Search

When a mouse zone of polarizing activity (ZPA) at the posterior margin of the limb bud was grafted at the anterior margin of the chick limb bud, we found that expression of the chick homeobox genes, Chox-4.7 and -4.8, was induced prior to the formation of chick extra digits. The induction was observed in the restricted domain close to both grafted mouse ZPA and the chick apical ectodermal ridge (AER). When the posterior half of the AER was removed, the normal expression was diminished in the posterodistal region. Thus, it is likely that at least two distinct factors, one from the ZPA and the other from the AER, provide positional information to induce cooperatively the sequential expression of the Chox-4 genes.

Expression patterns of three fibroblast growth factor receptor genes, FGFR1 (cek1), FGFR2 (cek3) and FGFR3 (cek2), were observed in limb and feather morphogenesis. Expression of FGFR1 was observed in the mesoderm of limb bud, in the mesenchyme just underneath the feather placode, and then in the anterior mesenchyme of the feather bud. While expression of FGFR2 was observed in both surface ectoderm and mesenchymal aggregates corresponding to the future bones of the limb, the mesenchyme between the feather placode, and surface ectoderm of feather buds. Expression of FGFR3 was observed rather ubiquitously over mesoderm of limb and feather buds. Differential expression of these FGF receptor genes suggested that differential roles of these receptors in epithelia-mesenchymal interactions of limb and feather morphogenesis.

Activin is known to induce axial mesoderm during early development in Xenopus embryo. Activin receptor was recently identified to be a member of transmembrane serine/threonine kinase family. We have studied the role of activin-mediated signaling in the limb morphogenesis by identifying the target cells. We isolated cDNAs encoding chicken activin receptors, cARIIA and cARIIB, and examined their expression patterns during chick embryogenesis. The cARIIA gene is expressed in the apical ectoderm of the limb bud at stage 20-21, whereas the cARIIB gene is expressed uniformly in the limb mesenchyme. Expression of the cARIIA gene is confined to dorsal and ventral mesenchyme at stage 23, and later confined to precartilaginous cells. Transcripts of the cARIIA gene are found in developing neuroepithelium of spinal cord, brain and eye, surface ectoderm differentiating to epidermis, and myoblasts differentiating to muscle. The IIB receptor gene is highly expressed in the developing brain. These results suggest that the activins and their receptors are implicated in the limb development, especially, in differentiation of muscle, skin and bone.