OBJECTIVE OF THE EXPERIMENT The chick embryo is extremely amenable to surgical manipulations such as bead implantation. Local delivery of signaling molecules is easily achieved in this experimental model and provides invaluable information as to their effect on a variety of developmental processes. Specifically, our knowledge about limb initiation was significantly increased when members of the Fibroblast Growth Factor (FGF) family were shown to induce ectopic limb formation (Cohn et al., 1995). In this experiment, recombinant FGF-8 is produced, purified, and applied to the flank of chick embryos, where it induces the formation of a fairly normal extra limb.

INTRODUCTION

The analysis of limb outgrowth and patterning during chick embryo development has become an excellent model for understanding the molecular mechanisms underlying general developmental processes. The choice of the chick as an experimental model system has greatly enhanced embryo manipulation and survival in the laboratory.

In most vertebrates, limb appendages originate in the flank of the early embryo where cells group in the so-called limb buds. The first step in limb development, known as limb initiation, comprises the induction of limb budding in specific locations of the embryo's flank. At this stage, most of the flank's extension is competent to give rise to limb buds (Figure 8.1). However, budding will occur only in distinct regions of the embryo's flank (four regions in tetrapods, two on each flank). The limb bud is formed by a mesodermal or mesenchymal core, covered by an ectodermal jacket (Figure 8.2).