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

The undifferentiated vertebrate limb bud is a selforganizing system. If transplanted to a favorable ectopic location, the limb bud is capable of developing into a morphologically normal limb (Harrison, 1918), and the anterior-posterior polarity of the transplanted limb is determined 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 undifferentiated 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 selforganization? Two recent papers (Charit~ et al., 1994; Cohn et al., 1995 [this issue of Cell]) give important clues that go a long way toward answering these questions. The Problem of Limb Bud Initiation Limb bud formation is initiated in the embryo bythe continued local proliferation of cells of the lateral plate mesoderm at the appropriate axial levels at a time of cessation of rapid growth of cells of the rest of the flank. These proliferating mesenchymal cells form a bulge under the ectoderm. As will be discussed below, there a~e extensive interactions between these mesenchymal cells and the overlying ectoderm during limb bud growth. However, at least in amphibian embryos, at this early stage it is the mesodermal cells that provide the signals to initiate the process of limb development. For example, grafting the presumptive limb mesenchyme to an ectopic location results in the formation of a new limb, while moving the presumptive limb ectoderm has no such effect (Harrison, 1918). Signals within the mesoderm thus produce a rapidly proliferating population of mesenchymal cells. Their continued proliferation, however, depends on interaction with the overlying ectoderm. In avian and mammalian limb buds, the rapidly dividing mesodermal cells induce the ectodermal cells along the anterior to posterior rim of the bud to elongate and form a specialized structure called the apical ectodermal ridge (AER). Once the AER has formed, it is responsible for maintaining the continued outgrowth of the limb bud (Saunders, 1972) and for elaborating pattern along the proximal-distal axis (Summerbell et al., 1973). These AER activities can be functionally replaced by ectopic application of members of the fibroblast growth factor (FGF) family (Niswander et al., 1993; Fallon et al., 1994). Several FGF family members are expressed in the AER, including FGF4, which has a posterior bias within the limb bud (Suzuki et al., 1992; Niswander and Martin, 1992). Thus, it seems likely that one or more of these factors are responsible for keeping the mesenchymal cells adjacent to the AER (a region referred to as the progress zone) in an undifferentiated, rapidly proliferating Minireview