The process by which trophoblastic tissue burrows into the uterine wall in order to obtain nourishment from maternal tissue and from the placenta and other fetal membranes such as chorion and amnion.
At the stage of implantation two distinct layers of the trophoblast become identified-
The syncytotrophoblasts contain many cells which are large and multinucleated that invade rapidly further in the endometrium. The single layer of the cells inner part of the trophectoderm forms the cytotrophoblasts. In hydatidiform mole which is a benign form of trophoblastic tumor the inner cell mass becomes unidentifiable and the trophoblastic shell continues a more vigorous invasion of the endometrium than usual.
The formation of the trophoblastic cells heralds the beginning of the production of hormones by the embryo through its membranes. Before the actual syncytiotrophoblast, outer cell mass begins to elaborate hCG which will become the most important source of gonadotropin during pregnancy as the pituitary ones loose their importance and are no longer produced. The hCG converts the corpus luteum of menstruation to the corpus luteum of pregnancy. With this conversion the ovarian steroids are maintained so that they do not reduce production until 16 weeks of gestation. At about the 12th week of gestation, placental steroids begin to be formed and they gradually take over from the ovarian source (corpus luteum). They become the main source of sex steroid production through out pregnancy.
Other hormones of less importance are produced by the placenta and they include human placental lactogen or human choronic somatotrophin (hCS) produced by the cytotrophoblasts. Also we have the so called posterior pituitary –like hormone being produced from the placenta. Certain embryos also produce some hormones such as blastocyst estrogen, prostaglandins etc.
Certain hormones are produced by the uterine decidua. They include FS, LH, sex steroids etc. We are not quite sure what these hormones do to the process of early pregnancy except to perhaps stimulate and maintain the process of nidation.