Chapter 6: Bones and Skeletal Tissue

Learning Objectives

• Describe the major structures of long bone.

• Identify the principal constituents of bone tissue.

• Explain the function of five types of bone cells.

• Discuss the homeostatic functions of bones.

• Outline the mechanism of bone development/ossification.

Importance of Bone Anatomy and Remodeling

Clinical Relevance

Understanding bone anatomy and the process of bone remodeling is essential for diagnosing and treating bone diseases such as osteoporosis and bone fractures. This knowledge is crucial for healthcare professionals working with patients who have musculoskeletal disorders.

Principal Constituents of Bone Tissue

Cartilage Structure and Function

Bone formation initially consists of cartilage, which is gradually replaced by bone except in specific areas. Cartilage is a resilient, molded tissue composed primarily of water and chondrocytes, and lacks blood vessels or nerves.

• Perichondrium: A dense connective tissue layer surrounding cartilage, acting as a girdle to resist outward expansion and deliver nutrients via blood vessels.

• Chondrocytes: Cells encased in lacunae within the jelly-like extracellular matrix of cartilage.

Types of Cartilage

• Hyaline Cartilage: Provides support, flexibility, and resilience; most abundant type; contains collagen fibers. Found in articular joints, costal (ribs), respiratory tract, and nasal cartilage.

• Elastic Cartilage: Similar to hyaline but contains more elastic fibers. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength. Found in menisci of the knee and intervertebral discs.

Cartilage Growth

• Appositional Growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

• Normal bone growth involves both types of cartilage growth, but cartilage growth can also occur in old age.

Functions of Bones

Major Functions

• Support: Provides structural support for the body and soft organs.

• Protection: Protects the brain, spinal cord, and vital organs.

• Movement: Serves as levers for muscle action.

• Mineral and Growth Factor Storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities of certain bones.

• Triglyceride (Fat) Storage: Fat used for energy is stored in bone cavities.

• Hormone Production: Osteocalcin secreted by bones helps regulate insulin secretion, glucose homeostasis, and energy metabolism.

Organization of the Human Skeleton

Axial and Appendicular Skeleton

• Axial Skeleton: Forms the long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

Classification of Bones by Shape

• Long Bones: Longer than they are wide (e.g., humerus, femur).

• Short Bones: Cube-shaped bones (e.g., wrist and ankle bones).

• Sesamoid Bones: Form within tendons (e.g., patella).

• Flat Bones: Thin, slightly curved bones (e.g., scapulae, ribs, most skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Major Structures of Long Bone

Bone Tissue Types

• Compact Bone: Dense outer layer that appears smooth and solid.

• Spongy Bone: Internal honeycomb structure made up of trabeculae, with spaces filled with bone marrow.

Structure of Long Bones

• Diaphysis: Tubular shaft forming the long axis, consisting of compact bone surrounding the medullary cavity (filled with yellow marrow in adults).

• Epiphyses: Bone ends consisting of compact bone externally and spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the childhood epiphyseal plate where bone growth occurred.

Membranes Covering Bone

• Periosteum: Double-layered membrane covering external bone surfaces except joints; outer fibrous layer and inner osteogenic layer (contains bone-forming cells).

• Endosteum: Connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone; contains osteogenic cells.

Bone Marrow

• Red Marrow: Found within trabecular cavities of spongy bone in flat bones and in the heads of femur and humerus; site of hematopoiesis.

• Yellow Marrow: Fat storage; can convert to red marrow if necessary (e.g., in anemia).

Bone Markings

Sites of muscle, ligament, and tendon attachment, as well as conduits for blood vessels and nerves. Examples include tuberosity, crest, trochanter, epicondyle, spine, and process.

Types and Functions of Bone Cells

Five Major Bone Cell Types

• Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid).

• Osteocytes: Mature bone cells that maintain bone matrix and act as stress/strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces, involved in bone maintenance.

• Osteoclasts: Large cells responsible for bone resorption (breakdown of bone).

Functions of Bone Cells

• Osteoblasts: Synthesize and secrete osteoid, initiate mineralization.

• Osteocytes: Monitor and maintain bone matrix, communicate mechanical stress to osteoblasts and osteoclasts.

• Bone-Lining Cells: Regulate calcium exchange and bone maintenance.

• Osteoclasts: Break down bone matrix via enzyme degradation, releasing calcium into the bloodstream.

Bone Tissue Composition

Organic and Inorganic Components

• Organic: Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid (ground substance and collagen fibers).

• Inorganic: Hydroxyapatites (mineral salts), primarily calcium phosphate crystals, responsible for bone hardness and resistance to compression.

Mechanism of Bone Development (Ossification)

Overview of Ossification

• Ossification (Osteogenesis): Process of bone tissue formation, beginning in the embryo and continuing through adolescence and adulthood.

• Bone growth occurs until early adulthood; bone remodeling and repair continue throughout life.

Types of Bone Formation

• Endochondral Ossification: Bone replaces hyaline cartilage; forms most bones below the skull.

• Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones such as the skull and clavicles.

Bone Growth

• Interstitial (Longitudinal) Growth: Increases bone length via growth at the epiphyseal plate.

• Appositional Growth: Increases bone thickness by adding new bone tissue to the surface.

• Epiphyseal plate closure occurs at the end of adolescence, leading to cessation of longitudinal growth.

Homeostatic Functions of Bone

Calcium Homeostasis

• Bone remodeling is regulated by two control loops: hormonal (negative feedback) and mechanical stress.

• Parathyroid Hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts to resorb bone, increasing blood calcium levels.

• Calcitonin: Produced by the thyroid gland in response to high blood calcium; inhibits bone resorption.

• Calcium is essential for nerve transmission, muscle contraction, blood coagulation, gland and nerve secretion, and cell division.

• Absorption of calcium requires vitamin D.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates epiphyseal plate activity during infancy and childhood.

• Thyroid Hormone: Modulates growth hormone activity for proper bone proportions.

• Sex Hormones (Testosterone and Estrogen): Promote adolescent growth spurts and induce epiphyseal plate closure.

• Imbalances in these hormones can cause abnormal skeletal growth.

Mechanical Stress and Bone Remodeling

• Bones remodel in response to mechanical stress (Wolff's Law): bones grow or remodel according to the demands placed on them.

• Bone mass is recycled weekly; compact bone replaced every 3-4 years, spongy bone every 10 years.

Bone Repair and Disease

Bone Fracture Repair

1. Hematoma Formation: Blood vessels break, forming a clot.

2. Fibrocartilaginous Callus Formation: New trabeculae form in the callus.

3. Bony Callus Formation: Spongy bone replaces the callus.

4. Bone Remodeling: Compact bone reconstructs shaft walls, restoring original structure.

Bone Diseases

• Osteoporosis: Bone resorption exceeds deposition, leading to fragile bones and increased fracture risk.

• Osteomalacia/Rickets: Bones are inadequately mineralized, causing softness and deformity; often due to vitamin D deficiency.

Summary Table: Types of Bone Cells

Key Equations

• Calcium Homeostasis (Negative Feedback):

• Bone Growth (Appositional):

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the detailed steps of bone repair and the summary table of bone cell types.