Osteoclasts are multinuclear cells of the monocyte macrophage lineage. They are responsible for bone remodeling by first resorbing packets of bone, which are subsequently replaced by new bone produced by osteoblasts. Osteoblasts are derived from mesenchymal stem cells, and thus osteogenesis can also be induced in various tissues at extra skeletal sites. Fifty years ago it was discovered that demineralized bone matrix is able to induce ectopic bone formation. Since that time the differentiation of bone cells has been studied intensively. The aim was to produce bone for the repair of bone defects. The molecular basis of bone remodeling has been established in great detail and the mechanism of how bone resorption and bone formation are coupled in bone remodeling sites has been delineated. Osteoclasts resorb bone, but they also secrete anabolic signals that induce mesenchymal stem cells and osteoblasts to initiate osteogenesis in resorption lacuna (remodeling) or another nonresorbed site (modeling). It is this osteoclast derived influence on mesenchymal stem cells and osteoblasts that could be utilized in tissue engineering. So far investigators have tried to find ways to induce bone formation by activating mesenchymal stem cells, but a better understanding of the remodeling paradigm of bone, the intrinsic regulation of bone formation through osteoclastic resorption, could be utilized for tissue engineering. Scaffold materials like decellularized natural tissue extracellular matrices or bone type resorbable mineral matrices induce resorption and simultaneously induce bone formation.

Bone marrow (BM) and peripheral blood (PB) derived mononuclear cells are precursors of in vitro osteoclast differentiation. However, few studies have compared the phenotypic and functional properties of osteoclasts generated from these sources and the effects of different growth factors on osteoclastogenesis. Both cell types differentiated into functional osteoclasts, but culturing the cells with or without transforming growth factor beta (TGF-β) and dexamethasone revealed differences in their osteoclastogenic capacity. When receptor activator for nuclear factor κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) were used for differentiation, we did not observe differences in bone resorption activity or expression of osteoclastogenic genes calcitonin receptor (CR) and nuclear factor of activated T-cells (NFATc1) between the osteoclasts formed from the two sources. Addition of TGF-β and dexamethasone led to higher number of nuclei in multinuclear cells and increased expression of tartrate resistant acid phosphatase (TRACP) 5a and 5b, CR and NFATc1 in PB- derived osteoclasts depicting the higher osteoclastogenic potential and responsiveness to TGF-β and dexamethasone in PB monocytes. These results conclude that the choice of the osteoclast precursor source as well as the choice of osteoclastogenic growth factors are essential matters in determining the phenotypic characteristics of heterogeneous osteoclast populations.

Background: The increasing demand for bone implants with improved osseointegration properties has prompted researchers to develop various coating types for metal implants. Atomic layer deposition (ALD) is a method for producing nanoscale coatings conformally on complex three-dimensional surfaces. We have prepared hydroxyapatite (HA) coating on titanium (Ti) substrate with the ALD method and analyzed the biocompatibility of this coating in terms of cell adhesion and viability. Methods: HA coatings were prepared on Ti substrates by depositing CaCO₃ films by ALD and converting them to HA by wet treatment in dilute phosphate solution. MC3T3-E1 preosteoblasts were cultured on ALD-HA, glass slides and bovine bone slices. ALD-HA and glass slides were either coated or non-coated with fibronectin. After 48h culture, cells were imaged with scanning electron microscopy (SEM) and analyzed by vinculin antibody staining for focal adhesion localization. An 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) test was performed to study cell viability. Results: Vinculin staining revealed similar focal adhesion-like structures on ALD-HA as on glass slides and bone, albeit on ALD-HA and bone the structures were thinner compared to glass slides. This might be due to thin and broad focal adhesions on complex three-dimensional surfaces of ALD-HA and bone. The MTT test showed comparable cell viability on ALD-HA, glass slides and bone. Conclusion: ALD-HA coating was shown to be biocompatible in regard to cell adhesion and viability. This leads to new opportunities in developing improved implant coatings for better osseointegration and implant survival.

Aims The aim of the study was to compare the influence of gap junctional communication (GJC) in osteoclastogenesis from bone marrow (BM) and peripheral blood (PB) monocytes. These widely used sources differ in purity, since BM cultures contain a significant number of stromal cells. We studied whether stimulation of GJC in BM monocyte/stromal cell cultures differs from the effect in pure PB monocyte cultures. We compared the differentiation also in acidosis, which is a known inducer of bone resorption. Main methods Human BM and PB monocytes were isolated from BM aspirates or whole blood samples. The cells were cultured on human bone slices with osteoclastogenic growth factors and a GJC modulator, antiarrhythmic peptide AAP10, at physiological and acidic pH. Key findings Both BM and PB monocytes differentiated into osteoclasts. Acidosis increased resorption in both cultures but stimulated cell fusion only in BM cultures, which demonstrates the role of stromal cells in osteoclastogenesis. At physiological pH, AAP10 increased the number of multinuclear cells and bone resorption in both BM and PB cultures indicating that GJC is involved in differentiation in both of these osteoclastogenesis assays. Interestingly, in PB cultures at pH 6.5 the stimulation of GJC with AAP10 inhibited both osteoclastogenesis and bone resorption suggesting a different role of GJC in BM and PB monocytes at stressed environment. Significance The study is conducted with primary human tissue samples and adds new knowledge on factors affecting osteoclastogenesis from different monocyte sources.