IntroductionWith increasing age, bone undergoes changes in remodeling that ultimately compromise the structural integrity ofthe skeleton. The presence of osteocalcin in bone matrix may alter bone remodeling by promoting osteoclast activity. Whether ageand/or gender-related differences exist in the distribution of osteocalcin within individual bone remodeling units is not known. In this study, we determined the immunohistochemical distribution of osteocalcin in the extracellular matrix of iliac crest bone biopsies obtained from normal male and female volunteers, 20-80 yr old. Normal bone metabolism is the result of a highly integrated relationship between bone resorption and formation. With age, the structural integrity of the skeleton declines along with the functional capacity of various organ systems. Although overall bone mass decreases with age in females and males, the rate of bone loss in females is greater than males ( 1-3). The mechanisms by which bone remodeling is altered with age and gender are poorly understood. The organic phase of normal bone consists of 90% type I collagen and the remaining 10% is composed of noncollagenous proteins (NCPs).' NCPs are synthesized and secreted by bone cells, and their expression is regulated, in part, by local growth factors and hormones (4-7). Accumulating evidence suggests that the extracellular matrix of bone exerts profound effects on cellular activity by retaining NCPs and growth factors that influence both immediate and long-term cell-matrix interactions (8-13). Reduced recruitment of osteoclasts to devitalized particles of bone from human donors of increasing age suggests that bone matrix components may be partially responsible for impaired skeletal remodeling associated with aging (9).Osteocalcin is a 6.5-kD vitamin K-dependent, gamma-carboxyglutamic acid-containing NCP secreted by osteoblasts and odontoblasts. Due to its high affinity for calcium and hydroxyapatite, osteocalcin is incorporated into the extracellular matrix of bone ( 14-16). A growing body of evidence suggests that this matrix protein is involved in bone remodeling. Significant differences in the concentration ofextractable osteocalcin between cortical and trabecular bone in humans provides evidence of distinct regulatory mechanisms among these areas (10). Osteocalcin-deficient bone particles obtained from warfarn-treated rats recruit fewer osteoclasts and are resorbed less than normal particles using in vivo assays ( 1 1, 17, 18). Osteopetrotic rat bone contains low concentrations of osteocalcin, and osteoclast activity in these animals is reduced ( 19).The possibility that NCPs are involved in regulating sitespecific cell-matrix interactions is supported by studies that have immunolocalized NCPs within the extracellular matrix of bone from several species (13,(20)(21)(22). Presently, it is not known whether age or gender differences exist in the microanatomical distribution of NCPs within bone remodeling units. In this study, the immunohistochemical distribution of osteocalcin in ...