Bones and Skeletal Tissues: Structure, Types, and Functions

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Bones and Skeletal Tissues

Introduction

The skeletal system is composed of bones and associated tissues, including cartilage. These structures provide support, protection, movement, and play key roles in mineral storage and blood cell formation. Understanding the types, structure, and functions of bones and cartilage is fundamental in Anatomy & Physiology.

Cartilages

General Properties of Cartilage

Cartilage is a resilient, molded connective tissue that initially forms the skeleton and is later replaced by bone in most areas.
Cartilage is avascular (contains no blood vessels or nerves).
Surrounded by perichondrium, a dense irregular connective tissue layer that resists outward expansion and supplies nutrients via blood vessels.
Composed of chondrocytes (cartilage cells) located in small cavities called lacunae within the extracellular matrix.

Types and Locations of Cartilage

Hyaline Cartilage – Provides support; most abundant type, contains only collagen fibers.
- Locations: Articular (joints), costal (ribs), respiratory (larynx), nasal cartilage (nose tip).
Elastic Cartilage – Provides elasticity; similar to hyaline but contains elastic fibers.
- Locations: External ear, epiglottis.
Fibrocartilage – Provides cushion; contains thick collagen fibers for great tensile strength.
- Locations: Menisci of knee, intervertebral discs.

Growth of Cartilage

Appositional Growth (on the surface): Cells in the perichondrium secrete new matrix against the external face of existing cartilage.
Interstitial Growth (from within): Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from the inside.

Additional info: Cartilage growth is most active during childhood and adolescence, but can occur after injury in adults.

Functions of Bones

Main Functions

Support: Provides structural framework for the body and soft organs.
Protection: Shields the 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): Occurs in red marrow cavities of certain bones.
Triglyceride (Fat) Storage: Fat stored in yellow marrow cavities serves as an energy source.
Hormone Production: Osteocalcin produced by osteoblasts regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

Axial Skeleton: Forms the long axis of the body; includes the skull, vertebral column, rib cage, sternum, and costal cartilages.
Appendicular Skeleton: Includes the bones of the upper and lower limbs, and the girdles (shoulder and hip) that attach limbs to the axial skeleton.

Bone Shapes

Long Bones: Longer than they are wide; e.g., femur, humerus.
Short Bones: Cube-shaped; e.g., bones of wrist (carpals) and ankle (tarsals). Sesamoid bones (e.g., patella) form within tendons.
Flat Bones: Thin, flattened, and usually curved; e.g., sternum, scapulae, ribs, most skull bones.
Irregular Bones: Complicated shapes; e.g., vertebrae, hip (coxal) bones.

Structure of Bone

General Structure

Compact Bone: Dense outer layer that appears smooth and solid.
Spongy Bone: Internal layer made up of a honeycomb of small needle-like or flat pieces called trabeculae. Spaces between trabeculae are filled with bone marrow.

Structure of Short, Irregular, and Flat Bones

Consist of thin plates of spongy bone (diploë) covered by compact bone.
Compact bone is sandwiched between connective tissue membranes: periosteum (outer) and endosteum (inner).
No shaft or expanded ends; bone marrow is scattered throughout spongy bone.

Structure of Long Bones

Have a shaft (diaphysis), bone ends (epiphyses), and membranes.
Diaphysis: Tubular shaft forming the long axis; compact bone surrounds the central medullary cavity (contains yellow marrow in adults).
Epiphyses: Bone ends; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.
Epiphyseal Line: Remnant of childhood epiphyseal plate where bone growth occurred.

Membranes of Bone

Periosteum: White, double-layered membrane covering external surfaces except joints.
- Outer layer: Dense irregular connective tissue (Sharpey's fibers anchor to bone).
- Inner layer: Osteogenic stem cells for bone growth and repair.
- Contains nerve fibers and blood vessels; provides attachment points for tendons and ligaments.
Endosteum: Delicate membrane lining internal bone surfaces, including trabeculae of spongy bone and canals of compact bone; contains osteogenic cells.

Microscopic Anatomy of Bone

Bone Cells

Osteogenic Cells: Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts.
Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed mainly of collagen and calcium-binding proteins.
Osteocytes: Mature bone cells in lacunae; monitor and maintain bone matrix, act as stress/strain sensors.
Bone-lining Cells: Flat cells on bone surfaces; help maintain bone matrix (periosteal cells on external surfaces, endosteal cells on internal surfaces).
Osteoclasts: Large, multinucleated cells derived from hematopoietic stem cells; responsible for bone resorption.

Compact Bone Structure

Osteon (Haversian System): Structural unit of compact bone; elongated cylinder parallel to long axis, consisting of concentric rings (lamellae) of bone matrix.
Lamellae: Collagen fibers in adjacent rings run in different directions to resist twisting and stress.
Central (Haversian) Canal: Runs through the core of each osteon; contains blood vessels and nerve fibers.
Perforating (Volkmann's) Canals: Perpendicular to central canals; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.
Lacunae: Small cavities containing osteocytes.
Canaliculi: Tiny canals connecting lacunae to each other and to the central canal; allow communication and nutrient transfer between osteocytes.

Spongy Bone Structure

Trabeculae align along lines of stress to provide strength.
No osteons present; trabeculae contain irregularly arranged lamellae and osteocytes connected by canaliculi.
Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

Includes bone cells (osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts) and osteoid (unmineralized bone matrix).
Osteoid consists of ground substance and collagen fibers, contributing to bone's tensile strength and flexibility.
Sacrificial bonds in collagen molecules allow bone to absorb energy and prevent fractures; bonds re-form after minor trauma.

Inorganic Components

Composed mainly of hydroxyapatites (mineral salts), primarily tiny calcium phosphate crystals.
Account for 65% of bone mass; responsible for bone hardness and resistance to compression.
Bones are as strong as steel in resisting compression and as strong as steel in tension.
Mineral composition allows bones to persist long after death, providing information about ancient peoples.

Summary Table: Types of Cartilage

Type	Main Fiber	Function	Locations
Hyaline	Collagen	Support, flexibility	Joints, ribs, larynx, nose
Elastic	Elastic fibers	Flexibility, elasticity	Ear, epiglottis
Fibrocartilage	Thick collagen	Strong support, cushion	Knee menisci, intervertebral discs

Key Equations

Bone Matrix Composition:

Hydroxyapatite Formula:

Example Application

Damage to cartilage, such as in osteoarthritis, impairs joint function due to loss of cushioning and support. Bone diseases like osteoporosis result from imbalances in bone resorption and formation, often involving osteoclast and osteoblast activity.