Synonyms
placenta, placenta
Definition
The placenta is an organ created during pregnancy, which consists of a fetal and a maternal part. The placenta assumes numerous functions. It provides nutrition and oxygen for the child, produces various hormones and is used for substance exchange.
The placenta is usually disc-shaped with a thickness of about 3 cm and a diameter of between 15 and 25 cm. It weighs about 500g. With an intact placenta, there is no contact between maternal and fetal blood.
This part is for especially interested people, otherwise skip this part! During the early development of the fertilized egg, from the 4th day after fertilization, two different types of cells, the embryoblasts and the trophoblasts, differ. The trophoblasts are important for the development of the placenta, especially the cells that emerge from them are called syncytiotrophoblasts.
The cell cluster of the syncytiotrophoblasts loosens up on the 9th day after fertilization and forms small cavities (lacunae). Since the fertilized egg has implanted in the uterine wall, the small maternal blood vessels (capillaries) in the uterine muscle layer are dilated and congested. This causes the formation of so-called sinusoids.
The increasingly growing syncytiotrophoblasts gnaw away at the maternal sinusoids, so that maternal blood seeps into the cavities. The syncytiotrophoblasts develop into villi, which transform and finally become tertiary villi at the end of the third week, in which fetal blood vessels are formed. The placenta consists of a fetal and a maternal part.
The maternal part is formed by the muscle layer of the uterus. The fetal part is the villi-rich egg membrane (chorion frondosum), which is located under the child and consists of the above-mentioned cells, the trophoblasts. Between these two parts there is a space filled with about 150-200ml of maternal blood.
This blood comes from maternal vessels in the uterine wall. In the blood-filled space there are many villi with their branches, which are then called villi trees. These villi trees are washed around by the mother’s blood, so that the exchange of substances between mother and child can take place on their surfaces due to various transport mechanisms.
It is important, however, that throughout the entire pregnancy the maternal blood remains separated from the fetal blood by a layer of cells. This filter membrane is therefore also called the placental barrier. In the direction of the maternal part, the placenta consists of up to 38 lobules (cotyledons), each of which houses at least two of these villi and is connected to each other.
At the 14th week of pregnancy (SSW), the placenta has its final structure. It continues to increase in thickness until the 5th month of pregnancy, while its surface area continues to increase after the 5th month of pregnancy and finally reaches a diameter of between 15 and 25 cm. In most cases, the placenta is a disc-shaped structure.
However, other forms are known. The placenta can be lobed, divided, with a secondary lobe or belt-shaped. Very rarely, only a diffuse distribution of the villi is observed.
An important function of the placenta is the exchange of substances between mother and child. Especially water and oxygen from the mother reach the fetal vessels of the villi due to differences in concentration. These vessels all ultimately unite in the vein of the umbilical cord (Vena umbilicalis), which carries the nutrient- and oxygen-enriched blood into the child’s body.
It is important that the blood passes by the liver, so that the entire organism has access to the substances it is supplied with and not all of them are used up by the liver. Sugars (glucose), proteins (amino acids and proteins) and fats also enter the child’s blood with the help of various transporters in the placenta. The absorption of a certain type of antibody (immunoglobulin G) is also worthy of special mention, as it provides the unborn child with a certain degree of protection against certain infections.
Nevertheless, some bacteria and viruses can penetrate the placental barrier and enter the child’s organism. Due to this transition, the unborn child can nevertheless become infected and fall ill with one or the other infection, especially those caused by viruses. In the same way, some drugs enter the child’s body via the placenta.For this reason it is very important during pregnancy to take care that such drugs are not taken, because they can harm the child’s development.
Substances that the child excretes are returned to the placenta via two arteries in the umbilical cord (Arteriae umbilicales) and can be released into the mother’s blood via the villi. The mother can completely break down or convert such excretion products and carry them out of her body. The second major task of the placenta is to produce the large quantities of hormones that are needed during pregnancy and cannot be additionally produced by the mother’s glands.
On the one hand the female sex hormones progesterone and estrogen are produced. Progesterone promotes breast development, milk production (lactogenesis) and inhibits the contraction of the muscles of the uterus. The growth of the breasts and uterus is due to the estrogen effect.
The concentration of estrogen in maternal blood and urine depends on the vitality of the child, as it converts precursors. Nevertheless, this method is hardly of importance today in the examination of pregnant women and their child. Another very well-known hormone is the so-called human chorionic gonadotropin (HCG).
It ensures that the muscle layer of the uterus with the fertilized egg is not rejected. It also causes the first maturation of eggs in the ovaries of a female child and the descent of the testicles into the scrotum in male children. In practice, this hormone is used to detect pregnancy by means of a pregnancy test.
This is because high concentrations of it can be detected in the urine of mothers in early pregnancy. In addition, the human placental actogen (HPL) is produced, which provides fats for the energy supply of the mother and reflects the functional state of the placenta, and the human chorionthyrotropin (HCT), whose function has not yet been fully clarified.