In mammals, a layer of tissue formed by cells forming the outer covering of the blastocyst. After implanation of the blastocyst, this layer is no longer cellular. At the 5th-5th day of human pregnancy, the blastocyst attaches to the luteinized endometrium and differentiates on its surface a syncytium which helps to penetrate the epithelium and then break through into the maternal mucous membrane (implanation).
As early as implanation (about the 8th-9th day), the trophoblast starts to establish a fetal-maternal exchange of gases and nutriments. When the blastocyst has completely penetrated into the maternal mucosa, the trophoblast consists of an external syncytial layer around the blastocyst, the syncytiotrophoblast, and an internal cellular layer, the cytotrophoblast.