Recent Insights into Long Bone Development: Central Role of Hedgehog Signaling Pathway in Regulating Growth Plate.

The longitudinal growth of long bone, regulated by an epiphyseal cartilaginous component known as the "growth plate", is generated by epiphyseal chondrocytes. The growth plate provides a continuous supply of chondrocytes for endochondral ossification, a sequential bone replacement of cartilaginous tissue, and any failure in this process causes a wide range of skeletal disorders. Therefore, the cellular and molecular characteristics of the growth plate are of interest to many researchers. Hedgehog (Hh), well known as a mitogen and morphogen during development, is one of the best known regulatory signals in the developmental regulation of the growth plate. Numerous animal studies have revealed that signaling through the Hh pathway plays multiple roles in regulating the proliferation, differentiation, and maintenance of growth plate chondrocytes throughout the skeletal growth period. Furthermore, over the past few years, a growing body of evidence has emerged demonstrating that a limited number of growth plate chondrocytes transdifferentiate directly into the full osteogenic and multiple mesenchymal lineages during postnatal bone development and reside in the bone marrow until late adulthood. Current studies with the genetic fate mapping approach have shown that the commitment of growth plate chondrocytes into the skeletal lineage occurs under the influence of epiphyseal chondrocyte-derived Hh signals during endochondral bone formation. Here, we discuss the valuable observations on the role of the Hh signaling pathway in the growth plate based on mouse genetic studies, with some emphasis on recent advances.

An intrinsic timer specifies distal structures of the vertebrate limb.

How the positional values along the proximo-distal axis (stylopod-zeugopod-autopod) of the limb are specified is intensely debated. Early work suggested that cells intrinsically change their proximo-distal positional values by measuring time. Recently, however, it is suggested that instructive extrinsic signals from the trunk and apical ectodermal ridge specify the stylopod and zeugopod/autopod, respectively. Here, we show that the zeugopod and autopod are specified by an intrinsic timing mechanism. By grafting green fluorescent protein-expressing cells from early to late chick wing buds, we demonstrate that distal mesenchyme cells intrinsically time Hoxa13 expression, cell cycle parameters and the duration of the overlying apical ectodermal ridge. In addition, we reveal that cell affinities intrinsically change in the distal mesenchyme, which we suggest results in a gradient of positional values along the proximo-distal axis. We propose a complete model in which a switch from extrinsic signalling to intrinsic timing patterns the vertebrate limb.

Resistance exercise and naproxen sodium: effects on a stable PGF2α metabolite and morphological adaptations of the upper body appendicular skeleton.

INTRODUCTION: Exercise-induced inflammation has been shown to be necessary for successful skeletal muscle regeneration post-injury. Accordingly, numerous investigations have demonstrated consequences of COX-inhibitors, anti-inflammatory drugs which prevent prostaglandin formation. In addition to its roles in inflammation, prostaglandin F2α (PGF2α) also mediates vital regenerative processes The majority of research to report consequences of suppressing inflammation has utilized acute injury models in combination with acute COX-inhibitor administration. To address the limited research investigating regular consumption of COX-inhibitors over time in exercising humans, the purpose of this study was to determine effects of a non-selective COX-inhibitor on a PGF2α metabolite and morphological adaptations of the upper body appendicular skeleton during periodized resistance training. Twenty-three (N = 23) recreationally trained college-aged males were randomly assigned to receive placebo (n = 11) or naproxen sodium (n = 12). Treatments were prophylactically administered in double-blind fashion with supervised upper body resistance exercise performed twice per week for 6 weeks. Venous blood was sampled pre- and post-exercise and analyzed for 13, 14-dihydro-15-keto PGF2α using enzyme immunoassay. Factorial mixed-design repeated-measures ANOVAs were utilized to examine relative changes in the plasma PGF2α metabolite and upper body appendicular morphology over the training period. RESULTS: Naproxen sodium significantly reduced the acute PGF2α metabolite response to exercise (p = 0.013); however, this effect diminished over time (p = 0.02), and both treatment groups exhibited significant increases in dominant arm skeletal muscle tissue (p = 0.037). CONCLUSION: Despite acute inhibition of the PGF2α metabolite at early time points, naproxen sodium did not hinder positive morphological adaptations of the upper body in response to resistance training.

Quantification of skeletal blood flow and fluoride metabolism in rats using PET in a pre-clinical stress fracture model.

PURPOSE: Blood flow is an important factor in bone production and repair, but its role in osteogenesis induced by mechanical loading is unknown. Here, we present techniques for evaluating blood flow and fluoride metabolism in a pre-clinical stress fracture model of osteogenesis in rats. PROCEDURES: Bone formation was induced by forelimb compression in adult rats. (15)O water and (18)F fluoride PET imaging were used to evaluate blood flow and fluoride kinetics 7 days after loading. (15)O water was modeled using a one-compartment, two-parameter model, while a two-compartment, three-parameter model was used to model (18)F fluoride. Input functions were created from the heart, and a stochastic search algorithm was implemented to provide initial parameter values in conjunction with a Levenberg-Marquardt optimization algorithm. RESULTS: Loaded limbs are shown to have a 26% increase in blood flow rate, 113% increase in fluoride flow rate, 133% increase in fluoride flux, and 13% increase in fluoride incorporation into bone as compared to non-loaded limbs (p < 0.05 for all results). CONCLUSIONS: The results shown here are consistent with previous studies, confirming this technique is suitable for evaluating the vascular response and mineral kinetics of osteogenic mechanical loading.