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Hormonal influence on bone remodeling and its implications for the pathophysiology of osteoporosis
A selection of talks on Physiology & Anatomy
I'm Stavros Manolagas, Professor of Medicine and the Director of the Division of Endocrinology and Metabolism and the Center for Osteoporosis and Metabolic Bone Diseases at the University of Arkansas for Medical Sciences in Little Rock, AR. The goal of the lecture is to review current understanding of the hormonal influences on bone remodeling and their implications for the pathophysiology and treatment of osteoporosis.
During development and growth, the skeleton is sculpted to achieve its shape and size by the removal of bone from one site and deposition at a different one, this process is called modeling. Once the skeleton has reached maturity, however, regeneration continues in the form of a periodic replacement of all bone with new at the same location, this process is called remodeling. Removal of bone, otherwise known as bone resorption, is the task of osteoclasts, formation of new bone is the task of osteoblasts. Bone resorption and bone formation, however, are not separate independent processes. In the adult skeleton, all osteoclasts and osteoblasts belong to a unique temporary structure known as a basic multicellular unit or BMU, which is depicted in this cartoon. The BMU is approximately 1 to 2 mm long and 0.2 to 0.4 mm wide, it comprises a team of osteoclasts in the front, a team of osteoblasts in the rear, depicted here with a yellow monolayer of cells, and a central vascular capillary. Osteoclasts adhere to bone and subsequently remove it by acidification and proteolytic digestion. As the BMU advances, in this cartoon from the top of the image to the bottom, osteoclasts leave the resorption site and osteoblasts move in to cover the excavated area and begin the process of new bone formation by secreting osteoid which is eventually mineralized into new bone. Importantly, some osteoblasts, after they have finished their bone-forming function are buried within the mineralized matrix that they have created and these are termed osteocytes. Osteocytes are depicted as individual cells in this cartoon with multiple processes connected with each other in the matrix surrounding the BMU. A microscopic image of an actual BMU in a section of murine trabecular bone is shown here. In this instance, the BMU travels from the left to the right of the section. Please note the group of red osteoclasts stained for tartrate-resistant acid phosphatase. Trailing behind the osteoclasts are teams of teal-colored osteoblasts bringing up the rear of the BMU. Pale-yellow erythrocytes are seen here in the vessel that brought the osteoclast precursors to the erosion cavity.