Placenta

Introduction

An organ of metabolic exchange between fetus & mother. It has both embryonic (fetal) & maternal components. The embryonic component is formed from the outermost embryonic membrane (chorion forndosum); the maternal component is formed from the uterine mucosa (decidua basalis).

Within the placenta, chorionic villi contain capillaries carrying blood of embryonic circulation. Maternal blood fills intervillous spaces creating an interface of contact. No direct mixing of blood occurs; bur the placental membrane separating the maternal & fetal circulation is thin & permeable to small molecules including nutrients, metabolites, many pharmaceutical agents, & IgG. The placenta secretes the hormone chorionic gonadotropin, found in the maternal circulation & measured clinically as an indicator of pregnancy.
At term, the human placenta is disk shaped, about 4 cm in thickness & 18 cm in diameter. Its weight is about 15% of the weight of the fetus. The fetal surface is smooth with the umbilical cord normally attached near its center. The maternal surface of a detached placenta is rough because of the torn decidual tissue adhering to the chorion. Lobular elevations in the maternal surface are called cotyledons or lobes.

Stedman's Medical Dictionary 27th ed, Williams & Wilkins, Baltimore, 1999

placenta