Is placenta an organ or tissue

The unexplored organ

In view of the importance of the organ, it has been massively researched, the authors write. The NICHD therefore launched the Human Placenta Project in 2014. The aim of the association of researchers is to elucidate the biology and pathology of the placenta and, above all, to develop technologies to observe the function of the organ in real time. So far it has mostly been examined after birth, when it has already lost its function. According to the initiators of the project, insufficient conclusions could be drawn from this about the organ's physiology. Animal models also provided limited information, as the placenta differs significantly from species to species. NICHD provided a total of 46 million US dollars in the first year to fund almost 20 research projects.

Although the placenta is also called placenta and is often attributed to the mother, it is mainly made up of embryonic cells. A few days after fertilization, the fertilized egg, which has since developed into a blastocyst (hollow germ) through multiple cell division, attaches to the wall of the uterus. The embryonic cells, called trophoblasts, penetrate the epithelium of the uterus and are very invasive: they release protein-degrading enzymes or induce apoptosis in the maternal cells in order to pave their way and dig into the tissue.

This decomposition nourishes the embryo until the blood supply is established, which is the case in the tenth to twelfth week of pregnancy. To do this, the trophoblasts penetrate the maternal blood vessels and modulate them, and they also form a total of around 50 km of capillaries. These open with the finest capillaries in finger-thick protuberances, the so-called chorionic villi, which bathe in the maternal blood in the intervillous space, i.e. between the maternal and embryonic side. The chorionic villi further branch into secondary and tertiary villi. In these, the embryonic blood is only separated from the maternal blood by a few layers of cells. The exchange of substances takes place via this placental barrier regulated by various mechanisms: diffusion, facilitated diffusion, active transport and pinocytosis.

Controlled exchange

Gases such as oxygen or carbon dioxide cross the placental barrier by diffusion, glucose by facilitated diffusion. Other substances such as amino acids are actively transported across the barrier. Macromolecules such as the mother's IgG antibodies overcome the barrier using the pinocytosis mechanism. In this process, vesicles are pinched off by the cell membrane and absorbed into the interior of the cell, where the substances or molecules contained are then released. In this way, the embryo is equipped with maternal antibodies that form its nest protection.

While the placental barrier is permeable for the necessary exchange of substances, it forms a strong barrier against pathogens. Although maternal infections are common during pregnancy, few pathogens have the ability to infect the embryo. These pathogens are summarized under the abbreviation TORCH. In addition to toxoplasmosis, there are the Treponema pallidum (syphilis), hepatitis B, hepatitis E, Coxsackie, Eppstein-Barr, varicella-zoster, rubella, cytomegalovirus and herpes simplex virus and the parvovirus B19. More recently, the Zika virus (ZIKV), which is known to infect the fetus and disrupt the development of the brain and skull, has also been added to the list.

Open questions

How the ZIKV crosses the placental barrier has not yet been fully clarified, write researchers led by Dr. Zachary A. Klase from the University of Sciences in Philadelphia in an article on the online portal »BioRxiv« (DOI: 10.1101 / 050674). One possible mechanism is that it uses the transport of immunoglobulins and, bound to antibodies, crosses the placental barrier by pinocytosis. However, since the transport of IgG does not start until the 16th week of pregnancy, but the main damage caused by ZIKV if the fetus is infected before this point in time, there must be another mechanism.