Chondrocyte differentiation is blocked prior to the separation into columnar and round chondrocytes.

<p>Sections of E14.5 forelimbs of control (A, D, G), <i>Hoxa11^−/−;d11^−/−</i> (B, E, H) and <i>Ulnaless</i> embryos (C, F, I) were hybridized with antisense riboprobes for <i>Col2a1</i> (A–C), <i>Ihh</i> (D–F) and <i>Ptch1</i> (G–I). At E14.5, <i>Col2a1</i> is expressed in all chondrocytes of ulna and radius of <i>Hoxa11^−/−;d11^−/−</i> and <i>Ulnaless</i> mutants similar as in control limbs (A–C). In contrast, <i>Ihh</i>, which is expressed in early hypertrophic chondrocytes in E14.5 control limbs (D, arrow), is not expressed in the ulna of both mutants (E, F). <i>Ptch1</i> is strongly expressed in wild type chondrocytes adjacent to the <i>Ihh</i> expression domain (G, yellow arrow), but only basal expression levels are detected in chondrocytes of the ulna of <i>Hoxa11^−/−;d11^−/−</i> and <i>Ulnaless</i> mice (G–I, red arrows). 80x magnification; R = radius, U = ulna.