Uterus

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Uterus
Figure 28 02 01.JPG
Image showing different structures around and relating to the human uterus.
Details
Precursor Paramesonephric duct
System Reproductive system
Artery Ovarian artery and uterine artery
Vein Uterine veins
Lymph Body and cervix to internal iliac lymph nodes, fundus to para-aortic lymph nodes, lumbar and superficial inguinal lymph nodes.
Identifiers
Latin uterus
Greek ὑστέρα (hystéra)
MeSH D014599
TA98 A09.1.03.001
TA2 3500
FMA 17558
Anatomical terminology
Different regions of Uterus displayed & labelled using a 3D medical animation still shot 3D Medical Animation Uterus.jpg
Different regions of Uterus displayed & labelled using a 3D medical animation still shot

The uterus (from Latin "uterus", plural uteri) or womb ( /wm/ ) is a major female hormone-responsive secondary sex organ of the reproductive system in humans and most other mammals. Things occurring in the uterus are described with the term in utero. In the human, the lower end of the uterus, the cervix, opens into the vagina, while the upper end, the fundus, is connected to the fallopian tubes. It is within the uterus that the fetus develops during gestation. In the human embryo, the uterus develops from the paramesonephric 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.

Contents

In medicine, and related professions the term uterus is consistently used, while the Germanic-derived term womb is commonly used in everyday contexts.

Structure

The uterus is located within the pelvic region immediately behind and almost overlying the bladder, and in front of the sigmoid 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. [1] [2] 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, the corpus (body), the cervix, and the cervical canal. The cervix protrudes into the vagina. The uterus is held in position within the pelvis by ligaments, 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. [3]

Diagram showing regions of the uterus Gray1167.svg
Diagram showing regions of the uterus

From outside to inside, regions of the uterus include:

Layers

Uterine wall thickness (cm) [4]
LocationMean (mm)Range (mm)
Anterior wall2317 - 25
Posterior wall2115 - 25
Fundus2015 - 22
Isthmus108 - 22

The uterus has three layers, which together form the uterine wall. From innermost to outermost, these layers are the endometrium, myometrium, and perimetrium. [5]

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 functional layer thickens and then is sloughed during the menstrual cycle or estrous cycle. During pregnancy, the uterine glands and blood vessels in the endometrium further increase in size and number and form the decidua. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus. [6] [7]

The myometrium of the uterus mostly consists of smooth muscle. The innermost layer of myometrium is known as the junctional zone, which becomes thickened in adenomyosis. [8]

The perimetrium is a serous layer of visceral peritoneum. It covers the outer surface of the uterus. [9]

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 organisms are present in the uterus and form the uterine microbiome. [10] [11] [12] [13]

Support

Uterus covered by the broad ligament Gray1161.png
Uterus covered by the broad ligament

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. [14]

Major ligaments

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

NameFromTo
Uterosacral ligaments Posterior cervixAnterior face of sacrum
Cardinal ligaments Side of the cervix Ischial spines
Pubocervical ligaments [14] Side of the cervix Pubic symphysis

Axis

Normally, the 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. [15] 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. [1]

Position

The uterus is in the middle of the pelvic cavity in frontal plane (due to ligamentum latum uteri). The fundus does not surpass the linea terminalis, 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 musculo-fibrous apparatus that consists of suspensory and sustentacular part. 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 meaning of these terms are described below:

DistinctionMore commonLess 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 larger pelvic diaphragm in the back and the smaller urogenital diaphragm in the front.

The pathological changes of the position of the uterus are:

In cases where the uterus is "tipped", also known as retroverted uterus, the person may have symptoms of pain during sexual intercourse, pelvic pain during menstruation, minor incontinence, urinary tract infections, fertility difficulties, [16] and difficulty using tampons. A pelvic examination by a doctor can determine if a uterus is tipped. [17]

Blood supply

Vessels of the uterus and its appendages, rear view. Gray589.png
Vessels of the uterus and its appendages, rear view.
Schematic diagram of uterine arterial vasculature seen as a cross-section through the myometrium and endometrium Uterine arterial vasculature.svg
Schematic diagram of uterine arterial vasculature seen as a cross-section through the myometrium and endometrium

The uterus is supplied by arterial blood both from the uterine artery and the ovarian artery. Another anastomotic branch may also supply the uterus from anastomosis of these two arteries.

