The Skeletal System: Structure, Function, and Disorders

The Skeletal System

Introduction to the Skeletal System

The skeletal system is a fundamental organ system in the human body, consisting of approximately 206 bones in adults. It also includes cartilages, ligaments, joints, and other connective tissues that stabilize or connect bones.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Support: Provides structural framework for the body.

• Blood cell production: Hematopoiesis occurs in bone marrow.

• Storage of lipids and minerals: Stores adipose tissue, calcium, and phosphate.

• Leverage: Muscles attach to bones for movement.

Bone Connective Tissue Composition

Bone tissue is a specialized connective tissue composed of collagen fibers and a calcified matrix.

• Collagen: Makes up one-third of bone weight; provides strength and flexibility.

• Calcified matrix (calcium phosphate): Makes up two-thirds of bone weight; provides hardness and resistance to shattering.

• Combination: Collagen and calcified crystals together make bone strong, flexible, and highly resistant to shattering.

Axial and Appendicular Skeleton

The skeleton is divided into two main regions:

• Axial skeleton: Includes bones of the skull, thorax, and vertebral column; forms the longitudinal axis of the body.

• Appendicular skeleton: Includes bones of the limbs and pectoral/pelvic girdles; attaches limbs to the axial skeleton.

Bone Classifications

Bones are classified based on their shape, which is closely related to their function.

• Flat bones: Protect internal organs (e.g., skull, ribs).

• Long bones: Support weight and facilitate movement (e.g., femur, humerus).

• Short bones: Provide stability and some movement (e.g., carpals).

• Irregular bones: Complex shapes (e.g., vertebrae).

• Sesamoid bones: Embedded in tendons (e.g., patella).

Structure of a Long Bone

Long bones have a distinct structure, including the diaphysis (shaft), epiphyses (ends), metaphysis, and epiphyseal plate/line.

• Diaphysis: Shaft of the bone.

• Epiphysis: Ends of the bone.

• Metaphysis: Region between diaphysis and epiphysis.

• Epiphyseal plate/line: Growth plate in children, becomes epiphyseal line in adults.

Types of Bone Tissue

Bone tissue is categorized into two main types:

• Compact bone: Covers the outside of bone surfaces; composed of osteons; resists stress in a specific direction.

• Spongy bone: Found inside bone; reduces bone weight and provides strength; consists of trabeculae with spaces filled with marrow.

Bone Cells

Both compact and spongy bone contain three main typecs of cells:

• Osteocytes: Mature bone cells that maintain the matrix.

• Osteoblasts: Cells that build new bone matrix.

• Osteoclasts: Cells that break down bone matrix.

Bone Development (Ossification)

Bone formation begins with cartilaginous models in embryos and continues into adulthood.

• Intramembranous ossification: Bone develops within a membrane.

• Endochondral ossification: Bone replaces a cartilage model.

Types of Bone Growth

Bone grows in two ways:

• Interstitial growth: Growth in length at the epiphyseal plate; closure results in the epiphyseal line.

• Appositional growth: Growth in diameter/thickness.

Bone Remodeling and Homeostasis

Bone tissue is constantly remodeled, with 3-5% of bone calcium exchanged each year.

• Osteocytes: Maintain matrix.

• Osteoclasts: Remove matrix.

• Osteoblasts: Build matrix.

• Factors: Nutrition (Vitamins A, C, D), exercise, hormones (GH, thyroid, sex hormones, calcitonin, PTH).

Hormonal Regulation of Calcium Homeostasis

• Calcitonin: Promotes calcium deposit in bone, decreases blood calcium, inhibits osteoclasts.

• Parathyroid hormone (PTH): Increases blood calcium, stimulates osteoclasts.

Skeletal Disorders

Atypical Skeletal Growth

• Achondroplasia: Slow growth of epiphyseal cartilage; short, stocky limbs.

• Gigantism: Overproduction of growth hormone before puberty; delayed puberty, excessive growth.

• Acromegaly: Overproduction of growth hormone after epiphyseal closure; bones thicken, especially in face, jaw, hands.

Osteopenia and Osteoporosis

• Osteopenia: Age-related loss of cartilage from joints.

• Osteoporosis: Disease resulting in loss of bone density and mass; bones become porous and fragile.

Bone Fractures

• Fracture: Crack or break in bone; classified as closed (simple) or open (compound).

Joints

Joints are classified by degree of movement and structure.

• Synarthrotic: No movement (e.g., sutures in skull).

• Amphiarthrotic: Little movement (e.g., symphysis between pubic bones).

• Diarthrotic: Free movement (e.g., synovial joints).

Functional Category	Structural Category and Type	Description	Example
Synarthrosis (No Movement)	Fibrous: Suture	Fibrous connections plus interlocked surfaces	Between bones of the skull
Synarthrosis (No Movement)	Fibrous: Gomphosis	Fibrous connection plus insertion in bone socket	Between teeth and bony sockets in the maxilla and mandible
Synarthrosis (No Movement)	Cartilaginous: Synchondrosis	Interposition of cartilage bridge or plate	Between first pair of ribs and the sternum; epiphyseal cartilage
Amphiarthrosis (Little Movement)	Fibrous: Syndesmosis	Ligamentous connection	Between the tibia and fibula
Amphiarthrosis (Little Movement)	Cartilaginous: Symphysis	Connection by fibrocartilage pad	Between adjacent vertebrae of spinal column
Diarthrosis (Free Movement)	Synovial	Complex joint bounded by joint capsule and containing synovial fluid	Numerous, subdivided by range of motion

Synovial Joints

Synovial joints are the most complex and allow free movement.

• Articular cartilage: Reduces friction and absorbs shock.

• Synovial fluid: Lubricates joint, nourishes cartilage, removes waste.

Types and Examples of Synovial Joints

Synovial joints are classified by their shape and movement:

• Ball-and-socket: Shoulder, hip.

• Hinge: Elbow, knee.

• Pivot: Atlas/axis vertebrae.

• Gliding: Wrist, ankle.

• Saddle: Thumb.

• Condyloid: Fingers.

Skeletal Disorders of Joints: Arthritis

• Arthritis: Inflamed, swollen joints; includes over 100 types of inflammatory or degenerative diseases.

• Rheumatoid arthritis: Chronic inflammatory disease; autoimmune disorder where the body attacks synovial joint membrane.

Summary Table: Skeletal System Functions and Disorders

Function/Disorder	Description
Protection	Shields vital organs
Support	Framework for body
Blood cell production	Hematopoiesis in marrow
Storage	Calcium, phosphate, lipids
Leverage	Muscle attachment for movement
Achondroplasia	Slow growth of epiphyseal cartilage
Gigantism	Excess GH before puberty
Acromegaly	Excess GH after puberty
Osteopenia	Age-related cartilage loss
Osteoporosis	Loss of bone density
Arthritis	Inflamed joints
Rheumatoid arthritis	Autoimmune attack on joints

Key Equations

• Calcium Homeostasis: $\text{Blood Calcium Level} = \text{Calcium Intake} - \text{Calcium Output}$

Additional info:

• Vitamin D is essential for calcium absorption and bone health.

• Exercise increases bone density and strength by stimulating osteoblast activity.