Development of the placenta
After the 4th month the cytotrophoblast slowly disappears from the walls of the tertiary villi (interactive diagram), 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.
- Decidual tissue
- Syncytiotrophoblast
- Cytotrophoblast islands
- Septum
The cytotrophoblast islands move into the periphery of the cotyledons and, together with the decidual tissue, are involved with formation of the placental inter-cotyledon 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 (interactive diagram).
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. (interactive diagram)
- A
- Subchorial Langhans' layer
- B
- Rohr's layer
- C
- Nitabuch's layer
The fibrinoid deposits form the sub-chorionic Langhans' layer A. Rohr's layer B is found at the level of the basal plate beneath the stem villi. Lying still deeper in the decidua basalis they form Nitabuch's layer C. This is located at the boundary between the zona spongiosa and the zona compacta (where the release of the placenta takes place).
The fibrinoid deposits are structurally and chemically closely related to fibrin that is formed by the activation of fibrinogen in the blood vessels. Recent research results emphasize the existence of two types of fibrinoid.
The fibrinoid deposits are present in all normal placentas, increase in thickness during the pregnancy, and can take up a maximum of 30% of the placental volume without affecting its function. Normally, at the end of the pregnancy, they do not occupy more than 10 to 20% of the placenta volume.
It seems their generation is connected with micro-lesions of the syncytiotrophoblast. Through these injuries, the mesenchyma comes into contact with maternal blood. This provokes the activation of local coagulation mechanisms. The mechanism resembles those which are brought about through endothelial lesions in the blood vessels of adults.
Fibrinoid deposits also envelop all necrotic material that ensues from placental degeneration. When these deposits are massive and block one or more vessels to the villi, they form white infarcts, which can be seen macroscopically on the placenta.
The functional importance of the fibrinoid seems to be quite complex. Besides their sealing effects, they also play a role in the immunologic "barrier" between feto-maternal tissue as well in the «anchoring» of the placenta.