21-P033 The role of the transcription factor Sox21 in the developing chick inner ear

delayed chondrocyte differentiation since Indian hedgehog (Ihh) is not expressed in the ulna of Ulnaless or Hoxa11//d11/ mice. Fibroblast growth factor 3 (Fgfr3), which is expressed at low levels in distal cells and at high levels in columnar chondrocytes, shows low expression in the Ulnaless and Hoxa11//d11/ zeugopod resembling the expression level in distal chondrocytes. However, the Unique cartilage matrix-associated protein (UCMA), a specific marker for distal chondrocytes, is only expressed in the outermost cell layers in ulna and radius of both mutant mouse lines. These results indicate that chondrocyte differentiation is arrested at an early step in Ulnaless and Hoxa11/d11 double mutant mice, before the differentiation of distal and columnar chondrocytes takes place.