2 - Effects of Aging on Bone Structure and Metabolism

Abstract

This chapter explores the effects of aging on bone structure and metabolism. Analyses of age-related bone loss patterns in females reveal progressive loss of axial (trabecular) bone mass, which begins sometime in the third decade; and a decrease in appendicular (cortical) bone mass beginning in the fifth decade with an accelerated rate of decline during the sixth and early part of the seventh decade. Although those factors that initiate and/or perpetuate this age-bone loss phenomenon are still not clearly defined, they result in an uncoupling of the cybernetic interplay between osteoblast-controlled bone formation and osteoclastic-osteocytic modulated bone resorption with the latter predominating. Differential patterns of trabecular and cortical bone loss may result in at least two distinct osteoporotic syndromes: one is characterized by accelerated and proportionate loss of trabecular bone and an increased incidence of vertebral or Colles' fractures in the immediate postmenopausal years; and the other is characterized by a proportionate loss of both cortical and trabecular bone resulting in fractures of the hip and pelvis in the seventh and eighth decades. Decrease in the osteoblastic mediated bone formation observed with advancing age in the elderly skeleton may result from acquired deficiencies in either the mobilization of immature marrow osteoprogenitor stem cells and/or in their conversion to mature bone forming osteoblasts.