The uterus (from
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''uterus'', plural ''uteri'') or womb () is the
organ
Organ may refer to:
Biology
* Organ (biology), a part of an organism
Musical instruments
* Organ (music), a family of keyboard musical instruments characterized by sustained tone
** Electronic organ, an electronic keyboard instrument
** Hammond ...
in the
reproductive system
The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are ...
of most
female
Female ( symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction.
A female has larger gametes than a male. Fema ...
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s, including humans that accommodates the
embryonic and
fetal development of one or more
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s until
birth. The uterus is a
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
-responsive
sex organ
A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
that contains
glands
In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream ( endocrine gland) or into cavities inside the body or its outer surface ( exocrine gland).
Structure
...
in its
lining that secrete
uterine milk for embryonic nourishment.
In the human, the lower end of the uterus, is a narrow part known as the
isthmus that connects to the
cervix
The cervix or cervix uteri (Latin, 'neck of the uterus') is the lower part of the uterus (womb) in the human female reproductive system. The cervix is usually 2 to 3 cm long (~1 inch) and roughly cylindrical in shape, which changes during ...
, leading to the
vagina
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
. The upper end, the body of the uterus, is connected to the
fallopian tubes
The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...
, at the
uterine horns
The uterine horns (cornua of uterus) are the points in the upper uterus where the fallopian tubes exit to meet the ovaries. They are one of the points of attachment for the round ligament of uterus (the other being the mons pubis). They also prov ...
, and the rounded part above the openings to the fallopian tubes is the fundus. The connection of the
uterine cavity with a fallopian tube is called the
uterotubal junction. The
fertilized egg is carried to the uterus along the fallopian tube. It will have
divided
Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new numbers. The other operations are addition, subtraction, and multiplication.
At an elementary level the division of two natural numbe ...
on its journey to form a
blastocyst
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called th ...
that will
implant
Implant can refer to:
Medicine
* Implant (medicine), or specifically:
**Brain implant
**Breast implant
** Buttock implant
**Cochlear implant
**Contraceptive implant
**Dental implant
** Fetal tissue implant
** Implantable cardioverter-defibrillato ...
itself into the lining of the uterus – the
endometrium
The endometrium is the inner 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 functional layer ...
, where it will receive nutrients and develop into the embryo proper and later fetus for the duration of the
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ...
.
In the
human embryo
Human embryonic development, or human embryogenesis, is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of d ...
, the uterus develops from the
paramesonephric duct
Paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo that run down the lateral sides of the genital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the ...
s 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.
In
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...
, and related professions the term ''uterus'' is consistently used, while the
Germanic-derived term ''
womb'' is commonly used in everyday contexts. Events occurring within the uterus are described with the term in utero.
Structure
In humans the uterus is located within the
pelvic region immediately behind and almost overlying the
bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine en ...
, and in front of the
sigmoid colon
The sigmoid colon (or pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus ' ...
. The human uterus is pear-shaped and about long, broad (side to side), and thick.
[Manual of Obstetrics. (3rd ed.). Elsevier 2011. pp. 1–16. .] A typical adult uterus weighs about 60 grams. The uterus can be divided anatomically into four regions: the fundus – the uppermost rounded portion of the uterus above the openings of the
fallopian tube
The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In o ...
s,
the body, the
cervix
The cervix or cervix uteri (Latin, 'neck of the uterus') is the lower part of the uterus (womb) in the human female reproductive system. The cervix is usually 2 to 3 cm long (~1 inch) and roughly cylindrical in shape, which changes during ...
, and the
cervical canal. The cervix protrudes into the
vagina
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
. The uterus is held in position within the pelvis by
ligaments
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal ...
, which are part of the
endopelvic fascia. These ligaments include the
pubocervical ligaments, the
cardinal ligaments, and the
uterosacral ligaments. It is covered by a sheet-like fold of peritoneum, the
broad ligament.
