Trabecular and endocortical bone surfaces in the rat: modeling or remodeling?

Area of Science:

• Bone Biology
• Skeletal Physiology
• Histomorphometry

Background:

• Conflicting evidence exists regarding site-specific coupling of bone resorption and formation in rat bone remodeling.
• Understanding this coupling is crucial for comprehending skeletal maintenance and age-related changes.

Purpose of the Study:

• To investigate the balance between bone remodeling and modeling in different skeletal sites and age groups in rats.
• To clarify the site-specific nature of bone turnover in relation to age.

Main Methods:

• In vivo fluorochrome labeling and cement line staining were used for bone histomorphometry in Fischer-344 rats.
• Tibial and vertebral bone samples from rats aged 3, 6, 9, and 12 months were analyzed.
• Bone formation sites were classified as remodeling, modeling, or uncertain based on label and cement line contours.

Main Results:

• Vertebral cancellous bone showed a progressive increase in remodeling activity with age (70.4% to 91.0%).
• In young rats (3 months), tibial metaphyseal bone primarily exhibited modeling (61.6%), whereas in older rats (12 months), remodeling dominated (66.3%).
• Both tibial and vertebral endocortical bone sites demonstrated a trend towards increased remodeling with advancing age.

Conclusions:

• Bone turnover in aged rats, similar to higher mammals, is predominantly characterized by remodeling in vertebral and tibial cancellous bone.
• In rapidly growing young rats, the proximal tibia shows a higher proportion of modeling activity in its cancellous bone.