Bone Tissue Structure and Remodeling - Anatomy and Physiology
- In an older adult, bone resorption exceeds formation, lowering bone density and raising fracture risk. What is this condition?
- Osteoporosis. Mineralization is normal, but there is too little bone mass.
- How is spongy bone organized?
- As a lattice of trabeculae without osteons; the spaces between them hold red or yellow marrow.
- An astronaut spends six months in microgravity. What happens to bone mass?
- It decreases. Without loading, resorption exceeds formation, especially in weight-bearing bones.
- What is the function and origin of osteoclasts?
- They resorb bone. They are large multinucleated cells derived from the monocyte-macrophage lineage.
- How do osteoblasts stimulate osteoclast formation?
- They express RANKL, which binds RANK on osteoclast precursors; osteoprotegerin (OPG) is a decoy receptor that blocks it.
- What are osteocytes, and how do they communicate?
- Mature bone cells trapped in lacunae that sense mechanical strain and communicate through canaliculi.
- What is the structural unit of compact bone?
- The osteon (Haversian system): concentric lamellae around a central canal.
- What is the function of osteoblasts?
- They secrete osteoid (the unmineralized organic matrix) and promote its mineralization, building new bone.
- What runs through the central canals of osteons, and how do perforating canals differ?
- Blood vessels and nerves. Perforating (Volkmann) canals run at right angles to connect central canals with each other and the periosteum.
- What does Wolff's law state?
- Bone remodels in response to the mechanical loads placed on it, thickening where stressed and thinning where unloaded.
- What membranes cover the outer and inner surfaces of a bone?
- The periosteum covers the outer surface; the endosteum lines the medullary cavity and trabeculae.
- Which bone matrix components give bone its hardness and its flexibility?
- Hydroxyapatite (calcium phosphate crystals) gives hardness; type I collagen gives flexibility and tensile strength.