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

## TL;DR

This study examines how mouse growth plates change with age, showing that they calcify but do not fully turn into bone like in humans.

## Contribution

The study provides new insights into the calcification process of mouse growth plates and their lack of complete epiphyseal fusion.

## Key findings

- Mouse growth plates calcify with age, as shown by high P and Ca concentrations in the hypertrophic zone.
- Aggrecan and type II collagen expression decreases in older mice, while type X collagen and MMP-13 are only present at younger ages.
- The mouse growth plate remains calcified cartilage without being replaced by bone, unlike in humans.

## Abstract

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.