The Indian Hedgehog - PTHrP System in Bone Development
197
Fig. 2. The genes of the IhhIPTHrP feedback loop are expressed in the postnatal growth plate. In situ hybridization shows the expression of Ihh,
PTHlPTHrP receptor, Pte, and Col-X in the primary and secondary ossification
center in the growth plate of a day 13 mouse tibia
until the ossification centers connect during puberty, resulting in a bony
skeletal element which is again flanked by caps of articulate cartilage.
We analyzed the expression of Ihh and its interactors during postnatal
bone growth in mice (Fig. 2) and found that Ihh and the PTH/PTHrP
receptor are still expressed in the differentiating, prehypertrophic chondrocytes at least up to postnatal day 21 (P21) (Iwasaki et al. 1997;
Vortkamp et al. 1998). Ptc and Gli are strongly expressed in the resting
and proliferating chondrocytes distal to the Ihh expressing cells in the
metaphysis. In addition we found a second domain of strong Ptc and Gli
expression in the zone of active ossification, directly flanking the hypertrophic chondrocytes (Vortkamp et al. 1998). The expression of Ptc and
Gli in both the proliferating chondrocytes and the zone of ossification
implies that hedgehog signaling regulates both chondrocyte differentiation and ossification. In this respect, the early expression of Ptc and Gli
in the perichondrium/periosteum could be involved in regulating the
differentiation of the perichondrium into a bone-producing periosteum,
whereas the expression in the proliferating chondrocytes might be responsible for regulating chondrocyte differentiation.
197
Fig. 2. The genes of the IhhIPTHrP feedback loop are expressed in the postnatal growth plate. In situ hybridization shows the expression of Ihh,
PTHlPTHrP receptor, Pte, and Col-X in the primary and secondary ossification
center in the growth plate of a day 13 mouse tibia
until the ossification centers connect during puberty, resulting in a bony
skeletal element which is again flanked by caps of articulate cartilage.
We analyzed the expression of Ihh and its interactors during postnatal
bone growth in mice (Fig. 2) and found that Ihh and the PTH/PTHrP
receptor are still expressed in the differentiating, prehypertrophic chondrocytes at least up to postnatal day 21 (P21) (Iwasaki et al. 1997;
Vortkamp et al. 1998). Ptc and Gli are strongly expressed in the resting
and proliferating chondrocytes distal to the Ihh expressing cells in the
metaphysis. In addition we found a second domain of strong Ptc and Gli
expression in the zone of active ossification, directly flanking the hypertrophic chondrocytes (Vortkamp et al. 1998). The expression of Ptc and
Gli in both the proliferating chondrocytes and the zone of ossification
implies that hedgehog signaling regulates both chondrocyte differentiation and ossification. In this respect, the early expression of Ptc and Gli
in the perichondrium/periosteum could be involved in regulating the
differentiation of the perichondrium into a bone-producing periosteum,
whereas the expression in the proliferating chondrocytes might be responsible for regulating chondrocyte differentiation.