Layers
The uterus has three layers, which together form the uterine wall. From innermost to outermost, these layers are the
endometrium
The endometrium is the inner 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 functional layer ...
,
myometrium, and
perimetrium.
The endometrium is the inner
epithelial layer, along with its
mucous membrane
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...
, of the
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
ian uterus. It has a basal layer and a functional layer; the functional layer thickens and then is shed during the
menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs ...
or
estrous cycle
The estrous cycle (, originally ) is the set of recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestro ...
. During
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ...
, the
uterine glands and
blood vessel
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...
s in the endometrium further increase in size and number and form the
decidua. Vascular spaces fuse and become interconnected, forming the
placenta
The placenta 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 the physically separate mate ...
, which supplies
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and nutrition to the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
and
fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
.
[Blue Histology - Female Reproductive System]
. School of Anatomy and Human Biology — The University of Western Australia Accessed 20061228 20:35
The myometrium of the uterus mostly consists of
smooth muscle
Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
. The innermost layer of myometrium is known as the junctional zone, which becomes thickened in
adenomyosis
Adenomyosis is a medical condition characterized by the growth of cells that proliferate on the inside of the uterus ( endometrium) atypically located among the cells of the uterine wall ( myometrium), as a result, thickening of the uterus occurs. ...
.
The perimetrium is a
serous
In physiology, serous fluid or serosal fluid (originating from the Medieval Latin word ''serosus'', from Latin ''serum'') is any of various body fluids resembling serum, that are typically pale yellow or transparent and of a benign nature. The fl ...
layer of visceral
peritoneum
The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
. It covers the outer surface of the uterus.
Surrounding the uterus is a layer or band of fibrous and fatty connective tissue called the
parametrium that connects the uterus to other tissues of the pelvis.
Commensal and
mutualistic organisms are present in the uterus and form the
uterine microbiome
The uterine microbiome is the commensal, nonpathogenic, bacteria, viruses, yeasts/fungi present in a healthy uterus, amniotic fluid and endometrium and the specific environment which they inhabit. It has been only recently confirmed that the u ...
.
Support
The uterus is primarily supported by the
pelvic diaphragm,
perineal body, and
urogenital diaphragm. Secondarily, it is supported by ligaments, including the
peritoneal ligament and the
broad ligament of uterus.
[The Pelvis]
University College Cork Archived fro
the original
on 2008-02-27
Major ligaments
It is held in place by several peritoneal ligaments, of which the following are the most important (there are two of each):
Axis
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. 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.
Position
The uterus is located in the middle of the pelvic cavity, in the
frontal plane
The coronal plane (also known as the frontal plane) is an anatomical plane that divides the body into dorsal and ventral sections. It is perpendicular to the sagittal and transverse planes.
Details
The coronal plane is an example of a longitud ...
(due to the
broad ligament of the uterus). The fundus does not extend above the ''
linea terminalis
The linea terminalis or innominate line consists of the pubic crest, pectineal line (pecten pubis), the arcuate line, the sacral ala, and the sacral promontory.
It is the pelvic brim, which is the edge of the pelvic inlet. The pelvic inlet is t ...
'', while the vaginal part of the cervix does not extend below the
interspinal 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:
The sustentacular part supports the pelvic organs and comprises the larger
pelvic diaphragm in the back and the smaller
urogenital diaphragm in 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 by
parametritis
Parametritis (also known as pelvic cellulitis) is an infection of the parametrium ( connective tissue adjacent to the uterus). It is considered a form of pelvic inflammatory disease
Pelvic inflammatory disease, also known as pelvic inflammatory ...
or 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 as
retroverted uterus
A retroverted uterus (tilted uterus, tipped uterus) is a uterus that is oriented posteriorly, towards the back of the body. This is in contrast to the typical uterus, which is oriented forward (slightly " anteverted") toward the bladder, with the ...
