Pax genes and organogenesis: Pax9 meets tooth development.

Pax genes encode a family of transcription factors that play key roles during embryogenesis. They are required for the development of a variety of organs including the nervous and muscular system, skeleton, eye, ear, kidney, thymus, and pancreas. Whereas the developmental roles of many of the nine known Pax genes have been analyzed in great detail, a functional analysis of Pax9 has just begun. During mouse embryogenesis, Pax9 exhibits a highly specific expression pattern in derivatives of the foregut endoderm, somites, limb mesenchyme, midbrain, and the cephalic neural crest. In the mandibular arch mesenchyme, the expression of Pax9 marks the prospective sites of tooth development prior to any morphological signs of odontogenesis and is maintained in the developing tooth mesenchyme thereafter. To understand the function of Pax9 during mouse embryogenesis, we recently have created a null allele by gene targeting. Preliminary analyses show that Pax9 is essential for the formation of teeth, and we conclude that Pax9 is required for tooth development to proceed beyond the bud stage. Here, we briefly summarize our current knowledge about Pax genes and introduce Pax9 to the growing family of factors which are involved in tooth development.