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Wang Y, Zhao S. Vascular Biology of the Placenta. San Rafael (CA): Morgan & Claypool Life Sciences; 2010.

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Vascular Biology of the Placenta.

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Chapter 3Structure of the Placenta

Villous “trees” are the main structure of the placenta. Based on the developmental stage, villous structure, vessel branches, histologic features, and vessel-cell type components, at least five types of villi have been described [14]. An illustration of the architecture of different villous trees is shown in Figure 3.1A.

Figure 3.1. Architecture of different villous and vessel branches of a cotyledon.

Figure 3.1

Architecture of different villous and vessel branches of a cotyledon. A: types of placental villi in human placenta. B: A villous tree connects to the fetal surface (chorionic plate) and the maternal surface (basal plate). The villous trees are so named (more...)

(1) Stem villi. Stem villi connect to the chorionic plate and are characterized by a condensed fibrous stroma containing large vessels and microvessels. Stem villi develop vessels with a smooth muscle investment and central stromal fibrosis. The trophoblast layer of stem villi is partly replaced by fibrin-type fibrinoid as gestation proceeds. The function of stem villi is to support the structures of the villous trees. Because of the low degree of fetal capillary and degeneration changes of the trophoblasts, fetal-maternal exchange and endocrine activity of stem villi are usually negligible [14].

(2) Immature intermediate villi. Immature intermediate villi are bulbous, peripheral, and immature continuations of stem villi. This type of villi has a loose or reticular stroma and Hofbauer cells, more prominent vessels and a discontinuous cytotrophoblast layer. The outer syncytiotrophoblast layer remains continuous throughout development. Immature intermediate villi are considered the growth centers of the villous trees. Immature intermediate villi are probably the principal sites of exchange during the first and second trimesters, as long as terminal villi are not yet differentiated [14].

(3) Mature intermediate villi. Mature intermediate villi are long, slender, peripheral ramifications that lack fetal vessels in the stroma. Mature intermediate villi produce the terminal villi. The high degree of fetal vascularization and the large share in the exchange surface make them important for fetal-maternal exchange.

(4) Terminal villi. Terminal villi are linked to stem villi by intermediate structures. These villi are grape-like structures characterized by a high degree of capillarization and highly dilated sinusoids. In term placenta, terminal villi are smaller with less stroma and a discontinuous cytotrophoblast layer, and contain 4–6 fetal capillaries per cross section. The fetal capillaries of the villous core oppose against thin attenuated syncytiotrophoblasts forming vasculosyncytial membranes. In the terminal villi, the fetal capillary vessels and syncytiotrophoblasts are separated by only a thin basement membrane with a minimal mean maternal-fetal diffusion distance ~3.7 µm, which make terminal villi the most appropriate place for diffusive exchange. In the normal mature placenta, the terminal villi comprise nearly 40% of the villous volume of the placenta. Because of their small diameters, the sum of their surfaces account for about 50% of the total villous surface and 60% of villous cross sections [14]. Terminal villi, the functional unit of the placenta, transfer electrolytes, O2, CO2 and nutrients between the mother and fetus.

(5) Mesenchymal villi. Mesenchymal villi are the most primitive type of villi during early stages of pregnancy. Mesenchymal villi have loose stroma, inconspicuous capillaries, and two complete surrounding trophoblast layers, a cytotrophoblast layer surrounding the villous core, and an outer syncytiotrophoblast on the villous surface. Fetal capillaries are poorly developed and never show sinusoidal dilatation. The unvascularized tips of mesenchymal villi are referred to as villous sprouts (Figure 3.2). The function of mesenchymal villi is very important during the first few weeks of pregnancy. Mesenchymal villi are the place of villous proliferation and they perform almost all endocrine activities. With advancement of pregnancy, their primary function is to sustain villous growth. At term, their share in total villous volume is less than 1% [14].

Figure 3.2. Mesenchymal villi and villous sprouts in first-trimester placentas.

Figure 3.2

Mesenchymal villi and villous sprouts in first-trimester placentas. Open arrowhead: cytotrophoblasts; solid arrowhead: syncytiotrophoblasts; and arrow: villous sprouts; V: fetal vessel; and IVS: intervillous space, respectively. A: bar = 100 micron, and (more...)

Placenta villous development starts with mesenchymal villi. Up to 5 weeks postconception (p.c.), all placental villi are of the ‘mesenchymal’ type (containing trophoblast and villous sprouts) [15]. Mesenchymal cells later invade these villi forming secondary villi (immature/intermediate villi) and also giving rise to placental blood vessels. Trophoblast syncytiolization leads to the formation of villous sprouts. Mesenchymal villi are continuously formed throughout pregnancy, but dominate during the first and second trimesters [15]. Villous sprouts further transform into immature/mature intermediate villi then to terminal villi [15,16]. Trophoblast sprouting, proliferation, and formation of finger-like trophoblast protrusions lead to mesenchymal invasion and local fetal angiogenesis [16]. Villous core fetal vessel formation and fetal-placental blood flow begins approximately around 6–8 weeks p.c.

The weight of the placenta is about 20 grams at 10 weeks of gestation and 150–170 grams at 20 weeks of gestation. A mature placenta weighs about 500–600 grams and consists of 15–28 “cotyledons.” The stem villus is the major structural unit of the fetal cotyledon. Each cotyledon begins with a stem villus that divides into 3-5 immature/mature intermediate villi, which further branches into 10–12 terminal villi (Figure 3.1B). Some terminal villi float freely in the intervillous space, whereas others are attached to the decidua, providing structural stability for the placenta.

Copyright © 2010 by Morgan & Claypool Life Sciences.
Bookshelf ID: NBK53256

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