Skeletal System: Structure, Function, and Joints

Skeletal System Overview

Main Organs and Functions

The skeletal system is a supportive connective tissue system that provides the framework for the body, protects internal organs, stores minerals and fat, produces blood cells, and enables movement through articulation with muscles. It consists of bones, cartilages, and ligaments.

• Bones: Provide structure, protection, and serve as levers for movement.

• Cartilage: Offers flexible support, reduces friction at joints, and forms the template for bone development.

• Ligaments: Connect bone to bone, stabilizing joints.

Cartilage: Structure and Function

Types and Roles of Cartilage

Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints and is a structural component of the rib cage, ear, nose, bronchial tubes, and intervertebral discs.

• Functions: Provides support, flexibility, and smooth surfaces for joint movement; forms the embryonic skeleton; and assists in bone growth.

• Types: Hyaline cartilage, elastic cartilage, and fibrocartilage.

• Matrix: Composed of glycosaminoglycans (GAGs), proteoglycans, collagen, and water.

• Cells: Chondroblasts (produce matrix) and chondrocytes (maintain matrix).

Comparison of Cartilage and Bone

Structural and Functional Differences

Cartilage and bone are both connective tissues but differ in structure, composition, and function.

Cartilage	Bone
Surrounded by perichondrium	Surrounded by periosteum
No blood vessels or nerves (except in perichondrium)	Blood vessels and nerves throughout
Chondrocytes in lacunae	Osteocytes in lacunae
Flexible extracellular matrix	Rigid extracellular matrix (due to inorganic calcium salts)
Matrix made by chondroblasts	Matrix (organic part) made by osteoblasts
Appositional and interstitial growth	Appositional growth only

Classification of Bones

By Shape and Structure

Bones are classified based on their shape and location in the body.

• By Shape:

  • Long bones (e.g., femur, humerus)

  • Short bones (e.g., carpals, tarsals)

  • Flat bones (e.g., sternum, skull)

  • Irregular bones (e.g., vertebrae, pelvis)

• By Structure: Compact (dense) and cancellous (spongy) bone.

Bone Cells

Types and Functions

Bone tissue contains four main types of specialized cells, each with distinct roles in bone formation, maintenance, and remodeling.

• Osteogenic cells: Stem cells from periosteum and endosteum; differentiate into osteoblasts.

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

• Osteocytes: Mature bone cells; maintain bone matrix and monitor mechanical stress.

• Osteoclasts: Bone-resorbing cells; break down bone matrix, derived from white blood cell lineage.

Bone Matrix

Composition and Properties

The bone matrix is composed of inorganic and organic components, providing both hardness and flexibility.

• Inorganic: Hydroxyapatite (calcium phosphate crystals) – gives hardness and strength.

• Organic: Collagen fibers, chondroitin sulfate, hyaluronic acid, and water – provide toughness and flexibility.

Anatomy of a Long Bone

Regions and Layers

Long bones have distinct regions and layers, each with specific functions in growth, support, and nutrient supply.

• Regions: Epiphysis (ends), diaphysis (shaft), epiphyseal line (growth plate remnant).

• Layers (superficial to deep): Periosteum (fibrous and osteogenic layers), compact bone, spongy bone, medullary cavity (contains marrow).

Microscopic Structure of Bone

Compact vs. Spongy Bone

Bone tissue is organized into compact and spongy types, each with unique microscopic features.

• Compact bone: Contains osteons (Haversian systems) with concentric lamellae, central canals, and canaliculi for nutrient delivery.

• Spongy bone: Composed of trabeculae; lacks central canals; nutrients diffuse through canaliculi.

Bone Formation (Ossification)

Intramembranous and Endochondral Ossification

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

• Intramembranous ossification: Osteoblasts form bone directly from mesenchymal tissue (e.g., skull, clavicle).

• Endochondral ossification: Bone forms by replacing hyaline cartilage (e.g., long bones, axial skeleton).

