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The placenta is a temporary fetal
organ Organ may refer to: Biology * Organ (anatomy), 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 ...
that begins developing from the
blastocyst 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 lay ...
shortly after
implantation Implantation may refer to: * Implantation (human embryo), in which the human embryo adheres to the wall of the uterus * Implant (medicine), insertion of implants * Endometrial transplantation, as part of the theory of retrograde menstruation in end ...
. 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 The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neura ...
producing
hormones A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate physiology and / or behavior. Hormones are required for the ...
that regulate both
maternal A mother is the female parent of a child. Mothers are women who inhabit or perform the role of bearing some relation to their children, who may or may not be their biological offspring. Thus, dependent on the context, women can be considered mo ...
and
fetal A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal de ...
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical and phys ...
during
pregnancy Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but can occu ...
. The placenta connects to the baby via the
umbilical cord In placental mammals, the umbilical cord (also called the navel string, birth cord or funiculus umbilicalis) is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologically an ...
, and on the opposite aspect to the maternal
uterus The uterus (from Latin "uterus", plural ''uteri'') or womb () 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 u ...
in a
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexe ...
dependent manner. In humans, a thin layer of maternal decidual (
endometrial 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 mammal ...
) tissue comes away with the placenta when it is expelled from the uterus following birth (sometimes incorrectly referred to as the 'maternal part' of the placenta). Placentas are a defining characteristic of
placental mammal Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The placentals are partly distinguished from other mammals in that the fetus is carried in the uterus of its mot ...
s, but are also found in
marsupials Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch. Well-kn ...

marsupials
and some non-mammals with varying levels of development.Pough ''et al.'' 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education, Inc., 2002. Mammal placentas probably first evolved about 150 million to 200 million years ago. The protein
syncytin Syncytin-1 also known as enverin is a protein found in humans and other primates that is encoded by the ERVW-1 gene (endogenous retrovirus group W envelope member 1). Syncytin-1 is a cell-cell fusion protein whose function is best characterized in ...
, found in the outer barrier of the placenta (the syncytiotrophoblast) between mother and baby, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient
retrovirus A retrovirus is a type of virus that inserts a copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme ...
: essentially a "good" virus that helped pave the transition from egg-laying to live-birth. The word ''placenta'' comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant language ...
word for a type of
cake Cake is a form of sweet food made from flour, sugar, and other ingredients, that is usually baked. In their oldest forms, cakes were modifications of bread, but cakes now cover a wide range of preparations that can be simple or elaborate, ...
, from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of ...
πλακόεντα/πλακοῦντα ''plakóenta/plakoúnta'', accusative of πλακόεις/πλακούς ''plakóeis/plakoús'', "flat, slab-like", in reference to its round, flat appearance in humans. The classical plural is ''placentae'', but the form ''placentas'' is common in modern English and probably has the wider currency at present.


Phylogenetic diversity

Although all mammalian placentae have the same functions, there are important differences in structure and function in different groups of mammals. For example, human, bovine, equine and canine placentae are very different at the both gross and the microscopic levels. Placentae of these species also differ in their ability to provide maternal immunoglobulins to the fetus.


Structure

Placental mammals, such as humans, have a chorioallantoic placenta that forms from the
chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline mem ...

chorion
and
allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo ...
. In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by an
umbilical cord In placental mammals, the umbilical cord (also called the navel string, birth cord or funiculus umbilicalis) is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologically an ...
of approximately 55–60 cm (22–24 inch) in length, which contains two
umbilical arteries The umbilical artery is a paired artery (with one for each half of the body) that is found in the abdominal and pelvic regions. In the fetus, it extends into the umbilical cord. Structure Development The umbilical arteries supply deoxygenated blo ...
and one
umbilical vein The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood volu ...
. The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called
cotyledon (''Cercis siliquastrum'') seedling. ; the cotyledon itself remains within the seed seeds split in half showing the embryos with cotyledons and primordial root. with seven cotyledons. Image:Mimosa pudica - cotyledon.jpg, ''Mimosa pudica'' seedli ...
s. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species. The placenta occasionally takes a form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, three, four, or more. Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, and so on. If there is a clearly discernible main lobe and auxiliary lobe, the latter is called a ''succenturiate placenta''. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during childbirth, labor, which is called ''vasa praevia, vasa previa''.


Gene and protein expression

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta. Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific. The corresponding specific proteins are mainly expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the Cancer/testis antigens, cancer testis antigen PAGE4 and expressed in cytotrophoblasts, Human placental lactogen, CSH1 and Kisspeptin, KISS1 expressed in syncytiotrophoblasts, and PAPPA2 and Major basic protein, PRG2 expressed in extravillous trophoblasts.


