Placental Development And Circulation

The placenta is where hormones needed for pregnancy are synthesized and serves as the link between the mother and the fetus. The growing placenta is larger and the fetus is smaller at first. The placenta's growth will eventually be surpassed by that of the fetus.

The placental weight and the fetus weight are equivalent at approximately 17 weeks of gestation. The placental to fetal weight ratio is approximately 1:6 at term. The placenta's fetal and maternal components are the two sources from which placental tissue grows. The decidua Basalis is the source of the placenta's maternal component. The chorion frondosum is the source of the fetal component.

Placental Circulation

The placenta has a volume of roughly 500 milliliters. There is 140 ml of blood in the intervillous area out of 500 ml. Maternal blood fills the intervillous area. Maternal blood is perfused into the intervillous area by the spiral artery.

The term "uteroplacental circulation" refers to a particular type of circulation. Additionally, there will be fetoplacental circulation.
The chorionic plate gives birth to several chorionic villi. The amnion, a fetal membrane, covers the chorionic plate on the fetal side. The placenta receives fetal membrane blood from the two umbilical arteries present in the fetus. The placental barrier is where gas exchange occurs. The fetus receives oxygenated blood through the umbilical vein.

Fetal blood enters the placenta at a rate of roughly 400 ml/min during fetal placental circulation. At term, the uteroplacental circulation flows between 700 and 900 milliliters per minute.

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Early Trophoblast Invasion

The decidua bacillus encases the entire implanting blastocysts in an embryo that is ten days old, indicating that placental and fetal development occur simultaneously. There is decidua capsularis on the upper side.

The trophoblast layer that invades is differentiated into the inner cytotrophoblast layer, which is a single-celled layer whose primary job is to synthesize cells that can be integrated into the syncytiotrophoblast layer. This is where these amnioblasts get their start, from the epiblasts.

By approximately day 12 of fertilization, the number of empty spaces, or lacuna, appears in the outer layer of the syncytiotrophoblast layer. The spiral arteries pierce through these gaps. Lacunae allow the entry of maternal blood. These gaps will eventually become intervillous areas.

 Extraembryonic mesoderm develops surrounds the embryo in the early stages of embryonic development, specifically between the embryo and the trophoblast layer.  Chorionic cavities will develop from extraembryonic mesoderm.  By day 15, when uteroplacental circulation is formed, lacuna is filled with maternal blood.  The complete mesenchyme layer and trophoblast layer combine to produce chorion upon the appearance of mesoderm.

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Chorionic Villi

The collective term "chorion" refers to extraembryonic mesoderm, cytotrophoblast, and syncytiotrophoblast. First, there are primary villi, which are made up of cytotrophoblast outside and syncytiotrophoblast inside a solid core.  Fetal vessels that are growing in the extraembryonic mesoderm layer will now start protruding inside the solid core and this will become the tertiary viilli.

Extraembryonic mesoderm invades these solid cores around the 12th day post-fertilization, at which point it is called the secondary villi. Chorionic vessels are another name for fetal vessels. As early as day 17 of fertilization, these chorionic arteries start to function. About day 21 after fertilization, the fetoplacental circulation begins to develop.

After 10 weeks of gestation, the umbilical artery's end diastolic flow begins.Chorionic villi have a lot of branches. There will be a single main stem villi that branches out several times. One portion of the placenta is perfused by a primary stem villi, and that segment is referred to as a placental lobule.The placenta's functional unit, also referred to as the cotyledon, is the lobule. There is only one major stem villi perfusing this area.

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Trophoblast Invasion Of Spiral Arteries

The superficial layer of the myometrium, the decidual basalis, is where the chorion villi attach themselves. Maternal blood perfuses the intervillous gaps, which are found between chorionic villi. The chorionic villi include a number of villous cytotrophoblasts. Some of these cytotrophoblasts, known as extra villous cytotrophoblasts, infiltrate the decidual basalis after emerging from the villous.

Extravillous interstitial cytotrophoblasts, also known as placental giant cells, are some of the extravillous cytotrophoblasts that continue to exist in the decidual basalis.

Cytotrophoblasts that cause modifications to the spiral arteries by invading the spiral arteries. The term "endovascular cytotrophoblasts" describes this. There is fibrinoid necrosis in the wall and the endovascular cytotrophoblast cell layer replaces the lining of the spiral arteries.

Throughout pregnancy, there must be an increase in uteroplacental perfusion. Vascular remodelling, the process by which the spiral artery transforms into a loose vascular channel, will preserve them. This remodeling of the arteries is unique to D. Basalis. It is not occurring in veins, but only in spiral arteries. Spiral arteries become loose vascular channels as a result of fibrinoid necrosis of the vessel wall. They permit greater uteroplacental perfusion because of this. Up to the point where the myometrium and decidual basalis meet, remodeling occurs. Vascular remodeling is completed in 16 weeks

 Chorionic villi encircle the early embryo inside the amniotic sac. There's also an outer layer of cytotrophoblasts. There are intervillous gaps in between the chorionic villi.  It has two ends; one is oriented toward the embryo and is next to the decidua basalis. This region has chorionic villi, which explains why there is vascularization of the villi and perfusion of the intervillous space there. This chorion frondosum is the location where the chorion grows into finger-like projections that give rise to the placenta.

In contrast, the side facing the uterine cavity is known as the abembryonic side. These are referred to as Chorion Laeve because the chorion is flattened out and does not get vascularized in those areas.The definitive chorionic plate, which forms by approximately 8 to 10 weeks of gestation, is the result of the amnion fusing with the chorionic plate as the fetus grows in size.

