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The intimate association or fusion of a tissue or organ of the embryonic stage of an animal to its parent for physiological exchange to promote the growth and development of the young. It enables the young, retained within the body or tissues of the mother, to respire, acquire nourishment, and eliminate wastes by bringing the bloodstreams of mother and young into close association but never into direct connection. Placentation characterizes the early development of all mammals except the egg-laying duckbill platypus and spiny anteater. It occurs in some species of all other orders of vertebrates except the birds. In fact, in certain sharks and reptiles it is almost as well developed as in mammals. A few examples are also known among invertebrates (Peripatus, certain tunicates, and insects). See Fetal membrane

Block removed from center of human placentaenlarge picture
Block removed from center of human placenta

Efficient interchange depends on close proximity of large areas of fetal tissues to maternal blood and glandular areas. This is provided in mammals by a remarkable regulatory cooperation between the developing outer layer (trophoblast) of the chorion, together with the vascular yolk sac or allantois or both, and the mother's uterine lining (endometrium). In the typical mammalian placenta, which is always formed by the chorion and the allantoic vessels, the fetal and maternal bloodstreams are as close as a few thousandths of a millimeter from each other (see illustration). The surface area of the fetal villi which contain the functional fetal capillaries is probably several times larger than the body surface of the female. In humans this ratio is known to be about 8:1.



the location of the placenta in the ovary of an angiospermous plant. A distinction is made between laminar placentation, in which the placentas are located in the inner surface of the carpel (on the sides or along the midrib), and marginal placentation, in which the placentas are located near the margins of the carpel.

Syncarpous gynoecia are marked by central marginal placentation: the placentas are located near the margins of the carpel, in the corners of the locules of the ovary, and along the ovary’s longitudinal axis. Parietal marginal placentation—the disposition of the placentas on the ovary wall near the margins of the carpel—is characteristic of paracarpous gynoecia. With free central placentation the ovules are borne by a central column; this arrangement characterizes lysicarpous gynoecia. In apical and basal placentation, the placentas are located at the apex or base of the ovary, respectively. The most primitive type of placentation is laminar, with placentas scattered over the inner surfaces of the carpel.


The attachment of ovules along the inner ovarian wall by means of the placenta.
The formation and fusion of the placenta to the uterine wall.
References in periodicals archive ?
1); second diagnostic criterion: at least one statistically significant MRI sign is indicative of MAP diagnosis and no sign or statistically nonsignificant signs is a normal result; third diagnostic criterion: at least two statistically significant MRI signs is indicative of MAP diagnosis and no sign, or statistically nonsignificant signs, or only one statistically significant sign are indicative of normal placentation.
These mechanisms are regarded as leading to a loss in placentation and placental dysfunction (Brook et al.
It has recently been proved that anti-CagA antibodies show an in vitro cross reaction on placental tissue that in turn decreases its invasiveness ability [23]; abnormal placentation might lead to a consequent IUGR.
Chorioallantoic and yolk sac placentation in Trichomys laurentinus [Echimydae] and the Evolution of histricognath Rodents.
The placentation period is from day 13 to 18 day, during which extensive organogenesis occurs followed by fetal growth stage from day18 to 28.
2009) Syncytin-A knockout mice demonstrate the critical role in placentation of a fusogenic, endogenous retrovirus-derived, envelope gene.
The characteristics of a typical Scrophularia include; asymmetrical mostly tubular flowers, ovaries with axile placentation and numerous ovules, capsular fruits, and seeds with endosperm, each are shared with one or several related families [3].
Some biological suggestions as to the cause of this include scarring, adhesions and abnormal placentation (Murphy, Stirrat, & Heron) and psychosocial factors, such as a reluctance to get pregnant (Gilliam, 2006).
Several studies were also performed to investigate IMA levels in complicated pregnancies such as preeclampsia, intrauterine growth restriction (IUGR), and recurrent pregnancy loss [6, 7] and serum IMA levels from women with preeclampsia as a consequence of defective endovascular trophoblast invasion were detected to be higher than those with normal placentation in first trimester [6] and serum IMA levels were significantly higher in women with recurrent pregnancy loss as an abnormally high hypoxic intrauterine environment which may be associated with abnormal placental development [7].
Repeat caesarean sections become increasingly complicated as the probability of internal abdominal adhesions, bladder injuries, and abnormal placentation (placenta praevia or placenta accreta) increases dramatically, with placenta accreta reportedly affecting 50-67% of women having three or more caesarean sections.