Abstract
Resorption of alveolar bone after tooth extraction is a common problem. Bone replacement materials are used to enhance socket healing and reduce alveolar bone loss. The success of the graft is dependent on multiple factors including the presence of growth factors. This is the first in vivo study to investigate the expression of the pleiotrophin family of cytokines in association with a grafting material during bone regeneration.
Objective: To investigate the role of the growth factor family of pleiotrophin/midkine and their receptors, during osteogenesis with and without a grafting material, after tooth extraction in a sheep model.
Methods: Thirty Romney-cross ewes were anaesthetised and all premolar teeth on the right side were extracted. The sockets were randomised to controls sites with no treatment and test sites with Bio-Oss® graft material and Bio-Gide® membrane. Samples were harvested after sacrificing animals 4, 8, and 16 weeks post-grafting (n=10 per time-point). Tissue for qRT2-PCR gene analysis was recovered from the socket next to the first molar using a trephine (Ø=2mm). Each socket was fixed, decalcified, paraffin-embedded and sectioned. Immunohistochemistry (IHC) was conducted to localise pleiotrophin and midkine and their receptors anaplastic lymphoma kinase (ALK), receptor-type tyrosine-protein phosphatase zeta (RPTPζ) and notch-2.
Results: Within the healing sockets high expression of genes for pleiotrophin, midkine, notch-2, and ALK were found at all time-points and in both grafted and non-grafted sites, while RPTPζ was only expressed at low levels. The relative gene expression of the PTN family of cytokines were not statistically different at the three time-points and between test and control groups (p>0.05). Immunohistochemistry found pleiotrophin and midkine in association with new bone, notch-2 in the connective tissue and intranuclear localisation of RPTPζ and ALK in association with cuboidal osteoblasts involved in bone formation.
Conclusions: The pleiotrophin family was expressed in both non-grafted and grafted sockets during osteogenesis in a sheep model of alveolar bone regeneration. The discovery of the pleiotrophin/midkine family as important during alveolar bone regeneration is novel and opens up new avenues of research. Growth factor supplementation with pleiotrophin and/or midkine during healing may be an approach for enhanced regeneration or to initiate healing where delayed. The activation of notch-2 and RPTPζ may be important to bone regeneration in vivo.