See also: placental barrier
The placenta is a deciduous, and therefore temporary, organ that forms in the uterus during pregnancy. The placenta is designed to nourish, protect and support fetal growth.
The placenta is common to the expectant mother and the fetus; part of it, in fact, has maternal origins (consisting of the modified or deciduous uterine endometrium), while the remainder has fetal origins (formed by chorionic villi). The placenta, therefore, represents the roots of the fetus in the soil of the mother.
The chorionic villi are very vascularized extensions generated by the outer layer of embryonic cells (the chorion), which branches out sinking into the uterine mucosa (endometrium).
Process of formation and development of the placenta
Around the seventh day the implantation (or nesting) of the blastocytes in the endometrium begins, thanks to the release of particular proteolytic enzymes by the blastocyst itself. This, after having penetrated it, is completely enveloped by the endometrium (twelfth day) and continues its development. The embryonic cells that will become placenta begin to form digitiform offshoots, called chorionic villi, which penetrate into the vascularized maternal endometrium releasing enzymes that corrode the walls of blood vessels. From this moment on, numerous villi will undergo further ramifications and structural transformations, sinking even more into the uterine mucosa, leading to an intimate system of exchanges which, under the name of placenta, unites the mother with the fetus [first the villi are distributed on the entire surface of the chorion but, as the pregnancy progresses (around the third month), only those adjacent to the basal decidua develop - forming the leafy chorion - while those facing the deciduous capsular degenerate (smooth chorion)].
At the end of their differentiation the chorionic villi are internally vascularized and immersed in blood lacunae filled with maternal blood. Despite this, embryonic blood and that of the mother do not mix, and most substances are exchanged through the thin walls of chorionic villi (placental barrier).
At the stage of definitive maturation the placenta is made up of a fetal portion, deriving from the frondous chorion, and a maternal portion, deriving from the basal decidua.
After the third month the placenta continues to grow, until it reaches, just before the birth, the 20-30 cm of diameter, the 3-4 cm of thickness (greater in the center) and the 500-600 grams of weight; as a whole, it will occupy 25-30% of the internal surface of the uterine cavity.
The placenta, as we said, is richly vascularized and receives up to 10% of total maternal cardiac output (about 30 liters / hour).
Placenta functions
The primary function of the placenta is to allow metabolic and gaseous exchanges between fetal and maternal blood. Fetus and placenta communicate through the umbilical cord or funiculus, while the mother communicates directly with the placenta through lacunae filled with blood (sanguine lacunae), from which they "catch" the chorionic villi.
The umbilical vessels include an umbilical vein - which carries oxygenated blood rich in nutrients from the placenta to the fetus - and the umbilical arteries, in which flows blood rich in catabolites that from the fetus go to the placenta.
The functions of this organ are very numerous, since it serves as:
- lung: supplies oxygen to the fetus and removes carbon dioxide; these gases spread easily through the thin layer of cells that separates chorionic villi from maternal blood.
- Kidney: purifies and regulates the body fluids of the fetus.
- Digestive system: it provides and supplies nutrients; the placenta is permeable to many nutrients present in the mother's blood, such as glucose, triglycerides, proteins, water and some vitamins and minerals.
- Immune system: allows the passage of antibodies by endocytosis but prevents that of many pathogens (exceptions, for example, the rubella viruses and protozoa of toxoplasmosis).
- Protective barrier: the placenta prevents the passage of many harmful substances, although some may still pass through it and harm the fetus (caffeine, cocaine, alcohol, some drugs, nicotine and other carcinogenic substances present in cigarette smoke ...).
The placenta also has a very important endocrine function. Since the early stages of its development, in fact, it secretes human chorionic gonadotropin (hCG), a hormone similar to the LH that supports the production of progesterone by the corpus luteum (not surprisingly, therefore, the dosage of Human Chorionic Gonadotropin in the blood or in urine is used in pregnancy tests). From the seventh week onwards, the placenta reaches a sufficient degree of development to produce all the necessary progesterone on its own; consequently, the corpus luteum degenerates and, along with it, the amount of hCG produced by the placenta.
Human chorionic gonadotropin is important for stimulating testosterone synthesis in developing testes of the male fetus.
In addition to hCG, the placenta secretes other hormones, such as human placental lactogen, estrogens (which inhibit the maturation of other follicles), progesterone (which prevents uterine contractions and supports the endometrium) and others (including inhibin, prolactin and pronenin). It is interesting to note that the placenta lacks some of the enzymes necessary to complete the synthesis of steroid hormones; however, these enzymes are present in the fetus. Thus, at least from the endocrine point of view, a relationship of "symbiosis" is established, so that we speak of "fetal-placental unity".
The placenta, therefore, provides for all the needs of the fetus, nourishing it, protecting it and building an intimate bond with the mother; a bond made of care and rejection, of dependence and autonomy which, in many respects, will accompany the two individuals even in extrauterine life.