, the woman may have symptoms of pain during sexual intercourse, pelvic pain during menstruation, minor incontinence, urinary tract infections, fertility difficulties, and difficulty using tampons. A pelvic examination by a doctor can determine if a uterus is tipped.
Blood, lymph, and nerve supply supply
The human uterus is supplied by arterial blood both from the
uterine artery and the
ovarian artery
The ovarian artery is an artery that supplies oxygenated blood to the ovary in females. It arises from the abdominal aorta below the renal artery. It can be found within the suspensory ligament of the ovary, anterior to the ovarian vein and uret ...
. Another anastomotic branch may also supply the uterus from anastomosis of these two arteries.
Afferent nerves supplying the uterus are
T11 and T12. Sympathetic supply is from the
hypogastric plexus and the
ovarian plexus. Parasympathetic supply is from the S2, S3 and S4 nerves.
Development
Bilateral
Müllerian ducts form during early human fetal life. In males,
anti-müllerian hormone
Anti-Müllerian hormone (AMH), also known as Müllerian-inhibiting hormone (MIH), is a glycoprotein hormone structurally related to inhibin and activin from the transforming growth factor beta superfamily, whose key roles are in growth differen ...
(AMH) secreted from the testes leads to the ducts' regression. In females, these ducts give rise to the
Fallopian tubes
The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...
and the uterus. In humans, the lower segments of the two ducts fuse to form a single uterus; in cases of
uterine malformations this 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 develop ''in utero''. Though uncommon, some of these are
didelphic uterus,
bicornate uterus and others.
See also
List of related male and female reproductive organs
This list of related male and female reproductive organs shows how the male and female reproductive organs and the development of the reproductive system are related, sharing a common developmental path. This makes them biological homologues. T ...
.
Function
The reproductive function of the human uterus is to accept a fertilized
ovum
The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete i ...
, which passes through the
uterotubal junction from the
fallopian tube
The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In o ...
. The
fertilized ovum divides mitotically to become a
blastocyst
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called th ...
, which implants into the
endometrium
The endometrium is the inner 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 functional layer ...
and derives nourishment from blood vessels which develop exclusively for this purpose. The fertilized ovum becomes an
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, attaches to the wall of the uterus, creates a placenta, and develops into a
fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
(gestates) until
childbirth
Childbirth, also known as labour and delivery, is the ending of pregnancy where one or more babies exits the internal environment of the mother via vaginal delivery or caesarean section. In 2019, there were about 140.11 million births glob ...
occurs. Due to anatomical barriers such as the
pelvis
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The ...
, the uterus is pushed partially into the abdomen due to its expansion during pregnancy. Even during pregnancy, the mass of a human uterus amounts to only about a kilogram (2.2 pounds).
The uterus also plays a role in
sexual response, by directing blood flow to the
pelvis
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The ...
,
ovaries
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
, and
genitals.
There is also some evidence from rat studies that the uterus plays a role in
cognition
Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, though ...
in a similar way to the ovaries. A study on rat models found that when the uterus was removed, the rats performed more poorly on
spatial memory
In cognitive psychology and neuroscience, spatial memory is a form of memory responsible for the recording and recovery of information needed to plan a course to a location and to recall the location of an object or the occurrence of an event. Sp ...
tasks. Prof. Bimonte-Nelson, the co-author of the study, explained: "the body's autonomic nervous system, which regulates 'automated' metabolic processes, such as heart rate, breathing, digestion, and sexual arousal, also has links to the uterus and brain." No similar studies have yet been conducted on humans.
Clinical significance
During
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ...
the growth rate of the fetus can be assessed by measuring the
fundal height.
Some
pathological
Pathology is the study of the causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in th ...
states include:
* Accumulation of fluids other than blood or of unknown constitution. One study came to the conclusion that
postmenopausal women with endometrial fluid collection on
gynecologic ultrasonography should undergo
endometrial biopsy if the endometrial lining is thicker than 3 mm or if the endometrial fluid is
echogenic. In cases of a lining 3 mm or less and clear endometrial fluid, endometrial biopsy was not regarded to be necessary, but
endocervical curettage to rule out endocervical cancer was recommended.
