Also found in: Dictionary, Thesaurus, Medical, Wikipedia.


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.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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 Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Accreta placentation: a systematic review of prenatal ultrasound imaging and grading of villous invasiveness.
The reproductive significance of these days is manifold; day 18 is the completion of placentation period in the rabbits and the placenta become matured.
Furthermore, attenuated expression of HAPLNs in malignant gliomas (38) results in similar characteristics with trophoblastic invasion, suggesting that HAPLNs are an important factor for normal placentation, which failures in PE.
Serum angiogenic profile in abnormal placentation. J Matern Fetal Neonatal Med 2016;29:3193-7.
Over the years, biophysical and biochemical markers were identified as possible early indicators of failures in the complex placentation process, which would lead to preeclampsia.
The regulation of metalloproteinase (MMP) secretion in the maternal-fetal interface plays a pivotal role in the process of trophoblast invasion and placentation. MMPs are known to be secreted by trophoblastic cells, and studies suggest that they are also expressed by maternal endometrial stromal cells and immunological cells which migrate into the uterus during perimenstrual and early pregnancy phases [10, 18].
On attempt to deliver the placenta, it was adherent to the uterus, consistent with invasive placentation. The placenta was left in situ and a supracervical hysterectomy was performed.
As a result, compromised placentation has been linked to the etiology of PE.
Patients who did not need hysterectomy because of noted clinical signs of physiologic placentation were considered normal.
Strong risk factors for abnormal placentation include previous caesarean scars and placenta previa.
Duvekot, "Conservative management of abnormally invasive placentation," Obstetrical and Gynecological Survey, vol.