Bone Development and the Nervous System

Innervation of bone influence bone modeling, growth and remodeling. Pro-inflammatory cytokines released after tissue trauma are recognized as neurotrophic factors as well as factors influencing bone formation. The Wnt signaling pathway, essential for cell migration during embryogenesis is found to influence bone formation during fracture healing. Alterations in growth and bone formation are seen in denervating disorders and in manipulated Wnt signaling. The aim of the present thesis was to study; sensory and autonomic innervation in the developing skeleton in rats and mice, a possible influence on bone formation in IL-4 and IL-13 depleted mice, and fracture healing in altered Wnt signaling by glycogen synthase-3β inhibition in rats. Bone innervation with sensory and autonomic nerves in modeling and growth follows a predictable and reproducible pattern both in the rat and in the mouse with sensory nerves occurring prior to autonomic nerves in areas with high chondrogenic and osteogenic activity. The time lag in occurrence between sensory and autonomic nerves indicates the importance of developmental timing between different nerve qualities in skeletal ontogeny. These findings give substantial morphologic support for important regulatory effects by the nervous system on bone development. Depletion of the anti-inflammatory cytokines IL-4 and IL-13 production in mice resulted in an inhibited autonomic innervation and lack of implant capillary ingrowth, studied by DXBM implants. In fracture healing no differences between IL-4/13 knockout mice and wild type mice were found concerning fracture callus parameters, biomechanical properties or histology except that sensory and autonomic nerves were found in the bone marrow in knockout mice but not in wild type mice. An altered canonical Wnt signaling was achieved by the GSK-3β inhibitor AR28. The increase in cytoplasmic β-catenin, due to inhibited degradation, resulted in a remarkable anabolic effect both on the fractured bone and on fracture healing. The histological analysis showed that the fractures healed without the usual formation of fibro-cartilage callus. This finding suggests that inhibition of GSK-3β inhibits the differentiation of chondrocytes and instead promotes the differentiation of mesenchymal progenitor cells into osteogenic cells