Expression of Indian hedgehog, parathyroid hormone-related protein, and their receptors in the postnatal growth plate of the rat: Evidence for a locally acting growth restraining feedback loop after birth

A focally acting growth restraining feedback loop has been identified in th e murine embryonic growth plate in which the level of parathyroid hormone-r elated peptide (PTHrP) expression regulates the pace of chondrocyte differe ntiation. To date, it is largely unknown whether this feedback loop also re gulates the pace of chondrocyte differentiation in the growth plate after b irth. We therefore characterized the spatio-temporal expression of Indian h edgehog (MH), PTHrP, and their receptors in the postnatal growth plate from female and male rats of 1, 4, 7, and 12 weeks of age. These stages are rep resentative for early life and puberty in rats, Using semiquantitative reve rse-transcription polymerase chain reaction (RT-PCR) on growth plate tissue , MH and components of its receptor complex, patched (PTC) and smoothened ( SMO), PTHrP and the type I PTH/PTHrP receptor messenger RNA (mRNA) were sho wn at all ages studied irrespective of gender. Using in situ hybridization, MH, PTHrP, and PTH/PTHrP receptor mRNA were detected in prehypertrophic an d hypertrophic chondrocytes in both sexes during development. In addition, especially in the younger age groups, faint expression of PTH/PTHrP recepto r mRNA also was shown in stem cells and proliferative chondrocytes. Immunoh istochemistry confirmed the observations made with in situ hybridization, b y showing the presence of IHH, PTC, PTHrP, and PTH/PTHrP receptor protein i n prehypertrophic and hypertrophic chondrocytes. In addition, staining for hedgehog, PTC, and PTHrP also was observed in growth plate stem cells. No d ifferences in staining patterns were observed between the sexes. Furthermor e, no mRNA or protein expression of the mentioned factors was detected in t he perichondrium. Our data suggest that in contrast to the proposed feedbac k loop in the early embryonic growth plate, which requires the presence of the perichondrium, a feedback loop in the postnatal growth plate can be con fined to the growth plate itself. In fact, two loops might exist: (1) a loo p confined to the transition zone and early hypertrophic chondrocytes, whic h might in part be autocrine and (2) a loop involving the growth plate stem cells.