Fibrinogen promotes resorption of chitosan by human osteoclasts

The osteoconductive and osteoinductive properties of materials intended for bone regeneration have been extensively tested, but the resorbability of these materials is often overlooked. Osteoclasts are responsible for bone resorption and play a crucial role in bone remodeling, which is essential for complete regeneration of bone tissue following injury. In this study we compare, for the first time, the ability of unmodified and fibrinogen (Fg)-modified chitosan (Ch) substrates to support the formation of multinucleated osteoclasts, and the potential of these cells to resorb the two substrates in vitro. Osteoclasts were differentiated from primary human peripheral blood monocytes directly on the substrates being investigated. Our results showed similar cell adhesion to unmodified and Fg-modified Ch substrates. Although the number of multinucleated osteoclasts on both Ch substrates increased throughout the culture period, by 21 days of culture significantly more highly multinucleated osteoclasts (>10 nuclei per cell) were observed on Fg-modified Ch, when compared to Ch alone. In addition, cells were tartrate-resistant acid phosphatase positive and secreted significantly more enzyme on Ch-based substrates than in control conditions. Unmodified and Fg-modified Ch resorption was investigated by fluorescence microscopy and confirmed by electron microscopy. Quantification of results obtained by fluorescence microscopy shows that Fg modification led to significantly higher substrate resorption by 17 days of culture. Our results show that osteoclasts, beyond resorbing mineralized substrates, successfully resorb a polymeric substrate (Ch), with Fg accelerating this process. Thus, in bone tissue regeneration strategies employing polymeric biomaterials, resorption may depend not only on macrophages, but also on osteoclasts.