Mechanism of Fundulus Epiboly--A Current View

This paper summarizes evidence for the following picture ofFundulus epiboly, with an eye toward laying groundwork for futureinvestigation. The major force in epiboly is the yolk syncytiallayer (YSL). Prior to epiboly, it spreads well beyond the borderof the blastoderm to form the wide external YSL (E-YSL). Thishas contractile properties, which, however, are restrained priorto epiboly by the attached enveloping layer (EVL) of the blastoderm.Epiboly begins when the E-YSL contracts and narrows, throwingits surface into folds and pulling the internal YSL (I-YSL)and the attached EVL vegetally. When the narrowing of the E-YSLhas ceased, it is postulated that its contractility continuesas a circumferential wave of vegetally directed contractionthat moves over the yolk toward the vegetal pole, dragging theI-YSL and the attached EVL (and blastoderm) with it. The mostobvious visible manifestation of this wave is a marked marginalconstriction, where the YSL joins the yolk cytoplasmic layer(YCL). As this contractile wave passes over the yolk, cytoplasmfrom the YCL mingles with that of the advancing E-YSL, and YCLsurface adds to the already highly convoluted surface of theE-YSL. This folded surface is the site of a thin, highly localizedband of rapid endocytosis that encircles the egg and passesover 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 forthe disappearance of the surface of the YCL. Since the YCL surfacestands in the way of the advancing YSL, its internalizationis part of the mechanism of epiboly. As the I-YSL expands inresponse to this marginal pull, its abundant microvilli graduallydisappear, providing surface for its epiboly. The firmly attachedEVL likewise expands toward the vegetal pole in response tothe pull of the autonomously expanding YSL. As epiboly of theEVL progresses, it adjusts to the geometric problems posed bya sheet expanding over a sphere by active cell rearrangementwithin the cell monolayer. Thus, epiboly of the EVL has an activeas well as a passive component. Deep cells are not causallyinvolved in epiboly, but move about in coordinated ways in theconstantly increasing space between the I-YSL and the EVL providedby epiboly and form the germ ring and the embryonic shield andeventually the embryo proper. An attempt is made to pull allof this together, and more, in order to achieve as comprehensivean understanding of epiboly as present evidence will allow.