TABLE 1.
Limb development genes discussed in text. This list is not intended to be exhaustive, but rather to serve as a guide to genes and proteins discussed in text.
| Gene or gene family | Putative function of product |
|---|---|
| Bone morphogenetic protein-5 (Bmp-5) | A growth factor that belongs to the transforming growth factor β (TGF-β) superfamily and bone morphogenetic protein family. This gene is involved in anteroposterior and proximodistal patterning during early limb development, and regulating cartilage growth in later development (Bailon-Plaza et al., 1999) |
| Engrailed 1 (En-1) | A transcription factor expressed in the ventral ectoderm which helps establish the dorso-ventral patterning of the limb. |
| Fibroblast growth factors | One of the major families of protein signaling molecules. FGF proteins play major roles in many different developmental contexts. |
| Fgf-4 | Involved in the regulation loop between FGF-8 and FGF-10. |
| Fgf-8 | Expressed in the limb ectoderm in the region of the apical ectodermal ridge (AER). FGF-8 is a signaling protein that promotes limb outgrowth through mitosis in the region underlying the AER. |
| Fgf-10 | A signaling protein which is expressed in the lateral plate mesoderm in the region where limb outgrowth takes place. This protein induces initial outgrowth of the limb. |
| Fibroblast growth factor receptor 2 (Fgfr-2) | Mediates the regulation loop between FGF-8 and FGF-10. |
| Growth differentiation factor-5 (Gdf-5) | A growth factor that belongs to the transforming growth factor β (TGF-β) superfamily. This gene is involved in anteroposterior and proximodistal patterning during early limb development, and joint formation later on (Strom and Kingsley, 1996). |
| Homeobox (Hox) genes | This family of genes is defined by a common “homeodomain” sequence code for transcription factors that play fundamental roles in specifying regional identity along the craniocaudal axis of the body. In the limb, various members of this family are involved in specifyious members of this family are involved in specifying regional identity along both the proximodistal and anteroposterior axes. |
| Pituitary homeobox 1 (Ptx-1) | A transcription factor that is a member of the Ptx homeobox gene family. Ptx1 is expressed in the lateral mesoderm in the hind limb region and is thought to act upstream of Tbx 4 in specifying hindlimb identity. |
| Radical Fringe (Rfng) | A signaling molecule that is expressed in the dorsal ectoderm of the limb bud and is involved in establishing the dorso-ventral patterning of the limb. |
| Sonic hedgehog (Shh) | A signaling molecule expressed in the posterior mesenchyme of the limb bud that may be anterposterior patterning of the limb. It may act as a diffusible morphogen. |
| T-box genes | A family of transcription factors that share a unique DNA-binding domain (T- box). Members of this family are crucial for pattern formation in many different developmental contexts. |
| Tbx-4 | A transcription factor that specifies hindlimb identity. |
| Tbx-5 | A transcription factor that specifies forelimb identity. It appears to initiate mesenchyme migration into the forelimb region (Ahn et al., 2002). |
| Wnt (Wingless Interactive) 7a | A signaling protein expressed in the dorsal ectoderm of the limb bud which helps establish the dorso-ventral patterning of the limb. |