Immunohistochemical localization of native and denatured collagen types I and II in fetal and adult rat long bones.

Collagen turnover during rat long bone development and growth was investigated using immunofluorescence methods with specific polyclonal antibodies against native (triple helix) and denatured (breakdown products) forms of type I and II collagen. Labeling of cryostat sections with anti-native and denatured collagen type II antibodies resulted in a positive staining throughout the cartilage matrix of fetal and adult long bones. Likewise, native and denatured type I collagen could be detected in mineralized and non-mineralized bone matrix. Moreover, labeling with anti-denatured type I antibody evoked a strong intracellular staining of osteoblasts, but not of osteocytes. Denatured type I was also localized intra-pericellularly in the small chondrocytes comprising the primitive cartilage cores and the epiphyses of older long bones. On the other hand, apart from its localization in the cartilage matrix, denatured type II collagen was found specifically within the chondrocytes. These observations indicate that a continuous turnover of the major collagen types takes place in fetal and adult rat long bone tissue. Degradation of collagen apparently occurs intra- and extracellularly, and is mainly independent of the presence and activity of osteoclasts. The presence of denatured type I collagen in cartilage suggests that chondrocytes synthesize small amounts of type I collagen, which is immediately degraded to a denatured form.