Mechanism ofFundulusEpiboly—A Current View

This paper summarizes evidence for the following picture of Fundulus epiboly, with an eye toward laying groundwork for future investigation. The major force in epiboly is the yolk syncytial layer (YSL). Prior to epiboly, it spreads well beyond the border of the blastoderm to form the wide external YSL (E-YSL). This has contractile properties, which, however, are restrained prior to epiboly by the attached enveloping layer (EVL) of the blastoderm. Epiboly begins when the E-YSL contracts and narrows, throwing its surface into folds and pulling the internal YSL (I-YSL) and the attached EVL vegetally. When the narrowing of the E-YSL has ceased, it is postulated that its contractility continues as a circumferential wave of vegetally directed contraction that moves over the yolk toward the vegetal pole, dragging the I-YSL and the attached EVL (and blastoderm) with it. The most obvious visible manifestation of this wave is a marked marginal constriction, where the YSL joins the yolk cytoplasmic layer (YCL). As this contractile wave passes over the yolk, cytoplasm from the YCL mingles with that of the advancing E-YSL, and YCL surface adds to the already highly convoluted surface of the E-YSL. This folded surface is the site of a thin, highly localized band of rapid endocytosis that encircles the egg and passes over it with the E-YSL in a wave throughout epiboly. This internalization, which is receptor independent and therefore somehow programmed, accompanies the putative contractile wave, and accounts for the disappearance of the surface of the YCL. Since the YCL surface stands in the way of the advancing YSL, its internalization is part of the mechanism of epiboly. As the I-YSL expands in response to this marginal pull, its abundant microvilli gradually disappear, providing surface for its epiboly. The firmly attached EVL likewise expands toward the vegetal pole in response to the pull of the autonomously expanding YSL. As epiboly of the EVL progresses, it adjusts to the geometric problems posed by a sheet expanding over a sphere by active cell rearrangement within the cell monolayer. Thus, epiboly of the EVL has an active as well as a passive component. Deep cells are not causally involved in epiboly, but move about in coordinated ways in the constantly increasing space between the I-YSL and the EVL provided by epiboly and form the germ ring and the embryonic shield and eventually the embryo proper. An attempt is made to pull all of this together, and more, in order to achieve as comprehensive an understanding of epiboly as present evidence will allow.