Bone Structure and Function Quiz

Bone Structure and Function

• Bone is a type of connective tissue, along with ligaments and tendons
• Bone is composed of specialized cells within an extracellular matrix composed of specific molecules
• Bone, ligaments, and tendons connect and support body parts
• Bone provides structural support for the body
• Bone protects soft tissues and internal organs from injury
• Bone facilitates movement by providing rigid rods and levers for muscles to act on, allowing interaction with the environment
• Bone plays a crucial role in calcium homeostasis, acting as a reservoir to regulate calcium levels
• Bone marrow is found within the central cavity of bone, responsible for blood cell production
• Bone is continuously remodeling and turning over
• Bone has two main types: compact (lamellar) and spongy (trabecular)

Bone Tissue—Cells and Matrix

• Bone is made of cells (osteocytes, osteoblasts and osteoclasts)
• Osteoblasts create bone, line bone surfaces, and produce osteoid
• Osteocytes are osteoblasts trapped in the bone matrix, maintaining the structure and assisting in bone nutrition
• Osteoclasts break down bone via phagocytosis, important in bone remodeling
• Osteoid, the organic component of bone matrix, comprises ~90% collagen type I and glycoproteins and is responsible for the tensile strength of bone
• Minerals predominantly hydroxyapatite (Ca10(PO4)6(OH)2), comprising ~60-70% dry weight of bone, provide compression strength and make the bone radio-opaque

Bone Tissue - Haversian System

• Compact bone is composed of units known as osteons (Haversian systems)
• Osteons are composed of concentric layers of osteocytes (lamellae) arranged around a central canal (containing blood vessels)
• Osteoblasts are on the outer and inner surfaces of the central canal
• Osteoclasts are located on surfaces where bone resorption occurs
• Osteons run parallel to the long axis of the bone
• The osteocytic-osteoblastic bone membrane (OOBM) is a network of fluid-filled canals (canaliculi) connecting osteocytes for exchange of substances

Bone Growth

• Most mammals have two periods of rapid growth: postnatal and pubertal
• Postnatal growth spurt occurs in early life, increasing body protein, mainly skeletal muscle
• Pubertal growth spurt increases growth hormone release and stimulates hypertrophy and hyperplasia of cells, leading to bone thickening and lengthening
• Chondrocytes (cartilage cells) proliferate in the outer edge of epiphyseal plates next to epiphysis resulting in temporary widening of the epiphyseal plate which is gradually replaced by bone as growth in length proceeds
• Osteoclasts remove dead chondrocytes and calcified matrix, followed by infiltration of osteoblasts from diaphysis, producing new bone and capillary blood supply
• Bone growth in thickness is achieved by osteoblasts adding new bone to the outer surface, concurrent with osteoclasts removing bone on the inside of bone (reducing the bone marrow cavity)
• Bone growth in length stops when the epiphyseal plate becomes completely ossified

Bone Remodeling

• Bone is constantly undergoing turnover due to concurrent bone deposition and resorption (remodeling)
• Remodeling maintains mechanical effectiveness and plasma Ca2+ levels
• Osteoclasts attach to osteoid (ECM) forming a ruffled membrane, increasing contact area with bone
• Osteoclasts secrete organic acids (e.g., HCl) to dissolve hydroxyapatite and enzymes to break down ECM
• Osteoblasts migrate into the cavity and fill it with osteoid, which mineralizes to form new bone
• Formation/resorption rates in the adult remain approximately equal, maintaining constant bone mass
• Remodeling facilitates changes in bone shape, material, and repair of damaged bone, and release of mineral ions

Regulation of Bone Growth

• Growth hormone (GH) stimulates insulin-like growth factor (IGF-I) production and paracrine action, stimulating chondrocyte and osteoblast activity
• Androgens (e.g., testosterone) promote protein synthesis, influencing bone growth but halt growth at the end of adolescence, closing the epiphyseal plate

Regulation of Plasma Ca2+ Levels

• Bone contains a labile pool of calcium in the bone fluid. Rapid exchange between the bone fluid and plasma maintains calcium homeostasis
• Osteocytic-osteoblastic membrane (OOBM) pumps calcium from the bone fluid into the plasma during fast exchange, stimulated by parathyroid hormone (PTH)
• Slow exchange involves PTH-induced bone dissolution, releasing calcium from the mineralized bone into the plasma
• The balance between RANKL and OPG signaling pathways regulates osteoclast activity and overall bone density