Fig. 8. Chondrocytes develop in the perichondrium of prx1cre;Ift88^fl/n conditional mutant tibiae. (A,B) In sections of wild-type E18.5 tibia (A), cartilage matrix, identified with Safranin O staining (red), is only seen in the growth region of the long bone, whereas bone, identified with Fast Green staining (blue-green), is present along the diaphysis. In conditional mutants (B), regions of cartilage matrix are evident along the diaphysis of the tibia (arrowheads) and appear to be derived from the overlying perichondrium. (C,D) In sections of the wild-type tibia (C) at E18.5, aggrecan is localized to the hypertrophic region and excluded from the perichondrium by immunofluorescence. Aggrecan expression is found ectopically in the perichondrium of the conditional mutant tibia (asterisk in D). Nuclei are stained blue and the epiphysis is up in both panels. (E) An adjacent section of the conditional mutant tibia shown in D was stained with Safranin O and Fast Green to confirm the presence of cartilage matrix in the perichondrium of the mutant sample.