Freeze-fracture image of microvilli (brush border) from the apical portion of an epithelial cell of the small intestine (cf. Figs. 72-74, 77, 78). In the freeze-fracture technique, membranes are split into two membrane complements. This makes it possible to investigate and interpret the structures of the two complementary inner membrane surfaces. It is hardly possible to render a picture of the real outer and inner surfaces of the plasmalemma with the freeze-etching technique alone. The freeze-fracture technique exposes the extracellular face (EF) and the protoplasmic face (PF) [2. EF is the exposed face of the half-membrane that started out as the border to the extracellular space; PF was the half-membrane toward the cytoplasmic side (cf. Figs. 75, 77).
1 Extracellular face (EF)
2 Protoplasmic face (PF)
3 Apical cytoplasm, location of the terminal web (see Fig. 75) Electron microscopy; magnification: x 35 200
Microvilli (brush border) around a duodenal enterocyte at large magnification (cf. Fig. 72-76, 415, 416). Each of the microvilli contains a central microfilament bundle (central bundle traversing the length of the villi) H, which emerges from an apical dense spot [2. Each longitudinal bundle consists of 20-30 actin filaments, which fan into the terminal web [3 area. Note the finely granular, sometimes filament-like material on the surfaces of the microvilli. This is the glycocalix (see Figs. 75, 79). The central actin filaments (inner skeleton) create a contractile element that can shorten the microvilli and presumably facilitate resorption. The microvilli, in conjunction with the terminal web, form a protein filament system, which achieves parallel movement and cytoskeletal stability. The apical, terminal web area of the epithelium contains few if any organelles. Among other structural proteins, the microvilli contain villin, which cross-links the actin filaments. Another component is calmodulin. Together with Ca2+, it apparently regulates the function of villin. There are actin filaments in the terminal web as well. These run parallel to the cell surface. Other elements of the terminal web are myosin, tropomyosin and 10-nm filaments, which are probably cytokeratin filaments that connect to the zonulae adherentes of the duodenal absorptive cells (cf. Fig. 56). This complex system apparently supports the microvilli, gives them rigidity, and anchors them in the terminal web H .On the other hand, this system also provides microvilli with a controlled level of motility. Duodenalenterocytes (absorptive cells).
1 Central microfilament bundle (actin filaments, diameter 7 nm)
2 Apical dense spot
3 Terminal web
Electron microscopy; magnification: x 95 000
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