Species-independent stimulation of osteogenic differentiation induced by osteoclasts.

The coupling of bone resorption and bone formation is well-recognized in the bone remodeling process, in which osteoblasts and osteoclasts are key players. However, the anabolic effect of human primary osteoclasts has rarely been reported as mouse and cell line derived osteoclasts were mostly used in previous reports. Therefore, a comprehensive comparison of mouse and human osteoclasts and their corresponding functions is needed to study cell-cell interactions between osteoclasts and osteoblasts. Osteoclasts from mouse and human origin were generated, characterized and compared, after which their anabolic effects on the osteogenic differentiation of mouse and human MSCs were assessed. Both murine RAW264.7 derived osteoclasts (mOCs) and primary human osteoclasts (hOCs) derived from buffy coats characteristically displayed multinuclearity, marked integrin ß3 expression and enhanced TRAP activity. Despite comparable cell size, mOCs showed higher osteoclast density (number of osteoclasts per cm2 culture dish) and osteoclast nuclearity (average number of nuclei per osteoclast), but lower TRAP activity compared to hOCs. Culturing primary rat and human bone marrow MSCs with the conditioned medium of mOCs or hOCs showed anabolic effects regarding the osteogenic differentiation of MSCs with superiority of hOCs over mOCs. We conclude that despite morphological and functional differences between mouse and human osteoclasts, their secretory factors evoke similar anabolic effects on MSC osteogenic differentiation.

Microfluidic-templating alginate microgels crosslinked by different metal ions as engineered microenvironment to regulate stem cell behavior for osteogenesis.

Cell microenvironment is a collection of dynamic biochemical and biophysical cues which functions as the key factor in determining cell behavior. Encapsulating single cell into micrometer-scale hydrogels which mimics the cell microenvironment can be used for single cell analysis, cell therapies, and tissue engineering. Here, we developed a microfluidics-based platform to engineer the niche environment at single cell level using alginate microgels crosslinked by different metal ions to regulate stem cell behavior for bone regeneration. Specifically, we revealed that Ca2+ in the engineered microenvironment promoted osteogenic differentiation of encapsulated stem cells and substantially accelerated the matrix mineralization compared to Sr2+in vitro. However, the superior osteoinductive capacity of Ca2+ compared with Sr2+ led to comparable bone healing in a rat bone defect model. This attributed to Sr2+ in microgels to inhibit the osteoclast activity and bone resorption after implantation. In summary, the present study demonstrates metal ions as a critical factor in the environmental cues to affect cell behavior and influence the efficacy of stem cell-based therapy in tissue regeneration, and provides new insights to engineer an expecting microenvironment for regenerative medicine.