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The slug gene encodes a zinc finger transcription factor expressed by neural crest cells (Nieto et al., Science 264: 835-839, 1994) and by certain non-crest derived mesenchymal cell populations, such as lateral mesoderm and sclerotome (Mayor et al., Development 121: 767-777, 1995; Buxton et al., Dev. Biol. 183: 150-165, 1997). We report here that slug is also expressed in developing chick limbs. The slug expression domain in the limb bud expands from posterior to anterior and marks cells that are predominantly destined to become chondrocytes but have not yet differentiated. Its expression is maintained in connective tissue, but is never observed in the premuscle masses. We show that removal of the apical ectodermal ridge results in loss of slug expression which can be arrested by the addition of an FGF-4 bead. Retinoic acid bead implants lead to down-regulation of slug expression, again accompanied by abolition of limb outgrowth. Dual bead implants demonstrate antagonism between these two factors, suggesting that a localized antagonistic effect between endogenous RA and FGF-4 on slug expression underlies the molecular mechanism controlling the transition between undifferentiated and differentiated state during normal limb development. The fact that slug expression pattern correlates with areas of growth in the limb, and is maintained by FGF-4 and down-regulated by retinoic acid, indicates that slug-expressing cells play a crucial role in growth and patterning of the chick limb. We propose that slug expression provides the best correlation to date between a molecular marker and the physical concept of the progress zone, defined as "a labile region where new positional values are successively engendered in the course of growth" (Summerbell et al., Nature 244: 492-496, 1973).

Expression of the transcription factor slug correlates with growth of the limb bud and is regulated by FGF-4 and retinoic acid.