The organization of actin filaments in the brush border of yolk sac epithelial cells

Abstract

The yolk sac epithelium offers a potentially interesting cell type in which to examine brush border cytoskeletal organization because it functions not only in absorption across the microvillous membrane, but also has numerous coated pits at the bases of the microvilli which function in endocytosis. In this study we have examined the organization of actin filaments in the brush border of yolk sac endodermal cells using myosin S1 subfragment decoration. Microvilli contained bundles of actin filaments polarized such that the arrowheads pointed away from the microvillar tips. The bundles continued into the apical cytoplasm as short rootlets that intermingled with a poorly developed layer of other filaments, some of which were actin, running parallel to the apical cell surface (i.e., in the terminal web region). Decorated microfilaments were sometimes associated with coated pits and vesicles, but it was difficult to distinguish direct membrane-filament interaction from coincidental associations brought about by close proximity of coated vesicles to actin filaments emerging from the microvilli. A prominent peripheral band of microfilaments of mixed polarity was present at the level of the zonula adherens. This band of filaments may act as a contractile ring. Tension of the contractile ring, together with a poorly developed terminal web, may produce the domeshaped apical surface characteristic of these cells.