Platelet-rich plasma is currently promoted to serve as an adjuvant for bone grafts to enhance quantity and quality of newly forming bone; however, the underlying cellular mechanisms are not fully understood. We show here that supernatants of leukocyte-depleted thrombin-activated platelets increase migration and proliferation, and decrease osteogenic differentiation of bone marrow-derived mesenchymal progenitor cells under in vitro conditions. Using neutralizing antibodies raised against platelet-derived growth factor (PDGF), the observed effects of platelet-released supernatants were diminished. The mitogenic response was also decreased when extracellular signal-regulated protein kinase (ERK) signalling was inhibited by PD98059; however, PD98059 did not reverse the effects of platelet-released supernatants on migration and osteogenic differentiation. Consistent with an ERK-mediated mitogenic activity, incubation of serum-starved mesenchymal cell progenitors with platelet-released supernatants increased phosphorylation of the kinase. Together, these observations indicate that PDGF is a key factor released upon platelet activation that can increase migration and proliferation, and decreases osteogenic differentiation of mesenchymal progenitor cells under in vitro conditions. The results further suggest that ERK signalling is required to mediate the mitogenic response to platelet-released supernatants.

The aim of this in vitro study was to determine whether the sinus mucosa holds cells with an osteogenic potential. Frozen sections of sinus mucosa from three adult pigs were investigated for the expression of STRO-1, a marker of mesenchymal progenitor cells, and alkaline phosphatase activity, an enzyme expressed by cells committed to the osteogenic lineage and by mature osteoblasts. To determine their osteogenic potential, mucosa-derived cells were incubated with bone morphogenetic protein (BMP)-6 and BMP-7, and alkaline phosphatase activity, osteocalcin expression, and mineralization of the extracellular matrix was measured. We found sinus mucosa cells staining positive for STRO-1 and alkaline phosphatase activity. When sinus mucosa tissue was placed in culture, alkaline phosphatase positive cells grew out from the explants and further increased alkaline phosphatase activity in response to BMP-6 and BMP-7. The expression level of the osteoblast-specific extracellular matrix protein osteocalcin, and the amount of calcium accumulation within the extracellular matrix was also increased in response to BMPs. We conclude that the sinus mucosa holds mesenchymal progenitor cells and cells committed to the osteogenic lineage that can respond to BMP-6 and BMP-7 by an increase of their osteogenic differentiation.

Bone formation in a sinus grafted with a cell-free scaffold requires the presence of local progenitor cells that differentiate into osteoblasts. The purpose of this study was to examine the effect of culture expanded autogenous bone-derived cells (ABC) with bovine bone mineral (BBM) on bone formation after single-stage sinus grafting in minipigs. Bone biopsies from the iliac crest were harvested 4 weeks prior to sinus grafting and ABC were culture expanded in vitro. The sinuses of five adult minipigs were grafted. In one sinus of each minipig the space between Schneider's membrane (SM) and the sinus wall was grafted with ABC (325,000 cells per sinus, on average) and BBM. In the other sinus, BBM alone was used. The animals were sacrificed after 12 weeks. One block of each grafted area was prepared by saw cutting and the amount of newly formed bone was analysed by micro-computed tomography (mu-CT). The addition of ABC to BBM significantly increased the amount of newly formed bone compared with BBM alone on mu-CT analysis (ABC+BBM: 29.86+/-6.45% vs. BBM: 22.51+/-7.28% (P=0.016)). Bone formation was increased near SM (ABC+BBM: 20.7+/-4.5% vs. BBM: 15.43+/-3.62% (P=0.009)) and in the middle zone of the grafting material (ABC+BBM: 31.63+/-7.74% vs. BBM: 22.5+/-7.91% (P=0.001)), but not near the local host bone (ABC+BBM: 37.23+/-8.23% vs. BBM: 28.42+/-12.54% (P=0.15)). These preliminary findings indicate that supplementation of cell-free grafting material with culture expanded ABC can stimulate bone formation in areas with low bone-forming capacity.