Expression and function of Bapxl during chick limb development

摘要

The homeobox-containing transcription factor Bapxl (also known as Nkx.3.2) is crucial for development of the axial skeleton and parts of the chondro-cranium,. Here we describe the detailed expression of Bapxl during chick limb development and show that in contrast to its expression in the axial skeleton, Bapxl is expressed after the commitment to chondrogenesis Bapxl is initially expressed throughout the developing skeletal elements prior to the overt differentiation of the distinct chondrogenic layers,. Once distinct layers (proliferating, prehypertrophic and hyper trophic) have formed, Bapxl expression is restricted to the proliferating chondrocytes, Bapxl transcripts are excluded from the articular cartilage, A second homeobox-containing transcription factor, Barxl, is expressed in a complementary fashion in the developing joint and articular cartilage. Interestingly, in vitro functional analyses showed that Bapxl over expression in micromass cultures increased both matrix production and nodule number suggesting that Bapxl is sufficient to promote chondrogenesis in the limb. In contrast, Barxl had the opposite effect on nodule number suggesting that it has an inhibitory effect on chondrogenic initiation consistent with its expression in the developing joint, A slight increase in matrix levels was also observed consistent with its expression in the articular chondrocytes. Finally, we show that Bapxl is also expressed in the soft tissues such as the developing tendons, muscle sheaths and surrounding mesenchyme, and therefore may have additional as yet uncharacterized roles in limb morphogenesis.