Cellular and Genetic Mechanisms of Convergence and Extension
The zebrafish blastula consists of a mound of blastomeres that lies atop a large syncytial yolk cell (Fig. 1). Around the mid-blastula stage, the initial dorsoventral polarity of the embryo has already been established (see Sakaguchi et al.; Hibi et al., this Vol.) and the induction of mesendoderm has begun (see Kimelman and Schier, this Vol.; Warga and Stainier, this Vol.). During the course of the next 10h of development, cells within the blastoderm will acquire specific fates (see Hammerschmidt and Mullins, this Vol.), as well as engage in morphogenetic movements to secure appropriate positions in the nascent embryo. These cell migrations and rearrangements transform the mound of blastomeres into an embryo that features a typical vertebrate body plan. During gastrulation, epibolic movements spread the blastoderm and the yolk syncytial layer towards the vegetal pole. Germ layer formation begins when mesendodermal cells move beneath ectoderm (see Kane and Adams, this Vol.). Epiboly and mesendoderm internalization take place in a relatively uniform manner along the entire dorsoventral axis, except for a small region in the gastrula organizer. This chapter focuses on the third gastrulation movement, convergence and extension, through which dorsoventral and anteroposterior embryonic polarity become morphologically distinct. The process of convergence and extension fashions the dorsoventral axis of the nascent embryo by narrowing its tissues from back to belly.
KeywordsExtension Movement Dorsal Midline Cell Fate Specification Fibroblast Growth Factor Signaling Vegetal Pole
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