gms | German Medical Science

Introduction: Fracture repair constitutes a sequential event following bone injury and is known to recapitulate the steps of endochondral ossification observed during skeletal development and growth. It is known that deletion of individual extracellular matrix components alters suprastructure, and hence stability and organization of mesenchymal tissues. Collagen IX, as a component of particular cartilage fibrils stabilizes and possibly directs organisation of the fibrillar network. The peripheral nervous system plays an important role in fracture healing and bone remodelling. Neuropeptide containing nerve fibres of sympathetic and sensory origin are known to innervate bone and fracture callus thereby influencing callus size and bone formation. The current research was launched to increase the understanding of the role of collagen IX protein for organization and differentiation of a cartilaginous callus and the impact of an altered callus structure on the release and distribution of specific neurotransmitters.

Methods: Tibial fractures were set in 8-10 week old wild-type or Col9a1^-/- mice. Bone calli were dissected on day 1, 5, 9, 13, 16 and 21 after setting the fractures, decalcified and embedded. The paraffin-sections were either stained with a combined matrix staining or with antibodies against specific cartilage components or neuroproteins

Results: Our fracture repair model demonstrated that the lack of collagen IX alters the time course of callus maturation, in particular that of chondrocyte hypertrophy. We found a delayed maturation of cartilage in the calli of Col9a1^-/- mice according to collagen X expression. Additionally, the morphology and organisation of hypertrophic chondrocytes and growth plates of Col9a1^-/- mice are affected. Our fracture model further demonstrated characteristic stage-specific localisations of tyrosine-hydroxylase (TH) -positive fibres during the healing progress. Notably, substance P (SP) -positive nerve fibres seem not to penetrate the cartilage tissue as well as the newly formed woven bone whereas chondrocytes originated from callus tissue express SP and its receptor neurokinin1 (NK1). Preliminary results suggest a difference in number and distribution pattern of SP-positive chondrocytes in the absence of collagen IX.

Discussion:We observed delayed cartilage maturation and bone formation and an altered morphology of hypertrophic chondrocytes and growth plates in Col9a1^-/- mice during fracture repair. Collagen IX, therefore constitutes a factor which is indispensable for proper organisation of growth plates and a timely maturation process in endochondral bone formation. Expression of SP and its receptor NK1 in chondrocytes originated from callus tissue implicates yet unknown functions of neuropeptides during cartilage differentiation and endochondral ossification.