Changes in type I, II, and X collagen immunoreactivity of the mandibular condylar cartilage in a naturally aging rat model

The effect of aging on type I, II, and X collagen in the mandibular condyle was histologically and immunohistochemically assessed in 1-, 4-, 9-, and 16-month-old rats. Hypertrophic chondrocytes, observed in the 4-month-old rat. were absent in the 9-month-old. In 9- and 16-month-old rats, a mineralizing front ran parallel to the surface of the condyle, and the calcified cartilage was thicker than in the younger rats. Type I collagen was observed from the fibrous layer to the upper maturative cell layer in the 1- and 4-month-old rats. In the 9-month-old. the type I collagen-positive area extended to the whole cartilaginous region. In the 16-month-old. type I collagen took on art archlike configuration around the lacunae. Intense type II collagen reactivity in the maturative and hypertrophic cell layers of the 1-month-old rat was only slightly changed in the 4-month-old. In the 9-month-old rat. immunoreaction was detected from the proliferative cell layer; this extended to the whole cartilage in the 16-month-old. Type X collagen was localized in the hypertrophic cell layer in the 1-month-old and had expanded over the maturative cell layer in the 4-month-old rat. It was detected beneath the proliferative cell layer in the 16-month-old. Type X collagen was always observed in the area immediately above the mineralizing front of the cartilage matrix. Thus, our study indicated that mandibular condylar cartilage becomes fibrocartilage-like tissue with advancing age and that type X collagen may play a pivotal role in the progression of the mineralized front.