Maturation of the placenta is one of the important stages in the development of a healthy baby in the future, since the placenta ensures the vital activity of your baby. It is necessary to understand how the placenta matures under normal conditions, and what its functions are in order to know what pathological changes and their signs can be.
What is the placenta for?
The function of the human placenta is of interest, both from a scientific and from a clinical point of view. The study of human placenta is very difficult, because the methodology carries unacceptable risks for both the mother and the fetus.
Placenta plays a vital role in all trimesters of pregnancy and ensures that your baby will develop safely. It performs many functions, such as:
- The placenta basically functions to provide adequate nutrition to your baby. Before blood from you reaches your baby, it travels through the placenta to reach the umbilical cord that connects you to your baby. The placenta is the only organ in the body with two separate blood supply, each of which comes from a separate organism. Since the placenta is a temporary organ, blood flow changes rapidly with each stage of pregnancy.
The uteroplacental circulation is a system with low resistance for the maternal organism. Uterine arterial blood flow in a non-pregnant state averages from 1% to 2% of the cardiac output in the mother. During pregnancy, uterine blood flow rises sharply until it grows to 17% of the maternal cardiac output.
The placental circulation of the fetus receives approximately two thirds of the total cardiac output of the fetus. This high flow rate is important when transporting oxygen and nutrients from the mother to the fetus and is supported by a number of anatomical differences in fetal circulation. Since the fetus’s lungs do not have any respiratory function, high vascular resistance is maintained in this organ due to the mechanical effects of the unexpanded alveoli on the vessel walls and the vasoconstrictor effect of low oxygen tension that prevails in fetal blood. These two factors combine to shunt approximately two-thirds of the output of the right ventricle from the lungs to systemic circulation through the arterial duct.
This is one of the most important functions of the placenta.
- Another important function that the placenta performs is that it acts as a kidney; It filters blood to eliminate harmful substances that may be hazardous to your baby’s health.
- The placenta also serves as a lung for children and allows you to pass on oxygen to your baby.
- The placenta returns your baby's biological waste to the mom's circulation system, which is later removed from your body through urine.
- Throughout pregnancy, the placenta retains the main role of all biological membranes (i.e., selective permeability). With particles, such as blood cells and macromolecules, the transport is severely limited, providing a “placental barrier”. At the other end of the spectrum, the transfer of many essential nutrients is accelerated by various transport mechanisms. This will save your child from possible infections by separating the blood from your child, acting as a filter.
- Many hormones are produced from the placenta in your body with the maximum amount of lactose, which provides an adequate level of glucose in the blood, which allows it to spread to the baby.
- The placenta also destroys the food particles you consume in order to ensure the proper nutrition of your baby.
- It captures the oxygen that you inhale to disperse in the blood to help it reach your child’s circulatory system, passing it through the umbilical cord. This is one of the important functions that the placenta performs, as it prevents the likelihood that your baby can inhale amniotic fluids, which can be catastrophic.
- The placenta secretes a huge amount of female hormones, such as progesterone and estrogen, which provides uterus tone, placenta growth, delay the next ovulation and support the pregnancy itself. It also opens the way for the preparation of maternal tissues and the uterus for the birth of a child.
- During pregnancy, the placenta moves while the uterus expands and grows. This is the overall function of the placenta to remain in the early stages of pregnancy, but in the later stages of pregnancy it moves to the top of the uterus to open the cervix for the birth of a baby.
Normal maturation of the placenta
The placenta is the fastest growing organ in the human body. The placenta grows from one cell to about 5 × 10 to the 10th grade of cells in 38 weeks. Implantation of a fertilized egg occurs on the seventh to tenth day after conception. The layer of cells forming the germ surface develops into the chorionic membrane, and the cells of the cytotrophoblast originate from it. Trophoblast cells are multinuclear aggregates of cytotrophoblast cells and are constantly being formed from them. These cells, plus the villi, are characteristic and unique features of the future human placenta.
The venous sinuses inside the endometrium very early invade the trophoblast cells. Within a few days, lacunae develop, surrounded by syncytial cells and filled with maternal venous blood and tissue fluid. Maternal spiral arterioles are destroyed on the 14th or 15th day, and maternal arterial blood enters the developing space. Fetal vessels form in situ inside the mesenchymal cores, and the resulting villi are called tertiary villi. At about the 17th day after conception, both fetal and maternal blood vessels function, and true placental circulation is established. This is the basis of the formation of the future placenta.
Fetal and maternal vascularization of the placenta is completed by the 17-20th day, and fetal erythrocytes can be detected inside the vessels of the fetus after the 21st day after conception. The placenta continues to grow in thickness and circumference until the end of the fourth month. An increase in the thickness of the placenta is a consequence of an increase in the length and size of the villi with an accompanying expansion of the intervertebral space. After the fourth month there is no noticeable increase in thickness, but growth along the circumference continues throughout most of the pregnancy.
