Pax genes are a family of development control genes that encode nuclear transcription factors. They are characterized by the presence of the paired domain, a conserved amino acid motif with DNA‐binding activity. Originally, paired‐box‐containing genes were detected in Drosophila malenogaster, where they exert multiple functions during embryogenesis. In vertebrates, Pax genes are also involved in embryogenesis. Mutations in four out of nine characterized Pax genes have been associated with either congenital human diseases such as Waardenburg syndrome (PAX3), Aniridia (PAX6), Peter's (...) anomaly (PAX6), renal coloboma syndrome (PAX2), Small eye (Pax6), (Pax21Neu), which all show defects in development. Recently, analysis of spontaneous and transgenic mouse mutants has revealed that vertebrate Pax genes are key regulators during organogenesis of kidney, eye, ear, nose, limb muscles, vertebral column and brain. Like their Drosophila counterparts, vertebrate Pax genes are involved in pattern formation during embryogenesis, possibly by determiing the time and place of organ initiation of morphogenesis. For most tissues, however, the nature of the primary development action of Pax transcription factors remains to be elucidated. One predominant theme is signal transduction during tissue interactions, which may lead to a position‐specific regulation of cell proliferation. (shrink)

During vertebrate limb development, various genes of the Hox family, the products of which influence skeletal element identity, are expressed in specific spatiotemporal patterns in the limb bud mesenchyme. At the same time, the cells also exhibit ‘self‐organizing’ behavior – interacting with each other via extracellular matrix and cell‐cell adhesive molecules to form the arrays of mesenchymal condensations that lead to the cartilaginous skeletal primordia. A recent study by Yokouchi et al.(1) establishes a connection between these phenomena. They misexpressed the (...) product of the Hoxa‐13 gene in chick limb buds and demonstrated both skeletal pattern perturbations and changes in cell‐cell adhesivity in mesenchyme aberrantly expressing this protein. (shrink)

The development of the vertebrate skeleton is under complex genetic control, and good progress is being made towards identifying the genes responsible. A recent paper(1) contributes to this progress by describing transgenic mice in which the homeobox‐containing MHox gene has been disrupted. MHox(−/−) mice have a range of skeletal defects, involving loss or shortening of structures in the skull, face and limb. Puzzling features of the MHox(−/−) mutation, which has similar effects on bones with very different embryological origins and yet (...) spares other bones completely, may hold clues to the mechanisms that shape the skeleton. MHox(−/−) mice, used in conjunction with other skeletal mutants, will be important tools for exploring these mechanisms further. (shrink)