Chapter 6: Bones and Skeletal Tissues

Introduction to Bones and Skeletal Tissues

Bones are vital organs that form the framework of the body, providing support, protection, and facilitating movement. Skeletal tissues include bone (osseous tissue), cartilage, and associated connective tissues. The study of bones involves understanding their structure, function, and chemical composition at various levels of organization.

• Bone organs consist of different tissue types: osseous tissue, nervous tissue, blood vessels, cartilage, fibrous connective tissue, and muscle.

• Three levels of bone study: Gross anatomy, Microscopic anatomy, and Chemical composition.

Functions of Bones

Major Functions

Bones perform several essential functions that are critical for maintaining homeostasis and overall health.

• Support: Bones provide the structural framework for the body and support soft organs.

• Protection: Bones protect vital organs such as the brain (skull), spinal cord (vertebrae), and thoracic organs (rib cage).

• Movement: Bones act as levers for muscles, enabling movement.

• Mineral and Growth Factor Storage: Bones store minerals (e.g., calcium, phosphorus) and growth factors.

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

• Triglyceride (Fat) Storage: Fat is stored in bone cavities as an energy reserve.

• Hormone Production: Bones produce hormones such as osteocalcin, which regulates bone formation and energy metabolism.

Classification of Bones by Shape

Main Types of Bones

Bones are classified based on their shapes, which relate to their functions and locations in the body.

• Long Bones: Longer than they are wide; found in limbs (e.g., femur, humerus). Function as levers for movement.

• Short Bones: Cube-shaped; found in wrists (carpals) and ankles (tarsals). Provide stability and support with limited movement.

• Flat Bones: Thin, flattened, and usually curved; found in the skull, ribs, and sternum. Protect internal organs and provide broad surfaces for muscle attachment.

• Irregular Bones: Complex shapes; found in vertebrae and certain facial bones.

• Sesamoid Bones: Specialized bones located within tendons (e.g., patella). Provide mechanical advantage and reduce friction.

Table: Classification of Bones by Shape

Structure of Bones

Long Bone Structure

Long bones have a unique structure that supports their function in movement and load-bearing.

• Diaphysis: Shaft of the bone; composed of compact bone surrounding a medullary cavity.

• Epiphyses: Ends of the bone; consist of spongy bone covered by compact bone. Articular cartilage covers joint surfaces.

• Membranes: Periosteum (outer double-layered membrane) covers external surfaces except joint surfaces; Endosteum lines internal bone surfaces.

• Perforating (Sharpey’s) fibers: Secure periosteum to underlying bone.

Structure of Short, Irregular, and Flat Bones

These bones have thin plates of spongy bone (diploë) covered by compact bone, with no shaft or epiphyses.

• Bone marrow is present between trabeculae.

• Periosteum covers compact bone externally; endosteum covers spongy bone internally.

Gross Anatomy of Bone

Bone Textures

• Compact Bone: Dense outer layer; smooth and solid.

• Spongy Bone (Cancellous or Trabecular): Honeycomb of small needle-like or flat pieces called trabeculae; spaces are filled with bone marrow.

Microscopic Anatomy of Bone

Compact Bone

Compact bone is organized into structural units called osteons (Haversian systems).

• Osteon: Cylindrical structure oriented parallel to the long axis of the bone; consists of concentric lamellae (layers of bone matrix).

• Central (Haversian) Canal: Runs through the core of each osteon; contains blood vessels and nerves.

• Perforating (Volkmann’s) Canals: Connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities that house osteocytes.

• Canaliculi: Tiny canals that connect lacunae to each other and to the central canal, allowing for nutrient and waste exchange.

Lamellae

• Interstitial Lamellae: Incomplete lamellae not part of an osteon; remnants of bone remodeling.

• Circumferential Lamellae: Extend around the entire surface of the diaphysis; resist twisting of the long bone.

Spongy Bone

• Trabeculae: Align along lines of stress to help resist it; contain irregularly arranged lamellae and osteocytes connected by canaliculi.

Bone Marrow and Hematopoietic Tissue

Red Marrow

• Found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In adults, red marrow is found in heads of femur and humerus, and in flat and irregular bones.

• Red marrow is the site of hematopoiesis (blood cell formation).

Yellow Marrow

• Contains a large percentage of fat; can convert to red marrow if necessary.

Chemical Composition of Bone

Inorganic Components

• Hydroxyapatites (Mineral Salts): Primarily calcium phosphate crystals; account for bone hardness and resistance to compression.

• Approximately 65% of bone mass is inorganic mineral.

Equation: Hydroxyapatite Formation

Organic Components

• Osteoid: Organic part of bone matrix; includes collagen fibers and ground substance.

• Collagen provides tensile strength and flexibility.

Summary Table: Bone Structure and Composition

Additional info:

• Bone remodeling is a continuous process involving osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).

• Bone health is influenced by nutrition, hormonal regulation, and mechanical stress.