The human placenta is a hemochronic placenta, which means that the maternal blood is in direct contact with the fetal trophoblast. Maternal blood circulates freely in space. The villus can be considered a functional unit of the placenta, it is here at the molecular level that the exchange of matter between mother and fetus takes place. Therefore, the development of placental villi is the basis for proper development and maturation of the placenta.
In early placentation, each placental villus passes through a similar initial developmental program. In late placentation, the villi are morphologically differentiated into a limited range of functional changes in the villi, reflecting their specialization. The main initial contribution consists of the trophoblast membrane that surrounds the embryo, and then through its development of extraembryonic mesoderm and the differentiation of blood vessels, it performs its function.
There are three main types of trophoblastic cells: fuzzy cytotrophoblasts, extravital cytotrophoblasts, and syncytiotrophoblasts, which are formed by fusion of fuzzy cytotrophoblasts.
The syncytiotrophoblast layer forms the epithelial covering of the entire fleecy tree. These cells are multinucleated, terminal-differentiated syncytium formed by the fusion of cytotrophoblast progenitor cells. Differentiation is regulated by chorionic gonadotropin, and cytotrophoblast cell fusion continues during the development of the placenta.
Cellular parts derived from syncytiotrophoblasts (apoptotic nuclei and microparticles) can be dropped into the maternal blood.
Mesenchymal villi are continuously formed from trophoblastic throughout pregnancy and are considered the basis for the growth and differentiation of nappy trees. They will form the basis of the functional unit of the future placenta.
Initially, the primary villi are formed. So, in the second week of placenta development, the first stage of development of chorionic villi, trophoblastic shell cells (syncytotrophoblasts and cytotrophoblasts) occurs, which form finger extensions to the maternal decidus.
Secondary villi develop in the third week - this is the second stage of the development of the chorionic villi. At the same time, extraembryonic mesoderm turns into villi and covers the entire surface of the chorionic sac.
Tertiary villi are formed for 4 weeks - this is the third stage of the development of the chorionic villi. In this stage, the mesenchyme differentiates into blood vessels and cells, forms an arteriocapillary network.
In the first two trimesters, the tertiary villi are the precursors of immature intermediate villi, whereas in the last trimester, the mesenchymal villi are transformed into mature intermediate villi. The immature intermediate fibers formed during the first two trimesters are stages of development in relation to the stem villi.
Mature intermediate villi develop during the last trimester, produce numerous terminal villi. Terminal villi are not active protuberances caused by trophoblast proliferation, but rather passive protrusions caused by capillary twisting due to excessive longitudinal growth of the capillaries of the fetus in mature intermediate villi.
The development of the placenta corresponds to the period of gestation. At week 4-5, a complex network of cords and vessels with redundant connections is formed initially. This network contains mostly cords already connected together. Vessels and cords are connected to each other without interruption.
At 6-7 week, the villi, in which the capillary network of vessels and cords prevails, form the basis of the villi.
At week 8-9, the villi have two large centralized vessels that are surrounded and connected to the peripheral capillary network. The capillary network contains vessels with a lumen in close contact with the overlapping trophoblatic layer. This ensures the development of the placental vascular network.
The maturation of the placenta begins from the very first moment of development of the first villi and lasts no less than thirty weeks.
The rate of maturation of the placenta has successive stages at the macroscopic level. Knowledge and distinction of such stages is very important for assessing the condition of the fetus and the functional development of the placenta itself. The degree of maturation of the placenta weekly distinguish the following:
0 (zero) degree is characterized by the formation of a clear correct structure, in which all the segments of the placenta are fully formed. Moreover, each villus has reached the final degree of growth, it has the weight of the cells and vessels necessary for gas exchange. This degree is typical for the complete completion of the formation of the placenta, and it should normally be in the thirtieth week of pregnancy. Such a placenta at this time can provide all the functions and needs of the infant at this period of gestation.
- the degree is characterized by a change in the homogeneity of the placental tissue and the formation of different echogenicity sites. This is a normal process and it talks about the consistent development of different parts of the placenta. This degree is characteristic for the thirtieth - thirty third week of pregnancy. There may be a variation of one week.
- degree develops in the thirty-fourth to thirty-seventh week. In this case, the chorionic plate becomes convoluted, echo-gene regions appear in larger numbers. This degree is considered the most mature and functionally active. The thickness of the placenta at this stage ranges from 29 to 49 millimeters. Such functional activity of the placenta allows the baby to get the most nutrients to store them for the period of childbirth.
- the degree of maturity indicates the complete readiness of the placenta to the process of physiological labor. At the same time, the processes of division of the placenta and the formation of its apical and distal end begin. This degree develops before the very birth and should be observed at least 39 weeks.