Nerve supply

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 fetal life. In males, anti-müllerian hormone (AMH) secreted from the testes leads to their regression. In females, these ducts give rise to the Fallopian tubes and the uterus. In humans the lower segments of the two ducts fuse to form a single uterus, however, in cases of uterine malformations this development may be disturbed. The different uterine forms in various mammals are due to various degrees of fusion of the two Müllerian ducts.

Various congenital conditions of the uterus can develop in utero. Though uncommon some of these are a double uterus, didelphic uterus, bicornate uterus and others. [18]

Function

The reproductive function of the uterus is to accept a fertilized ovum which passes through the utero-tubal junction from the fallopian tube. The fertilized ovum divides to become a blastocyst, which implants into the endometrium, and derives nourishment from blood vessels which develop exclusively for this purpose. The fertilized ovum becomes an embryo, attaches to a wall of the uterus, creates a placenta, and develops into a fetus (gestates) until childbirth. Due to anatomical barriers such as the pelvis, 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 and ovaries, and to the external genitals, including the vagina, labia, and clitoris.

There is also some evidence that the uterus plays a role in cognition 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 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." [19] No similar studies have yet been conducted on humans.

Clinical significance

A hysterectomy 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.

During pregnancy the growth rate of the fetus can be assessed by measuring the fundal height.

Some pathological states include:

Transvaginal ultrasonography showing a uterine fluid accumulation in a postmenopausal woman. Endometrial fluid accumulation, postmenopausal.jpg
Transvaginal ultrasonography showing a uterine fluid accumulation in a postmenopausal woman.

Other animals

Most animals that lay eggs, such as birds and reptiles, including most ovoviviparous species, have an oviduct instead of a uterus. However, recent research into the biology of the viviparous (not merely ovoviviparous) skink Trachylepis ivensi has revealed development of a very close analogue to eutherian mammalian placental development. [23]

In monotremes, mammals which lay eggs, namely the platypus and the echidnas, either the term uterus or oviduct is used to describe the same organ, but the egg does not develop a placenta within the mother and thus does not receive further nourishment after formation and fertilization.

Marsupials 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. [24] [25] Marsupial embryos form a choriovitelline 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, bandicoots also have a rudimentary chorioallantoic placenta, similar to those of placental mammals.

The fetus usually develops fully in placental mammals and only partially in marsupials including kangaroos and opossums. In marsupials the uterus forms as a duplex organ of two uteri. In monotremes (egg-laying mammals) such as the platypus, 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. [26]

In mammals, the four main forms of the uterus are: duplex, bipartite, bicornuate and simplex. [27]

Duplex
There are two wholly separate uteri, with one fallopian tube each. Found in marsupials (such as kangaroos, Tasmanian devils, opossums, etc.), rodents (such as mice, rats, and guinea pigs), and lagomorpha (rabbits and hares).
Bipartite
The two uteri are separate for most of their length, but share a single cervix. Found in ruminants (deer, moose, elk etc.), hyraxes, cats, and horses.
Bicornuate
The upper parts of the uterus remain separate, but the lower parts are fused into a single structure. Found in dogs, pigs, elephants, whales, dolphins, [28] and tarsiers, and strepsirrhine primates among others.
Simplex
The entire uterus is fused into a single organ. Found in higher primates (including humans 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

