Although the placenta appears as a physical barrier between maternal and foetal tissues, it brings the maternal and foetal circulation into close apposition for physiological exchange across a large area. Foetal well-being depends on good placental function for the supply of nutrients and the removal of waste products.
The ovum is fertilized in the Fallopian tube and it enters the uterine cavity where it rapidly converts to a blastocyst with an inner and outer cell mass. The outer cell layer of the blastocyst then proliferates to form the trophoblastic cell mass. At implantation, the trophoblast erodes into the surrounding decidua of the endometrium and its associated capillaries until the blastocyst is surrounded by circulating maternal blood (trophoblastic lacunae).
The placental tissue develops from the chorion that consists of the trophoblast and mesoderm of the developing blastocyst. The trophoblast differentiates into two layers, the thick outer syncytiotrophoblast and the thin inner layer, cytotrophoblast. In the second week of development, the (inner) cytotrophoblast layer begins to proliferate and extend cellular fingers into the (outer) syncytiotrophoblast (Figure PN.12). The cytotrophoblast cell columns and their covering syncytiotrophoblast extend as villous stems into the lacunae of maternal blood within the decidua. A mesodermal core appears within the villous stems. These villous stems form the framework from which the villous tree will later develop. Cellular differentiation of the villous mesoderm results in the formation of blood cells and blood vessels and form the villous vascular network. With development, the villi branch out extensively into the lacunae (intervillous spaces) forming the villous tree and thereby increasing their surface area (Figure PN.13).
Cytotrophoblastic cells grow into the lumens of the maternal spiral vessels within the decidua where they replace the endothelial cells, invade and destroy the musculo elastic medial tissue. As a result of the destruction of the smooth muscle, the walls of the spiral vessels in the decidua become thin and their vasoconstrictor activity is reduced. This wave of trophoblastic invasion starts at 10 weeks and is complete by 16 weeks. A second wave of vascular trophoblastic invasion occurs from 16 to 22 weeks and extends more deeply into the myometrial portions of the spiral arteries. These vessels are easily dilated as maternal flow to the placenta increases. Failure of this physiological change is found in pre-eclampsia and intra-uterine growth retardation. This means that these vessels still respond to vasoconstrictor stimulation and there is reduced flow to the intervillous space. Further maturation of the villi result in a marked reduction in the cytotrophoblast component and decrease the diffusional distance between the foetal villi and maternal intervillous blood. At term in humans, only a single layer of foetal chorionic tissue (syncytiotrophoblast) separates maternal blood and foetal capillary endothelium. Hence, the human placenta is classified as a haemo monochorial villous placenta.
Early development of the placenta showing formation of the villous tree and the intervillous space
Figure PN. 13
Placental circulation showing the villous tree (foetal placental circulation), spiral arteries and intervillous space (maternal placental circulation)
The placenta is connected to the developing embryo by a connecting stalk that subsequently becomes the umbilical cord containing the umbilical vessels. The placenta is supplied with maternal blood from the uterine blood vessels. Blood enters the intervillous space from the open ends of the uterine spiral arteries (Figure PN.13). The intervillous space is a large cavernous expanse into which the villous trees reach. Blood enters the intervillous spaces and flows into loosely packed areas, then into densely packed intermediate and terminal villi. It then empties into collecting veins. However, the relative direction of the blood flow is haphazard and behaves like a concurrent system, but with maternal blood flow exceeding foetal blood flow. The area of densely packed terminal villi is where placental exchange occurs. Maternal placental blood flow is a low pressure system; the pressure in the intervillous space is on average 10 mmHg. The increasing demands of the growing foetus require 100-150 spiral arteries to feed directly to the placenta. The maternal circulation through the intervillous space is fully developed by 20 weeks. Blood flow will increase from 50 ml/min at 10 weeks to between 500 and 800 ml/min at term.
Two umbilical arteries arising from the foetal internal iliac arteries carry de-oxygenated foetal blood via the umbilical cord to the placenta and a single umbilical vein returns oxygenated blood to the foetus. The umbilical arteries divide into chorionic arteries that feed the multiple placental lobules and these in turn subdivide into the villous tree which end as capillaries in the terminal villi (Figure PN.13). Foetal sinusoids formed within the terminal villi provide a large endothelial surface area and make it the ideal region for maternal foetal exchange. Each villous tree drains into a large vein that perforates the chorionic plate to become chorionic veins. Each of the venous tributaries course towards the umbilical cord attachment site where they empty into one umbilical vein.
The placenta grows dramatically from the third month of gestation until term. There is direct correlation between growth of the foetus and that of the placenta. By term, the mature placenta is oval and flat with an average weight of 500 g, average diameter of 20 cm and thickness of 3 cm.
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