Physiology


Development

The placenta begins to develop upon implantation of the
blastocyst 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 lay ...
into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta. The placenta grows throughout
pregnancy Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but can occu ...
. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).


Placental circulation


Maternal placental circulation

In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal Chorionic villi, villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal
chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline mem ...

chorion
, though no fluid is exchanged. As the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins. Maternal blood flow is approximately 600–700 ml/min at term. This begins at day 5 - day 12


Fetoplacental circulation

Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier"). Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation. On the other hand, there is no neural vascular regulation, and catecholamines have only little effect. The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive free radicals. This may contribute to pre-eclampsia and other pregnancy complications. It is proposed that melatonin plays a role as an antioxidant in the placenta. This begins at day 17 - day 22


Birth

Placental expulsion begins as a physiological separation from the wall of the uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor". The placenta is usually expelled within 15–30 minutes of birth. Placental expulsion can be managed actively, for example by giving oxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and the risk of postpartum bleeding may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary. The habit is to cut the cord immediately after birth, but it is theorised that there is no medical reason to do this; on the contrary, it is theorised that not cutting the cord helps the baby in its adaptation to extrauterine life, especially in preterm infants.


Microbiome

The placenta is traditionally thought to be Asepsis, sterile, but recent research suggests that a resident, Nonpathogenic organisms, non-pathogenic, and diverse population of microorganisms may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research.


Functions


Nutrition and gas exchange

The placenta intermediates the transfer of nutrients between mother and fetus. The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and Carbon dioxide#Human physiology, carbon dioxide back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via both Active transport, active and passive transport. Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients. Adverse pregnancy situations, such as those involving maternal diabetes or obesity, can increase or decrease levels of nutrient transporters in the placenta potentially resulting in overgrowth or restricted growth of the fetus.


Excretion

Waste products excreted from the fetus such as urea, uric acid, and creatinine are transferred to the maternal blood by diffusion across the placenta.


Immunity

IgG antibodies can pass through the human placenta, thereby providing protection to the fetus ''in utero''. This transfer of antibodies begins as early as the 20th week of gestational age, and certainly by the 24th week. This passive immunity lingers for several months after birth, thus providing the newborn with a carbon copy of the mother's long-term humoral immunity to see the infant through the crucial first months of extrauterine life. IgM, however, cannot cross the placenta, which is why some infections acquired ''during'' pregnancy can be hazardous for the fetus. Furthermore, the placenta functions as a selective maternal-fetal barrier against transmission of microbes. However, insufficiency in this function may still cause mother-to-child transmission of infectious diseases.


Endocrine function

* The first hormone released by the placenta is called the human chorionic gonadotropin hormone. This is responsible for stopping the process at the end of menses when the Corpus luteum ceases activity and atrophies. If hCG did not interrupt this process, it would lead to spontaneous abortion of the fetus. The corpus luteum also produces and releases progesterone and estrogen, and hCG stimulates it to increase the amount that it releases. hCG is the indicator of pregnancy that pregnancy tests look for. These tests will work when menses has not occurred or after implantation has happened on days seven to ten. hCG may also have an anti-antibody effect, protecting it from being rejected by the mother's body. hCG also assists the male fetus by stimulating the testes to produce testosterone, which is the hormone needed to allow the sex organs of the male to grow. * Progesterone helps the embryo implant by assisting passage through the fallopian tubes. It also affects the Fallopian tube, fallopian tubes and the
uterus The uterus (from Latin "uterus", plural ''uteri'') or womb () 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 u ...
by stimulating an increase in secretions necessary for fetal nutrition. Progesterone, like hCG, is necessary to prevent spontaneous abortion because it prevents contractions of the uterus and is necessary for implantation. * Estrogen is a crucial hormone in the process of proliferation. This involves the enlargement of the breasts and uterus, allowing for growth of the fetus and production of milk. Estrogen is also responsible for increased blood supply towards the end of pregnancy through vasodilation. The levels of estrogen during pregnancy can increase so that they are thirty times what a non-pregnant woman mid-cycles estrogen level would be. * Human placental lactogen is a hormone used in pregnancy to develop fetal metabolism and general growth and development. Human placental lactogen works with Growth hormone to stimulate Insulin-like growth factor production and regulating intermediary metabolism. In the fetus, hPL acts on lactogenic receptors to modulate embryonic development, metabolism and stimulate production of IGF, insulin, surfactant and Adrenocortical hormone, adrenocortical hormones. hPL values increase with multiple pregnancies, intact molar pregnancy, Diabetes mellitus, diabetes and Rh incompatibility. They are decreased with Toxemia of pregnancy, toxemia, choriocarcinoma, and Placental insufficiency.


