Chapter 6: Bones and Bone Structure

Introduction to the Structure and Growth of Bones

This chapter explores the anatomy and physiology of bones, focusing on their structure, classification, growth, and clinical significance. Understanding bone structure is essential for comprehending the skeletal system's role in support, movement, and mineral homeostasis.

Main Divisions of the Skeleton

• Axial Skeleton: Composed of 80 bones, including the skull, thorax, and vertebral column. Forms the longitudinal axis of the body.

• Appendicular Skeleton: Composed of 126 bones, including the limbs and girdles that attach them to the axial skeleton.

• Associated Structures: Cartilages, ligaments, and other connective tissues support and stabilize the skeleton.

Functions of the Skeletal System

• Support: Provides structural framework for the body.

• Mineral and Lipid Storage: Stores calcium, phosphate, and lipids (yellow marrow).

• Blood Cell Production: Occurs in red bone marrow (hematopoiesis).

• Protection: Shields vital organs (e.g., skull protects brain).

• Leverage: Acts as levers for muscle action, enabling movement.

Bone Classification and Surface Markings

Bones are classified by shape and surface features, which relate to their functions and anatomical locations.

Categories of Bone by Shape

• Flat Bones: Thin, parallel surfaces (e.g., cranial bones, sternum, ribs, scapulae).

• Sutural Bones: Irregular bones between cranial bones; variable in number and shape.

• Long Bones: Long and slender (e.g., limb bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis, facial bones).

• Sesamoid Bones: Small, flat, develop in tendons (e.g., patella).

• Short Bones: Small, boxy (e.g., carpals, tarsals).

Bone Markings (Surface Features)

• Elevations/Projections: Sites for muscle, tendon, and ligament attachment or articulation with other bones.

• Depressions/Grooves/Tunnels: Pathways for blood vessels and nerves.

Functional Anatomy of a Long Bone

Long bones are specialized for transmitting forces and have distinct anatomical regions and internal structures.

• Epiphysis: Expanded ends, mostly spongy bone, covered by articular cartilage.

• Metaphysis: Connects epiphysis to diaphysis.

• Diaphysis: Shaft, contains medullary (marrow) cavity.

• Medullary Cavity: Contains red marrow (hematopoiesis) and yellow marrow (fat storage).

• Blood Supply: Nutrient arteries/veins, metaphyseal arteries/veins, and periosteal vessels supply bone tissue.

• Innervation: Sensory nerves in periosteum, diaphysis, and epiphyses.

Bone Cells and Matrix

Bone tissue is dynamic, maintained by specialized cells and a mineralized matrix.

• Osteogenic Cells: Stem cells producing osteoblasts; important in repair.

• Osteoblasts: Produce new bone matrix (osteoid), initiate ossification.

• Osteocytes: Mature bone cells in lacunae; maintain matrix.

• Osteoclasts: Remove and remodel bone matrix via osteolysis.

Bone Matrix: Composed of collagen fibers (flexibility) and hydroxyapatite crystals (strength).

Compact vs. Spongy Bone

Bone tissue is organized into compact and spongy forms, each with distinct structures and functions.

• Compact Bone: Made of osteons (Haversian systems) with concentric lamellae around a central canal; strong along its length.

• Spongy Bone: Network of trabeculae; spaces filled with red marrow; no osteons; nutrients diffuse through canaliculi.

Appositional Bone Growth

Appositional growth increases bone diameter by adding new layers under the periosteum.

• Periosteum: Outer fibrous and inner cellular layers; isolates bone, provides route for vessels/nerves, participates in growth and repair.

• Endosteum: Incomplete cellular layer lining medullary cavity; active in growth, repair, and remodeling.

• Osteoclasts: Enlarge medullary cavity by removing matrix from inner surface.

Bone Formation: Ossification Processes

Bone develops through two main processes: endochondral and intramembranous ossification.

Endochondral Ossification

• Bone replaces a hyaline cartilage model.

• Primary ossification center forms in shaft; secondary centers in epiphyses.

• Growth in length occurs at epiphyseal plates until epiphyseal closure (formation of epiphyseal line).

Intramembranous Ossification

• Bone forms directly from mesenchymal tissue (no cartilage model).

• Occurs in dermal bones (e.g., skull, clavicle).

• Ossification centers form, producing spicules that fuse into trabeculae; compact bone forms at surfaces.

Clinical Module: Abnormalities of Bone Growth

Bone growth disorders can result in abnormal stature or skeletal deformities.

• Pituitary Growth Failure: Inadequate growth hormone; short bones.

• Achondroplasia: Slow epiphyseal cartilage growth; short limbs, normal trunk.

• Marfan Syndrome: Excessive cartilage formation; tall, slender limbs; cardiovascular risks.

• Gigantism: Excess growth hormone before puberty; abnormally tall stature.

• Acromegaly: Excess growth hormone after epiphyseal closure; thickened bones, especially in face, jaw, hands.

• Fibrodysplasia Ossificans Progressiva (FOP): Gene mutation causes bone formation in soft tissues.

• Clubfoot (Talipes Equinovarus): Abnormal muscle development distorts bones of the feet.

Physiology of Bones: Mineral Storage and Homeostasis

Bones act as reservoirs for minerals, especially calcium, which is tightly regulated by several organ systems and hormones.

Minerals Stored in Bone

• Calcium: Most abundant mineral in the body; essential for muscle contraction, nerve function, and blood clotting.

• Phosphate: Important for energy metabolism and bone structure.

Organs Involved in Calcium Homeostasis

• Intestines: Absorb dietary calcium and phosphate under hormonal control.

• Bones: Osteoclasts release calcium; osteoblasts deposit calcium.

• Kidneys: Regulate calcium and phosphate loss in urine.

Hormonal Regulation of Calcium Metabolism

Calcium levels are regulated by parathyroid hormone (PTH), calcitriol, and calcitonin.

Clinical Module: Bone Fractures and Repair

Fractures are breaks in bone due to mechanical stress. Healing involves a sequence of steps and can result in various fracture types.

Steps in Fracture Repair

1. Fracture Hematoma Formation: Blood clot forms at the site, closing injured vessels.

2. Callus Formation: Internal callus (spongy bone) unites inner edges; external callus (cartilage and bone) stabilizes outer edges.

3. Spongy Bone Formation: Cartilage replaced by spongy bone; dead bone removed.

4. Compact Bone Formation: Spongy bone replaced by compact bone; remodeling restores normal structure.

Types of Fractures

• Closed (Simple): Internal, no skin break.

• Open (Compound): Bone projects through skin; risk of infection and bleeding.

• Transverse: Break across long axis.

• Spiral: Twisting stress; spreads along bone length.

• Displaced/Nondisplaced: Abnormal/normal alignment.

• Compression: Vertebral collapse, often with osteoporosis.

• Greenstick: One side broken, one side bent; common in children.

• Comminuted: Bone shatters into fragments.

• Epiphyseal: At growth plate; may affect growth if not properly treated.

• Pott’s (Bimalleolar): Ankle fracture affecting both malleoli.

• Colles: Distal radius fracture.

Additional info: The included image is the cover of the textbook 'Visual Anatomy & Physiology,' which is directly relevant as the source of these study notes.