*
Hematometra, which is accumulation of blood within the uterus.
*
Prolapse of the uterus
*
Carcinoma of the cervix – malignant neoplasm
*
Carcinoma of the uterus – malignant neoplasm
*
Fibroids – benign neoplasms
*
Adenomyosis
Adenomyosis is a medical condition characterized by the growth of cells that proliferate on the inside of the uterus ( endometrium) atypically located among the cells of the uterine wall ( myometrium), as a result, thickening of the uterus occurs. ...
– ectopic growth of endometrial tissue within the myometrium
*
Endometritis
Endometritis is inflammation of the inner lining of the uterus (endometrium). Symptoms may include fever, lower abdominal pain, and abnormal vaginal bleeding or discharge. It is the most common cause of infection after childbirth. It is also pa ...
, infection at the uterine cavity
*
Pyometra – infection of the uterus, most commonly seen in dogs
*
Asherman's syndrome, also known as intrauterine
adhesions, occurs when the basal layer of the endometrium is damaged by instrumentation (e.g.,
D&C) or infection (e.g., endometrial
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, ...
) resulting in endometrial scarring followed by adhesion formation that partially or completely obliterates the uterine cavity
* Myometritis – inflammation of the muscular uterine wall.
Malformations
Uterine malformations are mainly
congenital malformations, and include ''
uterus didelphys'',
bicornuate uterus and
septate uterus. Congenital absence of the uterus is known as
Müllerian agenesis
Müllerian agenesis, also known as Müllerian aplasia, vaginal agenesis, or Mayer-Rokitansky-Küster-Hauser syndrome (MRKH syndrome), is a congenital malformation characterized by a failure of the Müllerian ducts to develop, resulting in a miss ...
.
Surgery
A
hysterectomy
Hysterectomy is the surgical removal of the uterus. It may also involve removal of the cervix, ovaries ( oophorectomy), Fallopian tubes ( salpingectomy), and other surrounding structures.
Usually performed by a gynecologist, a hysterectomy may b ...
is the surgical removal of the uterus which may be carried out for a number of reasons including the ridding of
tumours both benign and
malignant. 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 performed
gynecological surgical procedure.
Other animals
Most animals that lay
eggs, such as
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s and
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s, including most
ovoviviparous species, have an
oviduct instead of a uterus. However, recent research into the biology of the
viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
(not merely
ovoviviparous)
skink
Skinks are lizards belonging to the family Scincidae, a family in the infraorder Scincomorpha. With more than 1,500 described species across 100 different taxonomic genera, the family Scincidae is one of the most diverse families of lizards. ...
''
Trachylepis
''Trachylepis'' is a skink genus in the subfamily Mabuyinae found mainly in Africa. Its members were formerly included in the " wastebin taxon" ''Mabuya'', and for some time in ''Euprepis''. As defined today, ''Trachylepis'' contains the clade ...
ivensi'' has revealed development of a very close analogue to
eutherian mammalian placental development.
In
monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals ( Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brai ...
s, mammals which lay eggs, namely the
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
and the
echidna
Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the ...
s, either the term ''uterus'' or ''oviduct'' is used to describe the same organ, but the egg does not develop a
placenta
The placenta 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 the physically separate mate ...
within the mother and thus does not receive further nourishment after formation and
fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
.
Marsupials
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
have two uteri, each of which connect to a lateral vagina and which both use a third, middle "vagina" which functions as the birth canal.
Marsupial
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s form a
choriovitelline placenta A choriovitelline placenta is a placenta formed by the yolk sac and chorion. In a choriovitelline placenta, the yolk sac fuses with the chorion and, subsequently, wrinkles develop that hold the embryo to the uterine wall, thus forming the choriovite ...
