fertilizedovum,and maintenance of pregnancy if implantation occurs.[21]Traveling along thefallopian tubeon its way to the uterine cavity, the zygote dividesmitoticallyto become ablastocyst,which ultimately attaches to the uterine wall andimplantsinto theendometrium.[4]Theplacentalater develops to nourish the embryo, which grows throughembryonicandfetal developmentuntilchildbirth.During this process, the uterus grows to accommodate the growing fetus.[22]When normal labor begins, the uterus forcefully contracts as the cervix dilates, which results in delivery of the infant.[22]

In the absence of pregnancy,menstruationoccurs.[21]The withdrawal of female sex hormones,estrogenandprogesterone,which occurs in the absence of fertilization, triggers the shedding of the functional layer of the endometrium.[21]This layer is broken down, shed, and restored in anticipation of the next menstrual cycle.[21]The average bleeding duration during menses is 5-7 days after which the menstrual cycle begins again.[23]

Clinical significance

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Duringpregnancy,the growth rate of the fetus can be assessed by measuring thefundal height.

Somepathologicalstates include:

Transvaginal ultrasonographyshowing a uterine fluid accumulation in apostmenopausalwoman.
  • Accumulation of fluids other than blood or of unknown constitution. One study came to the conclusion thatpostmenopausalwomen with endometrial fluid collection ongynecologic ultrasonographyshould undergoendometrial biopsyif the endometrial lining is thicker than 3 mm or if the endometrial fluid isechogenic.In cases of a lining 3 mm or less and clear endometrial fluid, endometrial biopsy was not regarded to be necessary, butendocervical curettageto rule out endocervical cancer was recommended.[24]
  • Hematometra,which is accumulation of blood within the uterus.[25]

Malformations

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Four types ofuterine malformation

Uterine malformationsare mainlycongenital malformations,and includeuterus didelphys,bicornuate uterusandseptate uterus.Congenital absence of the uterus is known asMüllerian agenesis.

Surgery

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Ahysterectomyis the surgical removal of the uterus, which may be carried out for a number of reasons including the ridding oftumoursboth benign andmalignant.A complete hysterectomy involves the removal of the body, fundus, and cervix of the uterus. A partial hysterectomy may just involve the removal of the uterine body while leaving the cervix intact. It is the most commonly performedgynecological surgical procedure.

Transplants

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Uterus transplantationshave been successfully carried out in a number of countries. The transplant is intended to be temporary – recipients will have to undergo ahysterectomyafter one or two successful pregnancies. This is done to avoid the need to takeimmunosuppressive drugsfor life with a consequent increased risk of infection.[27]

The procedure remains the last resort: it is as of 2023, a relatively new and somewhat experimental procedure, performed only by certain specialist surgeons in select centres, it is expensive and unlikely to be covered by insurance, and it involves risk of infection and organ rejection. Someethicsspecialists consider the risks to a live donor too great, and some find the entire procedure ethically questionable, especially since the transplant is not a life-saving procedure.[28][29][30]

Other animals

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Most animals that layeggs,such asbirdsandreptiles,including mostovoviviparousspecies, have anoviductinstead of a uterus. However, recent research into the biology of theviviparous(not merelyovoviviparous)skinkTrachylepisivensihas revealed development of a very close analogue toeutherianmammalian placental development.[31]

Inmonotremes,mammals which lay eggs, namely theplatypusand theechidnas,either the termuterusoroviductis used to describe the same organ, but the egg does not develop aplacentawithin the mother and thus does not receive further nourishment after formation andfertilization.

Marsupialshave two uteri, each of which connect to a lateral vagina and both use a third, middle "vagina", which functions as the birth canal.[32][33]Marsupialembryosform achoriovitelline placenta(which can be thought of as something between a monotreme egg and a "true" placenta), in which the egg's yolk sac supplies a large part of the embryo's nutrition but also attaches to the uterine wall and takes nutrients from the mother's bloodstream. However,bandicootsalso have a rudimentary chorioallantoic placenta, similar to those of placental mammals.

Thefetususually develops fully inplacental mammalsand only partially inmarsupialsincludingkangaroosandopossums.In marsupials, the uterus forms as a duplex organ of two uteri. In monotremes such as theplatypus,the uterus is duplex and rather than nurturing the embryo, secretes the shell around the egg. It is essentially identical with theshell glandof birds and reptiles, with which the uterus ishomologous.[34]

In mammals, the four main forms of the uterus are: duplex, bipartite, bicornuate and simplex.[35]

Duplex
There are two wholly separate uteri, with one oviduct each. Found inmarsupials(such askangaroos,Tasmanian devils,opossums,etc.),rodents(such asmice,rats,andguinea pigs), andlagomorphs(rabbitsandhares).
Bipartite
The two uteri are separate for most of their length, but share a single cervix. Found inruminants(deer,moose,elk,etc.),hyraxes,cats,andhorses.
Bicornuate
The upper parts of the uterus remain separate, but the lower parts are fused into a single structure. Found indogs,pigs,elephants,whales,dolphins,[36]andtarsiers,andstrepsirrhineprimatesamong others.
Simplex
The entire uterus is fused into a single organ.[34]Found in higherprimates(includinghumansandchimpanzees). Occasionally, some individual females (including humans) may have abicornuate uterus,auterine malformationwhere the two parts of the uterus fail to fuse completely during fetal development.

Two uteri usually form initially in a female and usually male fetus, and in placental mammals, they may partially or completely fuse into a single uterus depending on the species. In many species with two uteri, only one is functional. Humans and other higher primates such as chimpanzees, usually have a single completely fused uterus, although in some individuals, the uteri may not have completely fused.

Additional images

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  • Schematic frontal view of female anatomy
    Schematic frontal view of female anatomy
  • Sectional plan of the gravid uterus in the third and fourth month
    Sectional plan of the gravid uterus in the third and fourth month
  • Fetus in utero, between fifth and sixth months.
    Fetus in utero, between fifth and sixth months.
  • Uterus
    Uterus

See also

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References

[edit]
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  36. ^Bernd Würsig; William F. Perrin; J.G.M. Thewissen (26 February 2009).Encyclopedia of Marine Mammals.Academic Press.ISBN978-0-08-091993-5.Archivedfrom the original on 10 April 2021.Retrieved19 October2020.
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