6 Basic Structure of the Villous Trees M. Castellucci and P. Kaufmann Nearly the entire maternofetal and fetomaternal exchange Syncytiotrophoblast takes place in the placental villi. There is only a limited contribution to this exchange by the extraplacental mem- Syncytium or Multinucleated Giant Cells? branes. In addition, most metabolic and endocrine activi- The syncytiotrophoblast is a continuous, normally unin- ties of the placenta have been localized in the villi (for terrupted layer that extends over the surfaces of all villous review, see Gröschel-Stewart, 1981; Miller & Thiede, 1984; trees as well as over parts of the inner surfaces of chori- Knobil & Neill, 1993; Polin et al., 2004). onic and basal plates. It thus lines the intervillous Throughout placental development, different types of space. Systematic electron microscopic studies of the syn- villi emerge that have differing structural and functional cytial layer (e.g., Bargmann & Knoop, 1959; Schiebler & specializations. Despite this diversification, all villi exhibit Kaufmann, 1969; Boyd & Hamilton, 1970; Kaufmann the same basic structure (Fig. 6.1): & Stegner, 1972; Schweikhart & Kaufmann, 1977; Wang • They are covered by syncytiotrophoblast, an epithelial & Schneider, 1987) have revealed no evidence that the surface layer that separates the villous interior from syncytiotrophoblast is composed of separate units. Rather, the maternal blood, which flows around the villi. Unlike it is a single continuous structure for every placenta. Only other epithelia, the syncytiotrophoblast is not com- in later stages of pregnancy, as a consequence of focal posed of individual cells but represents a continuous, degeneration of syncytiotrophoblast, fibrinoid plaques uninterrupted, multinucleated surface layer without may isolate small islands of syncytiotrophoblast from the separating cell borders (Fig. 6.1C; see Fig.6.21, and surrounding syncytial continuum; this happens mainly at Chapter 7, Fig. 7.11). the surfaces of chorionic and basal plates. Therefore, • Beneath the syncytiotrophoblast is an interrupted layer terms such as syncytial cells and syncytiotrophoblasts, of cytotrophoblast, which consists of single or aggre- widely used in experimental disciplines, are inappropriate gated cells, the Langhans’ cells, which are the stem cells and should be avoided. Their use points to a basic of the syncytiotrophoblast; they support the growth misunderstanding of the real nature of the and regeneration of the latter. syncytiotrophoblast. • The trophoblastic basement membrane separates syn- Where the syncytiotrophoblast due to degenerative cytiotrophoblast and cytotrophoblast from the stromal processes is interrupted, the gap is filled by fibrin-type core of the villi. fibrinoid (see Chapter 9). A few reports (Tedde et al., • The stroma is composed of varying numbers and types 1988a,b) have pointed to the existence of vertical cell of connective tissue cells, connective tissue fibers, and membranes that seemingly subdivide the syncytium into ground substance. syncytial units. These extremely rare findings have an • Additionally, the stroma contains fetal vessels of various accidental rather than a real basis. They represent the kinds and calibers. In the larger stem villi, the vessels likely results of a syncytiotrophoblastic repair mechanism are mainly arteries and veins; in the peripheral branches, (Figs. 6.1, 6.2B, and 6.3). That is, after syncytial rupture, most fetal vessels are capillaries or sinusoids. apical and basal plasmalemmas fuse at either side of the 50 Syncytiotrophoblast 51 Figure 6.1. Basic morphology of human placental villi. A: diate villi, the surface of which is densely covered with grape- Simplified longitudinal section across the uterus, placenta, and like terminal villi (4). C: Highly simplified light microscopic membranes in the human. The chorionic sac, consisting of pla- section of two terminal villi, branching off a mature intermedi- centa (left half) and membranes (right half), is black. B: Periph- ate villus (right). D: Schematic electron microscopic section of eral ramifications of the mature villous tree, consisting of a stem the placental barrier, demonstrating its typical layers. (Source: villus (1), which continues in a bulbous immature intermediate Modified from Kaufmann, 1983, with permission.) villus (3); the slender side branches (2) are the mature interme- 52 6. Basic Structure of the Villous Trees Figure 6.2. Remains of cell membranes in the syncytiotropho- have largely lost their microvilli and now establish a smooth blast result from two mechanisms: fusion of neighboring villous intercellular cleft bridged by numerous desmosomes that verti- surfaces forming bridges and repair of disrupted syncytial sur- cally traverse the