1. Vulva; 9. Vagina;
14. Uterus: Parts: 15. Cervix; 16. Body and 17. Fundus. 18. Orifices: External and Internal; 19. Cervical canal; 20. Uterine cavity; Layers: 21. Endometrium; 22. Myometrium and 23. Perimetrium
24. Fallopian tube
30. Ovary
31. Visceral pelvic peritoneum: 32. Broad ligament (with 35. Mesometrium)
Ligaments: 36. Round
Blood vessels: 40. Uterine artery and veins
Other: 42. Ureter; 46. Internal iliac vessels (anterior branches); 48. Abdominal cavity Scheme female reproductive system-number-full-cropped.svg
1. Vulva ; 9. Vagina ;
14. Uterus : Parts: 15.  Cervix; 16. Body and 17. Fundus. 18. Orifices: External and Internal; 19.  Cervical canal; 20.  Uterine cavity; Layers: 21.  Endometrium; 22.  Myometrium and 23.  Perimetrium
24. Fallopian tube
30. Ovary
31. Visceral pelvic peritoneum : 32.  Broad ligament (with 35.  Mesometrium)
Ligaments: 36.  Round
Blood vessels: 40.  Uterine artery and veins
Other: 42.  Ureter; 46.  Internal iliac vessels (anterior branches); 48.  Abdominal cavity

See also

Related Research Articles

Cervix The lower part of the uterus in the human female reproductive system

The cervix or cervix uteri is the lower part of the uterus in the human female reproductive system. The cervix is usually 2 to 3 cm long and roughly cylindrical in shape, which changes during pregnancy. The narrow, central cervical canal runs along its entire length, connecting the uterine cavity and the lumen of the vagina. The opening into the uterus is called the internal os, and the opening into the vagina is called the external os. The lower part of the cervix, known as the vaginal portion of the cervix, bulges into the top of the vagina. The cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago.

Endometrium Inner mucous membrane of the mammalian uterus

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 functional layer thickens and then is shed during menstruation in humans and some other mammals, including apes, Old World monkeys, some species of bat, the elephant shrew and the Cairo spiny mouse. In most other mammals, the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus. The speculated presence of an endometrial microbiota has been argued against.

Placenta Organ that connects the foetus to the uterine wall

The placenta is a temporary 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 maternal and fetal circulations, and is an important endocrine organ producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the baby via the umbilical cord, and on the opposite aspect to the maternal uterus in a species dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

Amnion Innermost membranous sac that surrounds and protects the developing embryo

The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus. The amnion, along with the chorion, the yolk sac and the allantois form a protective sac around the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus.

Blastocyst Structure formed around day 5 of mammalian embryonic development

The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos and κύστις kystis. In other animals this is called a blastula.

Female reproductive system Reproductive system of human females

The female reproductive system is made up of the internal and external sex organs that function in reproduction of new offspring. In humans, the female reproductive system is immature at birth and develops to maturity at puberty to be able to produce gametes, and to carry a foetus to full term. The internal sex organs are the uterus, Fallopian tubes, and ovaries. The uterus or womb accommodates the embryo which develops into the foetus. The uterus also produces vaginal and uterine secretions which help the transit of sperm to the Fallopian tubes. The ovaries produce the ova. The external sex organs are also known as the genitals and these are the organs of the vulva including the labia, clitoris, and vaginal opening. The vagina is connected to the uterus at the cervix.

Adenomyosis Extension of endometrial tissue into the myometrium

Adenomyosis is a medical condition characterized by the growth of cells that build up the inside of the uterus (endometrium) atypically located within the cells that put up the uterine wall (myometrium), as a result, thickening of the uterus occurs. As well as being misplaced in patients with this condition, endometrial tissue is completely functional. The tissue thickens, sheds and bleeds during every menstrual cycle.

Vaginal bleeding is any bleeding from the vagina. This bleeding may originate from the uterus, vaginal wall, or cervix. Generally, it is either part of a normal menstrual cycle or is caused by hormonal or other problems of the reproductive system, such as abnormal uterine bleeding.

Endometritis Medical condition

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 part of spectrum of diseases that make up pelvic inflammatory disease.

