Skeletal repair and regeneration involve a dynamic interplay of biological processes that result in spatially and temporally varying patterns of tissue formation and remodeling. For example, during bone fracture healing the cartilaginous callus that is formed initially in the fracture site is subsequently mineralized and remodeled to restore the original form and function to the injured bone. During much of this healing process, the fracture callus is comprised of a heterogeneous mixture of cartilage, fibrocartilage, multipotent mesenchymal tissue, and bone. Adding to this complexity, mechanical stimuli are known to influence the rate and type of tissues formed during skeletal healing [1]. Given the growing body of evidence that controlled mechanical stimulation may be used to enhance healing, it is of substantial interest to elucidate relationships between the distributions of local stresses and strains that develop within the healing region and the distribution of tissue types that form. While histomorphometry is a well established approach for characterizing the latter, it has historically been limited to analyses of a small number of two-dimensional sections of tissue. Such 2D sampling may be inadequate for quantitative characterization of the irregular geometry and heterogeneous composition of healing tissues. In this study, we report on a 3D histomorphometric method and apply this method to an in vivo model of skeletal repair [2] in which a bending stimulus delivered to a healing bone defect results in the formation of predominantly cartilage tissue, rather than bone.

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