Comparative Skeletal Structure

Abstract

The sophisticated organisation of the skeleton achieves a structure that can withstand the extremes of functional load‐bearing. The growth, development and repair of the skeletal structure are realised through the tightly regulated remodelling of bone tissue, orchestrated by cells that specifically form or resorb the matrix. Diseases such as osteoporosis, where bone formation is overwhelmed by bone resorption, the skeleton is at great risk of fracture. Key Concepts Bone regulates its mass and architecture to meet two critical but competing responsibilities: Structural and metabolic. Bone morphology is highly regulated, achieved by the coordinated formation of bone, by osteoblasts, and resorption of bone by osteoclasts. Bone is a composite material, with its strength coming – primarily – from the intricate integration of an organic constituent, collagen, and an inorganic component, hydroxyapatite. Skeletal tissue is comprised of two ‘types’: the dense outer shell and shafts of long bones, made of cortical bone, and the internal scaffolds of bone struts, called trabeculae. Bones are filled with marrow, which provides the niche for two critical stem cells, hematopoietic and mesenchymal stem cells, and thus the basis for the immune and regenerative capacity of the organism. The bone osteocytes, terminally differentiated osteoblasts encased in mineralized tissue, form a connective three dimensional syncytium that aids in recognition of physical signals. When functional loading increases, or decreases, osteocytes can signal to osteoblasts and osteoclasts to alter activity. Continual remodeling throughout lifespan requires that new osteoblasts and osteoclasts arise. While osteoclast activity rises during aging, the potential for osteoblast activity decreases, such that break down is greater than formation. This results in osteoporosis.