Decidua

The decidua is the modified mucosal lining of the uterus that forms in preparation for pregnancy. It is formed in a process called decidualization under the influence of progesterone. Endometrial cells become highly characteristic. The decidua forms the maternal part of the placenta and remains for the duration of the pregnancy. It is shed off during childbirth—hence why the term is used, "decidua" having the meaning of falling away, as in the word deciduous.

Round ligament of uterus

The round ligament of the uterus is a ligament that connects the uterus to the labia majora.

Cytotrophoblast

"Cytotrophoblast" is the name given to both the inner layer of the trophoblast or the cells that there live. It is interior to the syncytiotrophoblast and external to the wall of the blastocyst in a developing embryo.

Bicornuate uterus Medical condition

A bicornuate uterus or bicornate uterus, is a type of mullerian anomaly in the human uterus, where there is a deep indentation at the fundus (top) of the uterus.

Vesico-uterine pouch

In human female anatomy, the vesico-uterine pouch, also known by various names, is a fold of peritoneum over the uterus and the urinary bladder. Like the recto-uterine pouch, it is a female pelvic recess. However, it is a shallower pouch close to the anterior fornix of the vagina.

Endometrial biopsy

The endometrial biopsy is a medical procedure that involves taking a tissue sample of the lining of the uterus. The tissue subsequently undergoes a histologic evaluation which aids the physician in forming a diagnosis.

Arcuate arteries located in the uterus branch out and supply blood to different layers of the uterus. These arteries meet the myometrial-endometrial junction and lead to straight and endometrial arteries. The endometrium receives blood from endometrial arteries which are also called spiral arteries. Endometrial arteries proliferate rapidly and react to different hormones released. These hormones are progesterone and estrogen released by the ovaries and produced by the endocrine system. The endometrial arteries not only supply blood to the endometrium but are also important during pregnancy. They are the initial site of transportation of blood from the mother to the baby.

Placental expulsion

Placental expulsion occurs when the placenta comes out of the birth canal after childbirth. The period from just after the baby is expelled until just after the placenta is expelled is called the third stage of labor.

Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.

Mammalian reproduction Most mammals are viviparous, giving birth to live young

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.

Mullerian duct anomalies are those structural anomalies caused by errors in müllerian-duct development during embryonic morphogenesis. Factors that precipitate include genetics, and maternal exposure to teratogens.

