Trophoblast

Overview
Trophoblasts (from Greek threphein: to feed) are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg.

Function
Trophoblasts are invasive, eroding, and metastasizing cells of the placenta.

Trophoblasts mediate the implantation of the embryo into the endometrium, but they are never incorporated into the mother's body or the fetus. They are not "fetal" cells.

Trophoblasts become inert during pregnancy and are completely rejected by the fetus and mother at delivery. They can be seen as the thin membrane covering the fetus at birth, the caul.

Differentiation
The trophoblast proliferates and differentiates into 2 cell layers:

Pathology
The invasion of a specific type of trophoblast (extravillous trophoblast) into the maternal uterus is a vital stage in the establishment of pregnancy:
 * Failure of the trophoblast to invade sufficiently may be important in the development of some cases of pre-eclampsia.
 * Too firm an attachment may lead to placenta accreta.