Ihh signals the position of prehypertrophic and hypertrophic chondrocytes. (a–d) H&E staining of sections of the tibiae from d17.5 wild-type (a), PPR^–/–/wild-type chimera (b), and Ihh^–/–;PPR^–/–/wild-type chimera (c and d) embryos. The presence of ectopic hypertrophic chondrocytes in the PPR^–/–/wild-type chimeric growth plate induces elongation of the layer of wild-type columnar proliferating chondrocytes, leading to greater distance between the layers of periarticular proliferating chondrocytes and prehypertrophic/hypertrophic chondrocytes (compare a and b). In the Ihh^–/–;PPR^–/–/wild-type chimeric growth plate, however, the presence of ectopic hypertrophic chondrocytes does not induce elongation of the layer of wild-type columnar proliferating chondrocytes (c and d, in increasing order of chimerism). Perpendicular bars indicate the length of the layer of columnar proliferating chondrocytes. (e–h) In situ hybridization of sections of the tibiae from d17.5 embryos with a mouse PTHrP antisense probe. In the wild-type growth plate (e), PTHrP mRNA is weakly expressed in the periarticular proliferating chondrocytes, whereas in the PPR^–/– growth plate (f), its expression is strongly upregulated in the same area (bracket). In the PPR^–/–/wild-type chimeric growth plate (g), PTHrP mRNA expression is upregulated in the periarticular proliferating chondrocytes, but to a lesser extent compared with the PPR^–/– growth plate (bracket). In contrast, despite the presence of ectopic hypertrophic chondrocytes, there is no upregulation of PTHrP mRNA expression in the Ihh^–/–;PPR^–/–/wild-type chimeric growth plate (h). PTHrP mRNA expression is more clearly seen in sections of the wild-type and Ihh^–/–;PPR^–/–/wild-type chimeric growth plates exposed twice as long (data not shown); the sections shown in e and h, however, were exposed for the same time as those in f and g to allow direct comparison. Left, bright field; right, dark field. Horizontal bar = 100 μm.