Bones and Skeletal Tissues

Overview

The skeletal system is composed of bones and cartilage, which together provide structural support, protection, movement, and other essential functions for the human body. This chapter explores the types, structure, and function of bones and cartilage, as well as their microscopic and chemical composition.

Cartilage

Basic Structure, Types, and Locations

Cartilage is a resilient connective tissue consisting mainly of water, making it flexible and able to withstand compression. It contains no blood vessels or nerves and is surrounded by the perichondrium, a dense connective tissue layer that supplies nutrients and resists expansion. The main cell type is the chondrocyte, found in lacunae.

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant; found in joints, ribs, larynx, and nose.

• Elastic cartilage: Similar to hyaline but contains elastic fibers; found in external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; high tensile strength; found in menisci of knee and vertebral discs.

Growth of Cartilage

• Appositional growth: New matrix is laid down on the surface of cartilage.

• Interstitial growth: Chondrocytes divide and secrete new matrix from within.

• Calcification: Occurs during bone growth or aging; hardened cartilage is not bone.

Bones

Classification of Bones

The human skeleton contains 206 named bones, divided into two main groups:

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles attaching limbs to axial skeleton.

Bone Shapes

• Long bones: Longer than wide; found in limbs.

• Short bones: Cube-shaped; found in wrist and ankle; sesamoid bones form within tendons (e.g., patella).

• Flat bones: Thin, flat, slightly curved; found in sternum, scapulae, ribs, skull.

• Irregular bones: Complex shapes; found in vertebrae and hip bones.

Functions of Bones

• Support: Framework for body and soft organs.

• Protection: Shields brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis in red marrow cavities.

• Triglyceride storage: Fat stored in bone cavities for energy.

• Hormone production: Osteocalcin regulates insulin, glucose, and metabolism.

Bone Structure

Levels of Structure

Bones are organs containing various tissues: bone (osseous), nervous, cartilage, fibrous connective, muscle, and epithelial cells. Structure is studied at three levels: gross, microscopic, and chemical.

Gross Anatomy

• Compact bone: Dense outer layer, smooth and solid.

• Spongy bone: Honeycomb structure of trabeculae; spaces filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploe) covered by compact bone.

• Periosteum covers outside; endosteum covers inside.

• Bone marrow scattered throughout spongy bone; no defined cavity.

• Hyaline cartilage covers joint surfaces.

Structure of Long Bones

• Diaphysis: Shaft; compact bone surrounds medullary cavity (yellow marrow in adults).

• Epiphyses: Ends; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of growth plate.

Membranes

• Periosteum: Double-layered membrane covering external surfaces (except joints); fibrous layer (dense irregular connective tissue), osteogenic layer (stem cells).

• Contains nerves, blood vessels, and anchoring points for tendons/ligaments.

• Endosteum: Delicate membrane covering internal bone surfaces; contains osteogenic cells.

Hematopoietic Tissue

• Red marrow: Found in trabecular cavities of spongy bone and diploë of flat bones; in newborns, all spongy bone and medullary cavities contain red marrow; in adults, mainly in flat bones and some irregular bones.

• Yellow marrow: Can convert to red marrow if needed (e.g., anemia).

Bone Markings

Bone markings are sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves. They are classified as projections, depressions, or openings.

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells; secrete osteoid (collagen and calcium-binding proteins).

• Osteocytes: Mature bone cells in lacunae; maintain matrix and act as stress sensors.

• Bone-lining cells: Flat cells on bone surfaces; maintain matrix.

• Osteoclasts: Multinucleate cells; resorb bone; derived from hematopoietic stem cells.

Compact Bone Structure

• Osteon (Haversian system): Structural unit; elongated cylinder with concentric lamellae.

• Lamellae: Collagen fibers in different directions; resist stress and twisting.

• Canals and canaliculi: Central canal (blood vessels, nerves); perforating canals (connect periosteum, medullary cavity, central canal); canaliculi (connect lacunae).

• Interstitial and circumferential lamellae: Fill gaps and extend around diaphysis.

Spongy Bone Structure

• Trabeculae aligned along lines of stress; confer strength.

• No osteons; irregular lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes bone cells and osteoid (ground substance and collagen fibers).

• Osteoid provides tensile strength and flexibility.

• Resilience due to sacrificial bonds in collagen molecules.

Inorganic Components

• Hydroxyapatites (mineral salts): 65% of bone mass; mainly calcium phosphate crystals.

• Responsible for hardness and resistance to compression.

• Bones are strong and durable, lasting long after death.

Example: Forensic scientists use bone mineral composition to study ancient remains and detect illness or trauma.

Summary Table: Types of Bone Cells

Additional info: The notes expand brief points into full academic explanations and add context for clarity and completeness.