Immunological barrier

The placenta and fetus may be regarded as a foreign body inside the mother and must be protected from the normal immune response of the mother that would cause it to be Transplant rejection, rejected. The placenta and fetus are thus treated as sites of immune privilege, with immune tolerance. For this purpose, the placenta uses several mechanisms: *It secretes Neurokinin B-containing phosphocholine molecules. This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host (biology), host. *There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2. However, the Placental barrier is not the sole means to evade the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier.


Other

The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor.


Clinical significance

Image:CMV placentitis1 mini.jpg, 160px, Micrograph of a cytomegalovirus, cytomegalovirus (CMV) infection of the placenta (CMV placentitis). The characteristic large cell nucleus, nucleus of a CMV-infected cell (biology), cell is seen off-centre at the bottom-right of the image. H&E stain. Numerous pathologies can affect the placenta. * Placenta accreta, when the placenta implants too deeply, all the way to the actual muscle of uterine wall (without penetrating it) * Placenta praevia, when the placement of the placenta is too close to or blocks the cervix * Placental abruption/abruptio placentae, premature detachment of the placenta * Placentitis, inflammation of the placenta, such as by TORCH infections.


Society and culture

The placenta often plays an important role in various cultures, with many societies conducting rituals regarding its disposal. In the Western world, the placenta is most often incinerated. Some cultures Burial, bury the placenta for various reasons. The Māori people, Māori of New Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth. Likewise, the Navajo people, Navajo bury the placenta and umbilical cord at a specially chosen site, particularly if the baby dies during birth. In Cambodia and Costa Rica, burial of the placenta is believed to protect and ensure the health of the baby and the mother. If a mother dies in childbirth, the Aymara people, Aymara of Bolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life. The placenta is believed by some communities to have power over the lives of the baby or its parents. The Kwakiutl of British Columbia bury girls' placentas to give the girl skill in digging clams, and expose boys' placentas to ravens to encourage future prophetic visions. In Turkey, the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In Transylvania, and Japan, interaction with a disposed placenta is thought to influence the parents' future fertility. Several cultures believe the placenta to be or have been alive, often a relative of the baby. Nepalese think of the placenta as a friend of the baby; Malaysian Orang Asli regard it as the baby's older sibling. Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child. Various cultures in Indonesia, such as Javanese people, Javanese, believe that the placenta has a spirit and needs to be buried outside the family house. In some cultures, the placenta is eaten, a practice known as placentophagy. In some eastern cultures, such as China, the dried placenta (''ziheche'' , literally "purple river car") is thought to be a healthful restorative and is sometimes used in preparations of traditional Chinese medicine and various health products. The practice of human placentophagy has become a more recent trend in western cultures and is not without Human placentophagy#Controversy, controversy; its practice being considered cannibalism is debated. Some cultures have alternative uses for placenta that include the manufacturing of cosmetics, pharmaceuticals and food.


Additional images

Image:Gray30.png, Fetus of about 8 weeks, enclosed in the amnion. Magnified a little over two diameters. File:Placenta with fetal membranes.jpg, Placenta with attached fetal membranes, ruptured at the margin at the left in the image. Image:CMV_placentitis1.jpg, Micrograph of a placental infection (cytomegalovirus, CMV placentitis). Image:CMV_placentitis2.jpg, Micrograph of CMV placentitis. File:Placenta vasculature 3D power doppler 00001.gif, A 3D Power doppler image of vasculature in 20-week placenta File:2910 The Placenta-02.jpg, Schematic view of the placenta File:Human placenta 01.JPG, Maternal side of a whole human placenta, just after birth File:Human placenta 02.JPG, Fetal side of same placenta File:Fetal_side_close-up_of_freshly_delivered_placenta.jpeg, Close-up of umbilical attachment to fetal side of freshly delivered placenta File:Placenta weight by gestational age.svg, Placenta weight by gestational age File:Ziheche.jpg, ''Ziheche'' (紫河车), dried human placenta used in traditional Chinese medicine


See also

*Choriovitelline placenta *Caul *Zygote *Pregnancy in fish


References


External links


Placenta
a
the Human Protein Atlas




gynob.com, with quotes from Williams Obstetrics, 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.
Evaluation of Hydropic Placentas
by Miller RT, (PDF), ProPath.
Break on through: How some viruses infect the placenta
by Jyoti Madhusoodanan, ''Knowable Magazine''. {{Authority control Vertebrate developmental biology Embryology of cardiovascular system Organs (anatomy) Reproductive system Microbiomes Human female endocrine system