Development of the placenta
After the 4th month the cytotrophoblast slowly disappears from the walls of the tertiary villi (), whereby the distance between the maternal and fetal vessels diminishes. They also disappear from the chorionic plate. In the basal plate the cytotrophoblast remains mainly at the level of the cytotrophoblast layer. Together with the decidua and fibrin deposits, they form protrusions (inter-cotyledon septa) that project into the intervillous space, dividing it to some extent into so-called cotyledons.The formative mechanism of these inter-cotyledon septa remains speculative and probably depends on the folding together of the basal plate which, for its part, has resulted from the proliferation of the stem villi. They push the basal plate back. In addition, the spread of the placenta into the uterine cavity also appears to contribute to the creation of the septa.
These septa delimit the cotyledons but never merge with the chorionic plate. Maternal blood can accordingly circulate freely from one cotyledon to the other ().
The villus stems of the placenta lengthen considerably towards the end of the pregnancy and the fibrinoid deposits (extra-cellular substance made up of fibrin, placental secretions and dead trophoblast cells), accumulate in the placenta. This happens especially under the chorionic plate, where they form the subchorial Langhans' layer, as well as at the level of the basal plate beneath the stem villi and the cytotrophoblast layer, where the fibrin deposits form Rohr's layer. Still deeper in the decidua basalis these deposits form Nitabuch's layer. This is where the placenta detaches itself from the uterus at birth. ()