Segmental History

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Science  14 Nov 2003:
Vol. 302, Issue 5648, pp. 1117-1119
DOI: 10.1126/science.302.5648.1117d

Boundaries are established during embryonic development to define cellular compartments that underlie patterning and growth. In the developing Drosophila embryo, the epidermis becomes transiently divided into a series of segments, each with a deep groove that marks an anterior and posterior edge. These boundaries form just posterior to cells that express the transcription factor Engrailed.

Larsen et al. observed that boundary formation begins when specific Engrailed-expressing cells called the “groove founders” lose apical contact with neighboring cells, constrict, and migrate inward into the embryo, pulling adjacent cells with them to line the sides of the boundary. Once at the bottom of the groove, founder cells stop expressing Engrailed. Boundaries then regress rapidly during dorsal closure, when the epidermis must spread to enclose the entire embryo, suggesting that cells buried in the grooves may provide this extra surface area. Analysis of mutants indicates that groove formation requires Engrailed and the morphogens Hedgehog and Wingless. Hedgehog signaling is required posterior to the boundary but is repressed by Wingless anteriorly in order to prevent a boundary from forming on the wrong side of Engrailed-expressing cells.

Prud'homme et al. have found that Engrailed and Wingless are also expressed in epidermal cells on opposite sides of boundaries that define morphologically similar segments during the development of a primitive annelid. The molecular and morphological similarities suggest that a segmental unit body plan may have evolved from a common ancestor. — LDC

Development 130, 5625 (2003); Curr.Biol. 13, 1876 (2003).

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