Those cells then enter a path of chondrogenic differentiation which can be additionally promoted by mechanical instability at the fracture site [28]. After forming a callus the chondrocytes halt their dividing and begin increasing massively in volume becoming so called "hypertrophic chondrocytes". At this point the fate of the chondrocytes is debated, whether they conclude their life cycle by apoptosis or undergo trans-differentiation into osteogenic cells , however remaining cells surrounding the cartilage differentiate into osteoblasts, which begin replacing the cartilage with bone tissue trough mineralization of the matrix .