Early initiation of endochondral ossification of mouse femur cultured in hydrogel with different mechanical stiffness.

Mineralization is one of the most important processes in normal bone tissue development and in disease condition. Developing a novel and standardized in vitro model system that can readily monitor both cellular dynamics and mineralization is crucial for better understanding the bone tissue development and growth. Recent studies indicated that the mechanical environment is a critical condition in mineralization. We hypothesized that hydrogel with different mechanical stiffness can provide a biomimetic mechanical environment that can modulate bone tissue growth and mineralization. A femur of mouse embryo (embryonic day 16) was embedded in agarose hydrogel (2-60 kPa) and cultured in an osteogenic medium for a week. Microcomputed tomography (μCT) results revealed enhanced mineralization was detected in the femur head cultured in the gel condition, whereas no mineralization in the femur head cultured in the control (floating culture) condition. The mineralized region was corresponding to the region of secondary ossification center. Both histological and quantitative analyses indicated that the mineralized region of femur head cultured in 10 kPa gel condition was the highest and the mineralized area was significantly larger than that cultured in 2, 40, and 60 kPa gel condition. Immunofluorescence results indicated the enhanced mineralization caused by the higher chondrogenic differentiation at that region. This enhancement mainly relating to the mechanical forces and not to the oxygen tension was also confirmed. Since this system enhances and shortens the mineralization procedure compared with the conventional two-dimensional or three-dimensional cell culture system, this hydrogel system would be one of the unique models for better understanding the mineralized tissue development.