Mineral Content and Extracellular Matrix Protein Expression in Mouse Growth Plates During Epiphyseal Fusion: An Observational Study

In humans, the growth plate cartilage is completely replaced by bone in late puberty, resulting in epiphyseal fusion. However, in rats and mice, commonly used experimental model systems, the growth plate does not fuse completely even after sexual maturation, making it difficult to elucidate mechanisms involved in epiphyseal fusion. In this study, we investigated age-related changes in the mouse growth plate to better understand the process of epiphyseal fusion. We used scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) to examine the distributions and concentrations of minerals in the growth plate. In SEM images, the hypertrophic zone was observed as a bright area and other zones as dark areas at 10 weeks of age (W10). The bright area was further expanded at W55 than at W10. EDS analysis showed that P and Ca concentrations were high in this area, while C and O concentrations were low, indicating that the growth plate had calcified during aging. Alcian blue histochemistry revealed that the glycosaminoglycans of aggrecan were distributed in the growth plate at both W10 and W55. Immunohistochemistry showed that aggrecan and type II collagen were expressed throughout the growth plate at W10, but sparsely at W55. Type I collagen was expressed weak at both W10 and W55. Type X collagen and MMP-13 expression were observed in the hypertrophic zone at W10 but not at W55. This study demonstrated that although the mouse growth plate calcifies with age, it remains calcified cartilage for an extended period without being replaced by bone.