Bone Structure and Function

Bone Structure and Function

• Bone is a type of connective tissue that provides support to the body, protects internal organs, and acts as the body's calcium reservoir.

Cells and Matrix Components of Bone

• Osteoblasts differentiate from osteoprogenitor cells and secrete osteoid, which allows matrix mineralization to occur.
• Osteoid components include type I collagen, osteocalcin, and matrix vesicles with enzymes generating PO4-.
• High concentrations of Ca2+ and PO4- ions cause formation of hydroxyapatite crystals, which gradually calcify the entire matrix.
• Osteocytes differentiate from osteoblasts and become enclosed within matrix lacunae, maintaining the matrix and detecting mechanical stresses on bone.
• Osteocytes communicate with adjacent cells via a network of long dendritic processes that extend through the matrix via narrow canaliculi radiating from each lacuna.
• Osteoclasts are formed by fusion of several blood monocytes and locally erode bone matrix during osteogenesis and bone remodeling.

Periosteum

• A layer of dense connective tissue on the outer surface of bone
• Bound to bone matrix by bundles of type I collagen called perforating (or Sharpey) fibers
• Regions adjacent to bone are rich in osteoprogenitor cells and osteoblasts
• Mediates much bone growth and remodeling through osteoprogenitor cells and osteoblasts

Endosteum

• A thin layer of active and inactive osteoblasts
• Lines all internal surfaces within bone
• Osteoblasts in endosteum are required for bone growth

Types of Bone

• Dense bone beneath the periosteum is called compact bone
• Deep to compact bone are small bony trabeculae or spicules of cancellous (or spongy) bone

Bone Structure

• Long bones of the limbs have both compact and cancellous bone in epiphyses (knobby, bulbous ends) and diaphysis (intervening shaft)

Immature Bone

• Woven bone is formed during osteogenesis or repair and has a calcified matrix with randomly arranged collagen fibers

Bone Remodeling

• Osteoclasts and osteoblasts remodel woven bone into lamellar bone with new matrix deposited in distinct layers with parallel collagen bundles

Lamellar Bone

• Compact and cancellous bone are both types of lamellar bone
• Most lamellar bone consists of lamellae organized concentrically around small central canals containing blood vessels and nerves, called an osteon or Haversian system
• Osteocytic lacunae occur between the lamellae, with canaliculi radiating through the lamellae, allowing all cells to communicate with the central canal

Osteogenesis

• Bones of the skull and jaws develop through intramembranous ossification, where osteoblasts differentiate directly from mesenchyme progenitor cells.
• In contrast, all other bones form through endochondral ossification, involving osteoprogenitor cells surrounding and invading hyaline cartilage models in the embryo.
• Primary ossification centers in fetal long bones develop when chondrocytes die after being enclosed by a collar of woven bone, creating an initial cavity.
• Periosteal osteoblasts and vasculature then enter this cavity, marking the beginning of primary ossification.
• Secondary ossification centers develop later, within the epiphyses, with the cartilage of the epiphyseal growth plate situated between the primary and secondary ossification sites.

Bone Elongation During Childhood

• Growth plates consist of an interrelated series of developing zones that enable bone elongation during childhood

Zones of Growth Plate Development

• Reserve Zone: most distal zone, composed of typical hyaline cartilage that serves as a "resting" zone
• Proliferation Zone: adjacent to reserve zone, chondrocytes undergo mitosis and appear stacked within elongated lacunae
• Hypertrophy Zone: most mature chondrocytes swell up, compress the matrix, and undergo apoptosis, located closer to the primary ossification center
• Zone of Cartilage Calcification: spaces created in the matrix by apoptosis are invaded by osteoblasts, osteoclasts, and vasculature from the primary center
• Zone of Ossification: woven bone is initially laid down by osteoblasts and remodeled into lamellae bone

Bone Growth and Development

• Appositional bone growth increases the circumference of a bone through osteoblast activity at the periosteum
• Enlargement of the medullary marrow cavity accompanies appositional bone growth

Bone Growth and Remodeling

• Bone growth occurs throughout life, with continuous turnover of cells and matrix through the activities of osteoblasts and osteoclasts.
• Lamellae and osteons are temporary structures that are constantly replaced and rebuilt in a process of bone remodeling.
• Bone remodeling allows bones to change size and shape according to changes in mechanical stress.

Bone Repair

• Bone repair after fracture or injury involves the activation of periosteal fibroblasts to produce an initial soft callus of fibrocartilage-like tissue.
• The soft callus is gradually replaced by a hard callus of woven bone.
• The hard callus is soon remodeled to produce stronger lamellar bone.

Calcium Homeostasis

• Calcium (Ca2+) is a crucial ion for all cells, and its levels are maintained through bone storage and mobilization.
• When dietary calcium is sufficient, Ca2+ is stored in bone.
• When dietary calcium is deficient, Ca2+ is mobilized from bone.

Bone Cell Activity

• Maintenance of proper blood calcium levels involves the activity of osteoblasts, osteoclasts, and osteocytes.
• Subtle paracrine interactions among bone cells and other cells regulate this process.

Hormonal Regulation

• Parathyroid hormone (PTH) indirectly stimulates osteoclasts, increasing calcium levels in the blood.
• Calcitonin inhibits osteoclast activity, decreasing blood calcium levels.