The initiation of the limb bud: Growth factors, Hox genes, and retinoids

Abstract

Department of Genetics Harvard Medical School Boston, Massachusetts 02115 The undifferentiated vertebrate limb bud is a self- organizing system. If transplanted to a favorable ectopic location, the limb bud is capable of developing into a mor- phologically normal limb (Harrison, 1918), and the ante- rior-posterior polarity of the transplanted limb is deter- mined by the graft rather than the host environment. Great progress has recently been made in understanding the molecular steps by which pattern emerges within an undif- ferentiated limb bud (for recent reviews see Johnson et al., 1994; Tickle and Eichele, 1994). But what first initiates the formation of the limb bud? And what molecular steps provide the information necessary for polarized self- organization? Two recent papers (Charit~ et al., 1994; Cohn et al., 1995 [this issue of