Stretch but don't follow

杨柳

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% K+ K$ t# M: t9 bAs slow-twitch muscle cells migrate through fast-twitch muscle precursor cells, they act as morphogenesis tutors, according to Clarissa Henry and Sharon Amacher (University of California, Berkeley, CA).* q" ]; I! e  u0 ~% A5 x

4 b; D$ g4 Q) f) w. m+ fFast-twitch muscle cells in zebrafish start off life as round blobs but must undergo anterio-posterior elongation to become functional myofibers. Henry and Amacher had noticed that this elongation process spreads in a medial-to-lateral wave, i.e., cells near the midline change shape first.  n% x  \3 a# F. O' ?$ Y

( ]# r0 h" J, j2 o# v+ hThe authors first established that elongation required Hedgehog signaling. Transplantation experiments showed, however, that this effect was indirect. To restore fast-twitch elongation in a Hedgehog-signaling mutant, the critical ingredient was wild-type slow-twitch cells not wild-type fast-twitch cells.# [* z; N$ ~3 @, L% D
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In response to the Hedgehog signal, transplanted slow-twitch cells extended across the anterior–posterior width of a somite and migrated outwards from the notochord. Fast-twitch cells in their wake elongated. The effect could be relayed to other fast-twitch cells as long as the targets were in the same somite. In the wild type there are probably enough migrating cells to contact directly all of the fast-twitch cells.
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( ^2 Z( t# t# N* ^7 C8 SFuture work will show if the slow-twitch cell or its discarded extracellular matrix is acting as a scaffolding for the fast-twitch cell, or if other signals are at work. In other sites of cell migration, such as the developing limb and early embryo, there may be similar drop-offs of differentiation and morphogenesis cues.
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% |8 L( e& ~" s- y+ C( ZHenry, C.A., and S.L. Amacher. 2004. Dev. Cell. 7:917–923.(Ovoid fast muscle cells (white arrow) st)