syncytiotrophoblast. (See the survey picture faces. A: As a first step of formation of syncytial bridges between in Figure 6.3.) ¥11,000. C: As the next step of repair, the newly adjacent villi, the opposing microvillous membranes interdigi- developed separating cell membranes show disintegration tate. The identical initial step occurs as a first step of wound (arrows) so that the newly formed intrasyncytial cleft becomes repair when the disrupted ends of syncytiotrophoblast become focally bridged by syncytioplasm. After complete disintegration covered by microvillous membranes, which subsequently come of membranes and desmosomes, the syncytial villous cover is into contact and interdigitate. ¥14,500. B: Follow-up stage of again continuous. ¥13,800. (Source: Cantle et al., 1987, with that depicted in part A. The interdigitating syncytial surfaces permission.) wound, thus avoiding further loss of syncytioplasm. After- by desmosomes. Later, fusion occurs by disintegration of ward, either a fibrinoid plug closes the wound, or the the separating membranes (Fig. 6.2). Transitory stages of disconnected parts of the syncytium again come into this process may be misinterpreted as borders of syncytial close contact to form a lateral intercellular cleft bridged units. Syncytiotrophoblast 53 Figure 6.3. Survey electron micrograph of the section shown space (IVS) with the connective tissue core (CO) of the villi. in Figure 6.2B demonstrates that in intermediate stages of syn- Such intrasyncytial membranes must not be misinterpreted as cytial repair the villous syncytial trophoblast can be traversed stable intercellular clefts subdividing the syncytium in smaller by vertical “intercellular” clefts connecting the intervillous syncytial units. ¥7000. Many authors have described remainders of intercellular junc- as occludin in the apical part of syncytiotrophoblast (Marzioni et al., tions within the syncytiotrophoblast (Carter, 1963, 1964; Boyd & 2001). The authors of the publications considered above hypothesized Hamilton, 1966; Burgos & Rodriguez, 1966; Metz et al., 1979; Reale that these junctions represent stages of several dynamic processes: for et al., 1980; Wang & Schneider, 1987). Some investigators have example, interpreted these junctions as vestiges of a former cellular state of the • remainders of cytotrophoblastic fusion with the overlying syncytium, syncytium or as proof of recent cytotrophoblastic contribution to the • establishment of intervillous contacts by partial or complete syncytio- syncytiotrophoblast (Carter, 1963, 1964; Boyd & Hamilton, 1966). In trophoblastic surface fusion, a systematic study of these junctions, Metz et al. (1979) as well as • repair of syncytial disruption, or Reale et al. (1980) have described zonulae and maculae occludentes • decomposition of temporary surface invagination of the that were mostly located in apical membrane infoldings, as well as syncytiotrophoblast. numerous isolated desmosomes, without contact to any surface mem- branes. The structural evidence has been supported by recent immuno- The syncytiotrophoblast must be seen as a highly dynamic layer that is histochemical data that show zonula occludens type 1 (ZO-1) as well qualified for ameboid movements. Disconnection with subsequent re- 54 6. Basic Structure of the Villous Trees fusion and the formation of invaginations and bridges seem to be et al., 1969, 1970; Rovasio & Monis, 1973; Liebhart, 1974; Martin regular phenomena. et al., 1974; Okudaira & Hayakawa, 1975; Nelson et al., 1976; We not only find the ultrastructural remains of this process but also Wada et al., 1977, 1979; Kelley et al., 1979; King, 1981; Wasser- can easily observe it in vitro; when we (1) carefully trypsinize villi from man et al., 1983a,b; Fisher et al., 1984; Parmley et al., 1984; full-term placentas, (2) remove mesenchymal cells by means of CD9- Maubert et al., 1997). Various lectin-binding sites have been described antibodies, (3) remove cytotrophoblast by means of a hepatocyte by Whyte (1980), Gabius et al. (1987), Hoyer and Kirkeby (1996), growth factor-activator-inhibitor (HAI-1) antibody (Pötgens et al., 2003), and Lang et al. (1994a,b). and (d) let the remaining isolated tissues adhere on culture dishes, we Cytochemical demonstration of membrane-bound enzymes obtain a nearly pure population of mononucleated fragments of syncy- studied in the transmission electron microscope (TEM) comprise alkaline tiotrophoblast. These small CD105-positive mononucleated trophoblast phosphatase (Hempel & Geyer, 1969; Borst et al., 1973; Hulstaert et elements, which result in large numbers during any attempt of tropho- al., 1973; Kameya et al., 1973; Jemmerson et al., 1985; Matsubara blast isolation, should not be misinterpreted as cytotrophoblast (Pötgens et al., 1987c),