Cranial mesenchyme in the FGF9 transgenic mice differentiates into chondrocytes. Heads of nontransgenic (A, A', C, C', E, E', G, G', I, I', K and K') and FGF9 transgenic mice (B, B', D, D', F, F', H, H', J, J', L and L') were sectioned and stained with hematoxylin and eosin. Panels A'-L' are higher magnifications of the boxed regions in panels A-L respectively. In nontransgenic mice, the mesenchymal cells of the skull form a skeletogenic membrane (arrows in A', C', E' and G') within which the bones form. In the FGF9 transgenic mice the cranial mesenchymal cells (cm) differentiate into chondrocytes (ch) that initially form a structure resembling the hyaline cartilage (D, D', F and F') and later hypertrophy (hyp) (H, H', J and J'). Perichondrial cells, including blood cells (J', arrows) invade and replace the hypertophic chondrocytes forming bone and bone marrow (L and L'). Other abbreviations: b, brain; s, skin. The folding seen in the section in panel G is an artifact of the histology procedure. Note that the skin was removed from P7 and P18 mice to facilitate fixation and histological processing. Scale bar: 50 μm in A'-L'; 100 μm in A, B, C, D, E, H, I and K; 200 μm in F, G, J and L.