The Skeletal System

Overview

The skeletal system provides the structural framework for the human body, protects vital organs, enables movement, stores minerals, and houses bone marrow for blood cell production. This study guide covers the structure and function of bones, skeletal cartilages, the axial and appendicular skeleton, and joints.

Structure of Bone

Gross Anatomy of Bone

• Periosteum/Endosteum: Membranes covering the outer (periosteum) and inner (endosteum) surfaces of bone.

• Perforating (Sharpey's) Fibers: Collagen fibers anchoring periosteum to bone.

• Epiphysis: The ends of long bones, often containing spongy bone and red marrow.

• Diaphysis: The shaft of a long bone, primarily composed of compact bone.

• Metaphysis: The region between diaphysis and epiphysis, including the epiphyseal plate/line.

• Epiphyseal Plate/Line: Growth plate in children (cartilage), becomes epiphyseal line in adults (bone).

• Medullary Cavity (Marrow Cavity): Central cavity in long bones containing marrow.

  • Yellow Marrow: Fat storage (adults).

  • Red Marrow: Hematopoietic tissue (children and some adult bones).

• Osteons (Haversian System): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix; can be concentric, interstitial, or circumferential.

• Lacunae with Osteocytes: Small spaces housing mature bone cells (osteocytes).

Bone Tissue Composition

• Fibers, Matrix, Minerals: Collagen fibers provide flexibility; hydroxyapatite (calcium phosphate) gives hardness.

• Cells:

  • Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells; secrete osteoid.

  • Osteocytes: Mature bone cells maintaining bone matrix.

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

• Osteogenesis/Osteolysis and Bone Remodeling: Continuous process of bone formation and resorption.

Microscopic Structure

• Canaliculi: Tiny canals connecting lacunae, allowing communication between osteocytes.

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

• Perforating Canals (Volkmann's Canals): Connect central canals transversely.

Types of Bone

• Compact Bone: Dense, forms outer layer of bones; organized into osteons.

• Spongy Bone: Porous, found at ends of long bones and inside flat bones; contains trabeculae and marrow cavities.

Bone Marrow

• Medullary Cavity: Contains yellow or red marrow depending on age and bone type.

Skeletal Cartilages

Types and Structure

• Hyaline Cartilage: Most abundant; found in articular surfaces, costal cartilages, nose, trachea, and larynx.

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

• Fibrocartilage: Strongest; found in intervertebral discs, pubic symphysis, and menisci.

Axial Skeleton

Skull (22 Bones)

• Cranium (8): Frontal, Parietal (2), Temporal (2), Occipital, Sphenoid, Ethmoid.

• Facial Bones (14): Nasal (2), Lacrimal (2), Zygomatic (2), Palatine (2), Vomer, Inferior Nasal Concha (2), Maxilla (2), Mandible.

• Associated Bones: Auditory ossicles (6) – Malleus, Incus, Stapes (2 each).

Vertebral Column (26 Bones)

• Cervical (7): Includes Atlas (C1), Axis (C2), and Vertebra Prominens (C7).

• Thoracic (12): Articulate with ribs.

• Lumbar (5): Largest vertebrae, support lower back.

• Sacrum (1): Five fused vertebrae.

• Coccyx (1): Four fused vertebrae (tailbone).

Thoracic Cage

• Sternum: Manubrium, body, xiphoid process.

• Ribs (24): 7 true ribs, 5 false ribs (including 2 floating ribs).

• Costal Cartilage: Connects ribs to sternum.

Appendicular Skeleton

Pectoral Girdle (Shoulder Girdle)

• Clavicle (2): Sternal and acromial ends.

• Scapula (2): Superior/inferior angles, borders, glenoid cavity, acromion, coracoid process, spine, fossae.

Upper Limbs (60 Bones)

• Humerus (2): Arm bone; head, anatomical/surgical neck, greater/lesser tubercles.

• Radius (2): Lateral forearm; head, radial tuberosity, styloid process.

• Ulna (2): Medial forearm; olecranon, coronoid process, styloid process.

• Carpal Bones (16): Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate (each hand: 8).

• Metacarpals (10): Thumb (I) to little finger (V).

• Phalanges (28): Proximal, middle, distal (except thumb).

Pelvic Girdle

• Coxal Bones (Hip Bones, 2): Each formed by fusion of ilium, ischium, and pubis.

• Pelvis: Includes sacrum and coccyx; divided into greater (false) and lesser (true) pelvis.

• Acetabulum: Socket for femoral head.

• Obturator Foramen: Large opening formed by ischium and pubis.

Lower Limbs (60 Bones)

• Femur (2): Thigh bone; head, neck, greater/lesser trochanters, condyles.

• Patella (2): Kneecap.

• Tibia (2): Medial leg bone; condyles, tibial tuberosity, medial malleolus.

• Fibula (2): Lateral leg bone; head, lateral malleolus.

• Tarsal Bones (14): Talus, Calcaneus, Navicular, Cuboid, Cuneiforms (medial, intermediate, lateral).

• Metatarsals (10): Numbered I–V.

• Phalanges (28): Proximal, middle, distal (except hallux).

Spinal Curvatures

• Primary Curvatures: Thoracic and sacral (present at birth).

• Secondary Curvatures: Cervical and lumbar (develop after birth).

• Abnormal Curvatures: Kyphosis (hunchback), lordosis (swayback), scoliosis (lateral curvature).

Joints (Articulations)

Classification by Range of Motion

• Synarthroses: Immovable joints (e.g., sutures of the skull).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., shoulder, knee).

Histological Structure

• Fibrous Joints: Bones joined by dense connective tissue; no joint cavity.

  • Sutures: Skull bones.

  • Syndesmoses: Tibia/fibula or radius/ulna.

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity.

  • Synchondroses: Hyaline cartilage (e.g., epiphyseal plate, first rib-sternum).

  • Symphyses: Fibrocartilage (e.g., pubic symphysis, intervertebral discs).

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

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

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

Major Synovial Joints

• Hinge Joint: Elbow, knee, interphalangeal joints.

• Condylar Joint: Wrist, knuckles.

• Ball-and-Socket Joint: Shoulder, hip.

Knee Joint

• Structures to Identify: Lateral/medial meniscus, anterior/posterior cruciate ligaments, deep infrapatellar bursa, tibial collateral ligament, fibular collateral ligament.

Body Movements

• Flexion/Extension/Hyperextension/Lateral flexion

• Abduction/Adduction

• Internal/External Rotation

• Circumduction

• Pronation/Supination

• Inversion/Eversion

• Dorsiflexion/Plantar flexion

• Protraction/Retraction

• Elevation/Depression

• Opposition/Reposition

• Luxation (Dislocation) vs. Subluxation

Table: Comparison of Synovial Joint Types

Additional info:

• Bone remodeling is regulated by hormones (e.g., parathyroid hormone, calcitonin) and mechanical stress.

• Ossification occurs via intramembranous (flat bones) or endochondral (long bones) processes.

• Fontanelles in the infant skull allow for growth and flexibility during birth.