Tbx4 function during hindlimb development reveals a mechanism that explains the origins of proximal limb defects.

We dissect genetically a gene regulatory network, including the transcription factors Tbx4, Pitx1 and Isl1 that act cooperatively to establish the hindlimb bud and identify key differences in the pathways that initiate formation of the hindlimb and forelimb. Using live image analysis of limb mesenchyme cells undergoing chondrogenesis in micromass culture, we distinguish a series of changes in cellular behaviours and cohesiveness that are required for chondrogenic precursors to undergo differentiation. Furthermore, we provide evidence that the proximal hindlimb defects in the Tbx4 mutant result from a failure in the early differentiation step of chondroprogenitors into chondrocytes, providing a novel explanation for the origins of proximally-biased limb defects.