placenta
The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.
DEFINITION:
- placenta
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A fetomaternal organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble, bloodborne substances through an apposition of uterine and trophoblastic vascularized parts. According to species, the area of vascular apposition may be diffuse , cotyledonary , zonary , or discoid ; the nature of apposition may be labyrinthine or villous ; and the intimacy of apposition may vary according to what layers are lost of those originally interposed between maternal and fetal blood (maternal endothelium, uterine connective tissue, uterine epithelium, chorion, extraembryonic mesoderm, and endothelium of villous capillary). The chorion may be joined by and receive blood vessels from either the yolk sac or the allantois, and the uterine lining may be largely shed with the chorion at birth ( deciduate placenta ) or may separate from the chorion and remain ( nondeciduate placenta ). The human placenta is discoid , villous , hemochorial , chorioallantoic , and deciduate . After birth, it weighs about 600 g and is about 16 cm in diameter and 2 cm thick, discounting the decidua basalis and the maternal blood in the intervillous space (a principal functional part into which the chorionic villi dip and which leaks out at birth). The villi are grouped into adjoining cotyledons making about 20 velvety bumps on the side of the placenta facing the uterus; the side facing the fetus is smooth and covered with amnion, a thin avascular layer that continues past the edges of the placenta to line the entire hollow sphere of chorion except where it is reflected to cover the umbilical cord. The umbilical cord joins fetus and placenta and usually joins the placenta near the center, although it sometimes inserts at the edge, on the nonplacental chorion, or on an accessory placenta.
Used in 1 Article abstract
Used in 1 Article abstract