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Regulation Of Trophoblast Invasion

The extracellular matrix proteins of Decidua basalis and trophoblasts adhere to one another more readily when integrins are present.Matrix metalloproteinases will be stimulated by these integrins. And in the decidua basalis region, that results in matrix deterioration.The trophoblast cells will release chemicals like IGF-II to activate integrins when the invasion is necessary. The expression of Integrin molecules is also stimulated by factors released by Decidua.

Intregrin-mediated adhesion sets off a number of events, including matrix metalloproteinase activation and matrix degradation. Additionally, plasmin can raise MMPs. The plasminogen activator enzyme converts plasminogen into plasmsmin. Matrix breakdown is restricted when decidual TGF-β produces plasminogen activator inhibitor, which in turn inhibits plasminogen activator.

Additionally, residual TGF-β stimulates tissue inhibitors of matrix metalloproteinases, which inhibit MMPs and limit the extracellular matrix.  NK cells are essential for controlling the invasion process because they secrete pro-invasion materials like chemokines and vascular endothelial growth factor. They might later produce chemicals that slow down the invasion process.

The decidua contain decorin, a proteoglycan, which is linked to decidual TGF-β. Although decorin binds to decidual TGF-β, plasmin will release this decidual TGF-β, which has an anti-invasive impact on its own.  As pregnancy progresses, E2 levels rise. Early placental development and early trophoblast invasion occur when E2 concentrations are lower; it has been proposed that this is due to a decrease in the expression of integrin receptors and VEGFs.

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Placental Barrier

Fetal capillaries go from the fetal side to the maternal side, chorionic villi are present, and maternal blood is found in intervillous gaps. The endothelium and basement membrane of the fetal capillaries are present. Extra embryonic mesoderm are present in the second place. The cytotrophoblast layer and its basement membrane are next.

The syncytiotrophoblast layer is next, followed by the intervillous space and mommy blood. Hemochorial placentas are placentas that form when maternal blood comes into indirect touch with chorionic cells.Spiral arterioles, d. basalis, and an outer cytotrophoblastic shell are present. The area where the cytotrophoblastic shell and D. basalis meet is where a zone of fibrinoid necrosis will form.We refer to the area of fibrinoid necrosis as the Nitabuch layer. This layer is crucial because it prevents the placenta from penetrating the myometrium more deeply.

The infant experiences stress during labor because placental perfusion diminishes with uterine contractions. The procedure maintains the fetus's oxygenation; as you may know, veins and spiral arteries are parallel to the myometrium's fibers. Because the veins are parallel, uterine contractions cause them to compress more in the intervillous spaces. Because uterine contractions compress the veins more, blood enters the intervillous gaps. As a result, blood stays in these spaces, supporting placental perfusion even during uterine contractions.

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What Prevents Fetal Rejection?

 Since antigen-presenting cells are responsible for identifying any foreign cell, it is crucial to identify HLA molecules, which are expressed by all cells.  Invading cytotrophoblast cells display only class I MHC molecules; they do not express the full system.

Secondly, because syncytiotrophoblast cells lack HLA cells and MHC molecules, the mother's immune system is unable to recognize them. They exhibit a distinct form of HLA-G I, suggesting a function in immunological tolerance. Because pregnancy is an immunomodulated state, there is a reduction in cytotoxic immunity. Pregnancy enhances immunological tolerance. The hormones released during pregnancy are in charge of preserving an immune-regulated state. The regulation of trophoblastic invasion by deft NK cells.

Placental Barrier At Term

 The cytotrophoblast and syncytiotrophoblast layers initially line the chorionic villi and all of their terminal branches. The cytotrophoblastic cells lose continuity as they divide further. At 16 weeks of gestation, this continuity is lost. The cytotrophoblast layer is a very thin and sparse layer; in the transfer area, where actual substance transfer is required, a thin layer of syncytiotrophoblast is present; and there is also a thick layer of syncytiotrophoblast, where thickness is maintained because it is the area of synthesis where hormones are synthesized. In relation to the placental barrier, macrophages are also present. We refer to these fetal macrophages as Hofbaeur cells.

At 8 ml/min/kg of fetal weight, oxygen is given to the fetus. The fetal capillaries receive a steady supply of oxygen from the intervillous area, whereas carbon dioxide diffuses in different ways. The oxygen saturation between the umbilical vein and the fetal capillaries is not significantly altered when there is continuous oxygen transport from the intervillous space.

Because fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin and because the partial pressure of oxygen in the umbilical venous system is slightly lower than in the IVS, more oxygen is bound to the fetal capillaries. This helps to diffuse oxygen from high pressure to low pressure.

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Amnion

It is the avascular membrane; it is protective; it produces tensile strength.  An amnion epithelial layer is present toward the fetal side. The epithelial layer's basement membrane is the second. The third layer is the collagen-filled compact layer. There is a layer of mesenchymal cells that resembles fibroblasts, followed by a spongy layer.

Foetal ectoderm from epiblast cells is the source of the amnion epithelial layer.  The fetal mesodermal layer gives rise to the fibroblast-like mesenchymal layer.  There is metabolic activity in the amnion epithelial layer. It conceals a number of important compounds, such as PGE2. Additionally, it produces fetal fibronectin, which functions as a glue between the nearby decidual and the fetal membrane.

Locally synthesized by amnion epithelial cells, brain natriuretic peptide (BNP) has a smooth muscle relaxing effect that blocks pain stimulants. Since (BNP) expression is downregulated at term, the membrane is more prone to stretching.

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