Epiphyseal Plate and Bone Growth

Zones and Hormonal Regulation

The epiphyseal plate enables longitudinal bone growth through distinct cellular zones. Growth is regulated by hormones such as growth hormone, sex hormones, thyroxine, and insulin.

• Zones: Reserve cartilage, proliferation, hypertrophy, calcification, ossification.

• Hormones: Stimulate or inhibit growth and closure of the growth plate.

Bone Remodeling and Calcium Regulation

Dynamic Process and Hormonal Control

Bone remodeling is a lifelong process involving osteoblast and osteoclast activity. Calcium homeostasis is tightly regulated by hormones.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts and increasing intestinal absorption.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and promoting calcium deposition in bone.

• Calcitriol (Vitamin D): Enhances calcium absorption from the gut.

Fractures and Bone Healing

Types and Stages of Healing

Fractures are breaks in bone continuity, classified as simple, compound, comminuted, or greenstick. Healing occurs in four stages:

1. Hematoma formation (2 days)

2. Soft callus formation (2 weeks)

3. Hard callus formation (2 months)

4. Remodeling (up to 2 years)

Joints (Articulations)

Definition and Classification

Joints are sites where two or more bones meet. They are classified by structure and function.

• Functional classification: Synarthroses (immovable), amphiarthroses (slightly movable), diarthroses (freely movable).

• Structural classification: Fibrous, cartilaginous, and synovial joints.

Fibrous Joints

Types and Examples

Fibrous joints are connected by dense connective tissue and allow little or no movement.

• Sutures: Found in the skull; immovable.

• Syndesmoses: Bones joined by ligaments; slight movement (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., teeth in jaw).

Cartilaginous Joints

Types and Examples

Cartilaginous joints are united by cartilage and allow limited movement.

• Symphyses: Bones joined by fibrocartilage (e.g., intervertebral discs, pubic symphysis).

• Synchondroses: Bones joined by hyaline cartilage (e.g., epiphyseal plate, first rib-sternum joint).

Synovial Joints

Characteristics and Types

Synovial joints are the most movable type of joint, characterized by a joint cavity filled with synovial fluid, articular cartilage, a fibrous capsule, and supporting ligaments.

• Types: Plane (gliding), hinge, pivot, condylar (ellipsoid), saddle, ball-and-socket.

• Features: Menisci, bursae, fat pads for shock absorption and stability.

Movements at Synovial Joints

Types of Movement

Synovial joints allow a variety of movements, including gliding, angular, and rotational movements.

Movement	Definition
Gliding	Sliding flat surfaces of two bones across each other
Flexion	Decreasing the angle between two bones
Extension	Increasing the angle between two bones
Abduction	Moving a limb away from the body midline
Adduction	Moving a limb toward the body midline
Circumduction	Moving a limb or finger so that it describes a cone in space
Rotation	Turning a bone around its longitudinal axis
Medial rotation	Rotating toward the median plane
Lateral rotation	Rotating away from the median plane

Major Synovial Joints

Examples and Features

• Temporomandibular Joint (TMJ): Combination plane/ellipsoid; allows elevation, depression, protraction, retraction; contains fibrocartilage disc.

• Shoulder (Glenohumeral) Joint: Ball-and-socket; stabilized by rotator cuff muscles and ligaments; allows wide range of motion.

• Elbow Joint: Hinge joint; stabilized by collateral and annular ligaments.

• Hip (Coxal) Joint: Ball-and-socket; very stable due to deep socket and strong ligaments.

• Knee Joint: Largest, most complex; hinge with some rotation; stabilized by menisci, cruciate and collateral ligaments, and bursae.

Joint Disorders

Common Pathologies

• Dislocations: Bones forced out of alignment (e.g., shoulder, fingers).

• Osteoarthritis: Degeneration of articular cartilage due to wear and tear; non-inflammatory.

• Rheumatoid arthritis: Autoimmune inflammation of synovial membrane, leading to cartilage destruction.