References

  1. 1 2 Manual of Obstetrics. (3rd ed.). Elsevier 2011. pp. 1–16. ISBN   9788131225561.
  2. Donita, D'Amico (2015). Health & physical assessment in nursing. Barbarito, Colleen (3rd ed.). Boston: Pearson. p. 645. ISBN   9780133876406. OCLC   894626609.
  3. Gray's Anatomy for Students, 2nd edition
  4. Nandita Palshetkar; Rishma Dhillon Pai; Hrishikesh D Pai (2012-09-30). Textbook of Hysteroscopy. JP Medical Ltd. pp. 135–. ISBN   978-93-5025-781-4.
  5. Tortora, G; Derrickson, B (2011). Principles of anatomy & physiology (13th. ed.). Wiley. p. 1105. ISBN   9780470646083.
  6. Blue Histology - Female Reproductive System Archived 2007-02-21 at the Wayback Machine . School of Anatomy and Human Biology — The University of Western Australia Accessed 20061228 20:35
  7. Guyton AC, Hall JE, eds. (2006). "Chapter 81 Female Physiology Before Pregnancy and Female Hormones". Textbook of Medical Physiology (11th ed.). Elsevier Saunders. pp. 1018ff. ISBN   9780721602400.
  8. "NCI Dictionary of Cancer Terms". National Cancer Institute. Archived from the original on 2017-12-26. Retrieved 2017-12-27.
  9. Ross, Michael H.; Pawlina, Wojciech. Histology, a text and atlas (Sixth ed.). p. 848.
  10. Franasiak, Jason M.; Scott, Richard T. (2015). "Reproductive tract microbiome in assisted reproductive technologies". Fertility and Sterility. 104 (6): 1364–1371. doi: 10.1016/j.fertnstert.2015.10.012 . ISSN   0015-0282. PMID   26597628.
  11. Verstraelen, Hans; Vilchez-Vargas, Ramiro; Desimpel, Fabian; Jauregui, Ruy; Vankeirsbilck, Nele; Weyers, Steven; Verhelst, Rita; De Sutter, Petra; Pieper, Dietmar H.; Van De Wiele, Tom (2016). "Characterisation of the human uterine microbiome in non-pregnant women through deep sequencing of the V1-2 region of the 16S rRNA gene". PeerJ. 4: e1602. doi:10.7717/peerj.1602. ISSN   2167-8359. PMC   4730988 . PMID   26823997.
  12. Mor, Gil; Kwon, Ja-Young (2015). "Trophoblast-microbiome interaction: a new paradigm on immune regulation". American Journal of Obstetrics and Gynecology. 213 (4): S131–S137. doi:10.1016/j.ajog.2015.06.039. ISSN   0002-9378. PMC   6800181 . PMID   26428492.
  13. Payne, Matthew S.; Bayatibojakhi, Sara (2014). "Exploring Preterm Birth as a Polymicrobial Disease: An Overview of the Uterine Microbiome". Frontiers in Immunology. 5: 595. doi:10.3389/fimmu.2014.00595. ISSN   1664-3224. PMC   4245917 . PMID   25505898.
  14. 1 2 The Pelvis University College Cork Archived from the original on 2008-02-27
  15. Snell, Clinical Anatomy by regions, 8th edition
  16. "Retroverted Uterus: What it is & How it Affects Pregnancy". www.womens-health.co.uk. Archived from the original on 2013-10-05.
  17. Tipped Uterus:Tilted Uterus Archived 2011-02-24 at the Wayback Machine AmericanPregnancy.org. Accessed 25 March 2011
  18. "Surgical Correction of Uterovaginal Anomalies | GLOWM". www.glowm.com. Archived from the original on 2017-12-28. Retrieved 2017-12-27.
  19. "The uterus plays a role in memory, study finds". Medical News Today. medicalnewstoday.com. Retrieved 24 September 2019.
  20. "Cervical Stenosis - Women's Health Issues - Merck Manuals Consumer Version". Merck Manuals Consumer Version. Retrieved 2018-11-07.
  21. Takacs P, De Santis T, Nicholas MC, Verma U, Strassberg R, Duthely L (November 2005). "Echogenic endometrial fluid collection in postmenopausal women is a significant risk factor for disease". J Ultrasound Med. 24 (11): 1477–81. doi: 10.7863/jum.2005.24.11.1477 . PMID   16239648. S2CID   20258522.
  22. "Myometritis - Medical Definition from MediLexicon". www.medilexicon.com. Archived from the original on 2016-12-29.
  23. Blackburn, D. G.; Flemming, A. F. (2011). "Invasive implantation and intimate placental associations in a placentotrophic African lizard, Trachylepis ivensi (scincidae)". Journal of Morphology. 273 (2): 137–59. doi:10.1002/jmor.11011. PMID   21956253. S2CID   5191828.
  24. Hugh Tyndale-Biscoe; Marilyn Renfree (30 January 1987). Reproductive Physiology of Marsupials. Cambridge University Press. ISBN   978-0-521-33792-2.
  25. Ronald M. Nowak (7 April 1999). Walker's Mammals of the World. JHU Press. ISBN   978-0-8018-5789-8.
  26. Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, Pennsylvania: Holt-Saunders International. pp. 390–392. ISBN   0-03-910284-X.
  27. Lewitus, Eric, and Christophe Soligo. "Life-history correlates of placental structure in eutherian evolution Archived 2017-09-06 at the Wayback Machine ." Evolutionary Biology 38.3 (2011): 287-305.
  28. Bernd Würsig; William F. Perrin; J.G.M. Thewissen (26 February 2009). Encyclopedia of Marine Mammals. Academic Press. ISBN   978-0-08-091993-5.