(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,
bandicoots
Bandicoots are a group of more than 20 species of small to medium-sized, terrestrial, largely nocturnal marsupial omnivores in the order Peramelemorphia. They are endemic to the Australia–New Guinea region, including the Bismarck Archipelago ...
also have a rudimentary chorioallantoic placenta, similar to those of placental mammals.
The
fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
usually develops fully in
placental mammals and only partially in
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s including
kangaroo
Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern ...
s and
opossum
Opossums () are members of the marsupial order Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 93 species in 18 genera. Opossums originated in South America and entered No ...
s. In marsupials the uterus forms as a duplex organ of two uteri. In monotremes such as the
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
, the uterus is duplex and rather than nurturing the embryo, secretes the shell around the egg. It is essentially identical with the
shell gland of birds and reptiles, with which the uterus is
homologous.
In mammals, the four main forms of the uterus are: duplex, bipartite, bicornuate and simplex.
[Lewitus, Eric, and Christophe Soligo.]
Life-history correlates of placental structure in eutherian evolution
." Evolutionary Biology 38.3 (2011): 287-305.
; Duplex: There are two wholly separate uteri, with one fallopian tube each. Found in
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s (such as
kangaroo
Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern ...
s,
Tasmanian devil
The Tasmanian devil (''Sarcophilus harrisii'') ( palawa kani: purinina) is a carnivorous marsupial of the family Dasyuridae. Until recently, it was only found on the island state of Tasmania, but it has been reintroduced to New South Wales ...
s,
opossum
Opossums () are members of the marsupial order Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 93 species in 18 genera. Opossums originated in South America and entered No ...
s, etc.),
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s (such as
mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
,
rats, and
guinea pig
The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent belonging to the genus '' Cavia'' in the family Caviidae. Breeders tend to use the word ''cavy'' to describe the ...
s), and
lagomorpha (
rabbit
Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit sp ...
s and
hare
Hares and jackrabbits are mammals belonging to the genus ''Lepus''. They are herbivores, and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The g ...
s).
; Bipartite: The two uteri are separate for most of their length, but share a single cervix. Found in
ruminants (
deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...
,
moose
The moose (in North America) or elk (in Eurasia) (''Alces alces'') is a member of the New World deer subfamily and is the only species in the genus ''Alces''. It is the largest and heaviest extant species in the deer family. Most adult ma ...
,
elk etc.),
hyrax
Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simila ...
es,
cats, and
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s.
; Bicornuate: The upper parts of the uterus remain separate, but the lower parts are fused into a single structure. Found in
dogs,
pigs,
elephant
Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantida ...
s,
whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s,
dolphin
A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the b ...
s,
and
tarsier
Tarsiers ( ) are haplorhine primates of the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all of its species living today are found in Maritime Southeas ...
s, and
strepsirrhine primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...
s among others.
; Simplex: The entire uterus is fused into a single organ. Found in higher
primates
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
(including
humans" \n\n\n\n\nThe robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visi ...
and
chimpanzees). Occasionally, some individual females (including humans) may have a
bicornuate uterus, a
uterine malformation where 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
File:Scheme female reproductive system-en.svg, Schematic frontal view of female anatomy
File:Gray34.png, Sectional plan of the gravid uterus in the third and fourth month
File:Gray38.png, Fetus in utero, between fifth and sixth months.
File:Slide4DDD.JPG, Uterus
See also
*
Menopause
Menopause, also known as the climacteric, is the time in women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause usually occurs between the age of 47 and 54. Medical professionals often d ...
*
Artificial uterus
*
Social uterus
*
Unicornuate uterus
*
Uterus-like mass
References
External links
* – "The Female Pelvis: Organs in the Female and male Pelvis in situ"
Encyclopedia.comUterus AnatomyUterus Pregnancy
{{Authority control
Organs (anatomy)
Pelvis
Mammal female reproductive system
Human female reproductive system
Women's health