Placentation
Encyclopedia
In biology, placentation refers to the formation, type and structure, or arrangement of placenta
s. The function of placentation is to transfer nutrients from maternal tissue to a growing embryo
. Placentation is best known in pregnant female mammals (eutheria
), but also occurs in other animals, eggs (yolk sac
placentation) and flowering plant
s.
. The developing fetus
is connected to it via an umbilical cord
. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are:
In hemochorial placentation maternal blood comes in direct contact with the fetal chorion, which it does not in the other two types. It may avail for more efficient transfer of nutrients etc., but is also more challenging for the systems of gestational immune tolerance to avoid rejection of the fetus.
During pregnancy
, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessel
s in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.
In the case of twin
s, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentation occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in pre-eclampsia
.
s, placentation occurs where the ovule
s are attached inside the ovary
. The ovules inside a flower
's ovary (which later become the seed
s inside a fruit
) are attached via funiculi
, the plant part equivalent to an umbilical cord. The part of the ovary where the funiculus attaches is referred to as the placenta.
In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include:
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
s. The function of placentation is to transfer nutrients from maternal tissue to a growing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. Placentation is best known in pregnant female mammals (eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
), but also occurs in other animals, eggs (yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
placentation) and flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s.
Placentation in mammals
In placental mammals, the placenta forms after the embryo implants into the wall of the uterusUterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
. The developing fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
is connected to it via an umbilical cord
Umbilical cord
In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are:
- endotheliochorial placentation (e.g. in most carnivoreCarnivoreA carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
s like catCatThe cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s and dogDogThe domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s) - epitheliochorial placentation (e.g. in ruminantRuminantA ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...
s, horseHorseThe horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, whaleWhaleWhale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, lower primatesStrepsirrhiniThe clade Strepsirrhini is one of the two suborders of primates. Madagascar's only non-human primates are strepsirrhines, and others can be found in southeast Asia and Africa...
) - hemochorial placentation (e.g. in higher order primatesCatarrhiniCatarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...
, including humanHumanHumans are the only living species in the Homo genus...
s, and also in guinea pigGuinea pigThe guinea pig , also called the cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not in the pig family, nor are they from Guinea...
s, rabbitRabbitRabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s, miceMICE-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...
, and ratsRATSRATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
)
In hemochorial placentation maternal blood comes in direct contact with the fetal chorion, which it does not in the other two types. It may avail for more efficient transfer of nutrients etc., but is also more challenging for the systems of gestational immune tolerance to avoid rejection of the fetus.
During pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.
In the case of twin
Twin
A twin is one of two offspring produced in the same pregnancy. Twins can either be monozygotic , meaning that they develop from one zygote that splits and forms two embryos, or dizygotic because they develop from two separate eggs that are fertilized by two separate sperm.In contrast, a fetus...
s, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentation occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in pre-eclampsia
Pre-eclampsia
Pre-eclampsia or preeclampsia is a medical condition in which hypertension arises in pregnancy in association with significant amounts of protein in the urine....
.
Placentation in plants
In flowering plantFlowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s, placentation occurs where the ovule
Ovule
Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
s are attached inside the ovary
Ovary (plants)
In the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the pistil which holds the ovule and is located above or below or at the point of connection with the base of the petals and sepals...
. The ovules inside a flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
's ovary (which later become the seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s inside a fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
) are attached via funiculi
Funiculus
The cerebrospinal fibers consist of numerous nerve fibers collected together and enclosed in membranous sheaths. A small bundle of fibers, enclosed in a tubular sheath, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are...
, the plant part equivalent to an umbilical cord. The part of the ovary where the funiculus attaches is referred to as the placenta.
In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include:
- Basal placentation: The placenta is at the base (bottom) of the ovary. Simple or compound carpel.
- Apical placentation: The placenta is at the apex (top) of the ovary. Simple or compound carpel.
- Parietal placentation: The placentas are in the ovary wall within a non-sectioned ovary. Compound carpel.
- Axile placentation: The ovary is sectioned by radial spokes with placentas in separate loculeLoculeA locule is a small cavity or compartment within an organ or part of an organism ....
s. Compound carpel. - Free or central placentation: The placentas are in a central column within a non-sectioned ovary. Compound carpel.
- Marginal placentation: There is only one elongated placenta on one side of the ovary. This is conspicuous in legumes. Simple carpel.