Skeletal System Overview

Functions of the Skeletal System

The skeletal system provides the framework for the body, supports and protects organs, enables movement, stores minerals, and houses bone marrow for blood cell production.

• Support: Provides structural support for the entire body.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Mineral Storage: Stores calcium and phosphate ions.

• Blood Cell Production: Houses red bone marrow, which produces blood cells (hematopoiesis).

• Energy Storage: Yellow bone marrow stores lipids.

Bone Structure and Composition

Structural Components of a Long Bone

Long bones have distinct regions and structures that contribute to their function and growth.

• Diaphysis: The shaft of the bone, composed mainly of compact bone.

• Epiphysis: The ends of the bone, containing spongy bone and red marrow.

• Medullary Cavity: Central cavity within the diaphysis, containing yellow marrow.

• Periosteum: Dense connective tissue covering the outer surface of bone.

• Endosteum: Thin membrane lining the medullary cavity.

• Articular Cartilage: Hyaline cartilage covering joint surfaces.

• Epiphyseal Plate/Line: Growth plate in children; becomes the epiphyseal line in adults.

Types of Bone Tissue

Bone tissue is classified as either compact (cortical) or spongy (cancellous) bone.

• Compact Bone: Dense, strong, forms the outer layer of bones.

• Spongy Bone: Porous, contains trabeculae, found in epiphyses and flat bones.

Microscopic Anatomy of Bone

• Osteon (Haversian System): Structural unit of compact bone.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae.

• Central (Haversian) Canal: Contains blood vessels and nerves.

Bone Matrix Composition

The bone matrix consists of organic and inorganic components.

• Organic: Collagen fibers (provide flexibility and tensile strength).

• Inorganic: Hydroxyapatite crystals (calcium phosphate) provide hardness.

Bone Cells

Types of Bone Cells

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Bone Marrow

Types and Functions

• Red Bone Marrow: Site of hematopoiesis (blood cell formation); found in spongy bone of flat bones and epiphyses of long bones.

• Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones.

Comparison Table: Red vs. Yellow Bone Marrow

Bone Development and Growth

Ossification Processes

• Intramembranous Ossification: Bone develops from fibrous membranes (e.g., flat bones of the skull).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

Epiphyseal Plate: Site of longitudinal bone growth during childhood and adolescence.

Bone Remodeling

• Continuous process of bone resorption and formation.

• Regulated by hormones (e.g., parathyroid hormone, calcitonin) and mechanical stress.

Calcium Homeostasis

Regulation of Blood Calcium Levels

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclast activity.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Negative Feedback Loop: Maintains calcium homeostasis.

Example: When blood calcium drops, PTH is released to increase levels; when calcium is high, calcitonin is released to decrease levels.

Cartilage

Types and Functions

• Hyaline Cartilage: Most common; found in articular surfaces, nose, trachea.

• Elastic Cartilage: Flexible; found in ear and epiglottis.

• Fibrocartilage: Strong; found in intervertebral discs, pubic symphysis.

Functions: Provides support, flexibility, and reduces friction at joints.

Axial and Appendicular Skeleton

Major Divisions

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

• Appendicular Skeleton: Limbs and girdles (pectoral and pelvic).

Bone Markings

• Foramina: Openings for nerves and blood vessels.

• Processes: Projections for muscle and ligament attachment.

• Fossae: Depressions in bones.

Joints (Articulations)

Classification of Joints

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue (e.g., sutures in skull).

  • Cartilaginous Joints: Bones joined by cartilage (e.g., intervertebral discs, pubic symphysis).

  • Synovial Joints: Bones separated by a fluid-filled cavity; most movable type.

• Functional Classification:

  • Synarthrosis: Immovable joints.

  • Amphiarthrosis: Slightly movable joints.

  • Diarthrosis: Freely movable joints (all synovial joints).

Synovial Joints

• Components: Articular cartilage, joint (synovial) cavity, articular capsule, synovial fluid, ligaments, nerves, and blood vessels.

• Types: Ball-and-socket, hinge, pivot, condyloid, saddle, plane.

Movements at Synovial Joints

• Angular Movements: Flexion, extension, hyperextension, abduction, adduction, circumduction.

• Rotational Movements: Medial and lateral rotation.

• Special Movements: Pronation, supination, inversion, eversion, elevation, depression, protraction, retraction.

Joint Disorders

• Osteoarthritis: Degenerative joint disease.

• Rheumatoid Arthritis: Autoimmune inflammation of joints.

• Osteoporosis: Loss of bone mass, increased fracture risk.

• Osteopenia: Reduced bone density, precursor to osteoporosis.

• Achondroplasia: Genetic disorder affecting bone growth.

Vertebral Column and Thoracic Cage

Vertebral Column

• Regions: Cervical (7), thoracic (12), lumbar (5), sacral (5 fused), coccygeal (4 fused).

• Functions: Protects spinal cord, supports head, attachment for ribs and muscles.

Thoracic Cage

• Components: Sternum, ribs, thoracic vertebrae.

• Functions: Protects heart and lungs, supports shoulder girdles and upper limbs.

Appendicular Skeleton: Upper and Lower Limbs

Upper Limb

• Shoulder Girdle: Clavicle and scapula.

• Arm: Humerus.

• Forearm: Radius and ulna.

• Hand: Carpals, metacarpals, phalanges.

Lower Limb

• Pelvic Girdle: Hip bones (ilium, ischium, pubis).

• Thigh: Femur.

• Leg: Tibia and fibula.

• Foot: Tarsals, metatarsals, phalanges.

Fontanelles of the Skull

Definition and Function

• Fontanelles: Soft spots on infant skulls where bones have not yet fused.

• Function: Allow for brain growth and passage through the birth canal.

• Closure: Anterior fontanelle closes by 18-24 months; posterior by 2-3 months.

Summary Table: Types of Synovial Joints

Additional info:

• Bone remodeling is influenced by mechanical stress and hormones such as growth hormone, sex hormones, and vitamin D.

• Specific bone markings serve as attachment points for muscles, ligaments, and tendons, and as passageways for nerves and blood vessels.

• Joint stability is determined by the shape of articular surfaces, ligaments, and muscle tone.