Skeletal System Overview

Main Components of the Skeletal System

The skeletal system provides the framework for the body, supports movement, and protects vital organs. It consists of several key structures:

• Bones: The rigid organs forming the skeleton.

• Joints: Articulations where two or more bones meet.

• Cartilages: Flexible connective tissue found in joints and other areas.

• Ligaments: Connective tissue bands that link bones together.

The skeleton is subdivided into:

• Axial skeleton: Skull, vertebral column, and thoracic cage.

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

Functions of Bones

• Support: Provides structural support for the body.

• Protection: Shields soft organs (e.g., skull protects brain, rib cage protects thoracic organs).

• Movement: Acts as levers for muscles to produce movement.

• Storage: Stores minerals (calcium, phosphorus) and fats (yellow marrow).

• Blood cell formation: Hematopoiesis occurs in red marrow.

Classification of Bones

Types of Osseous Tissue

• Compact bone: Dense, smooth, homogeneous tissue.

• Spongy bone: Composed of small needlelike pieces (trabeculae) and open spaces.

Bone Shapes

Bones are classified by shape into four groups:

• Long bones: Longer than wide, shaft with enlarged ends, mostly compact bone (e.g., femur, humerus).

• Flat bones: Thin, flattened, usually curved, two layers of compact bone with spongy bone between (e.g., skull, ribs, sternum).

• Short bones: Cube-shaped, mostly spongy bone, includes sesamoid bones (e.g., carpals, tarsals, patella).

• Irregular bones: Complex shapes, do not fit other categories (e.g., vertebrae, hip bones, facial bones).

Structure of Bone

Gross Anatomy of Long Bones

• Diaphysis: Shaft, composed of compact bone.

• Epiphysis: Ends, mostly spongy bone enclosed by compact bone.

• Periosteum: Outer fibrous membrane covering diaphysis, secured by Sharpey's fibers.

• Articular cartilage: Hyaline cartilage covering epiphyses, reduces friction and absorbs shock.

• Epiphyseal plate/line: Plate of hyaline cartilage in growing bones (site of lengthwise growth); line is its remnant in adults.

• Endosteum: Connective tissue lining the inner shaft surface.

• Medullary cavity: Central cavity in shaft, contains yellow marrow (fat) in adults, red marrow (hematopoiesis) in children.

Bone Markings

Bone markings are features on bones that serve as attachment sites or passageways:

• Projections/processes: Grow out from bone surface (e.g., crest, spine).

• Depressions/cavities: Indentations (e.g., foramen, fossa, meatus).

Common Bone Markings Table

Microscopic Anatomy of Bone

• Osteocytes: Mature bone cells in bone matrix.

• Lacunae: Cavities housing osteocytes.

• Lamellae: Concentric rings of bone matrix around central canal.

• Osteon (Haversian system): Structural unit containing central canal and lamellae.

• Central (Haversian) canal: Runs lengthwise, carries blood vessels and nerves.

• Canaliculi: Tiny canals connecting lacunae to central canal, facilitating nutrient transport.

• Perforating (Volkmann's) canals: Perpendicular canals carrying blood vessels and nerves.

• Trabeculae: Needlelike structures in spongy bone, surrounded by marrow and vessels.

Bone Composition

• Organic components: Collagen fibers provide flexibility and tensile strength.

• Inorganic components: Calcium salts make bone hard and resistant to compression.

Bone Formation, Growth, and Remodeling

Ossification

Ossification is the process of bone formation, occurring on hyaline cartilage models or fibrous membranes.

• Phase 1: Osteoblasts cover cartilage model with bone matrix.

• Phase 2: Cartilage is digested away, forming the medullary cavity.

After birth, cartilage remains only at articular surfaces and epiphyseal plates.

Bone Growth

• Lengthwise growth: Occurs at epiphyseal plates.

• Appositional growth: Bones grow in width; osteoblasts add bone to periosteum, osteoclasts remove bone from endosteum.

• Hormonal control: Growth hormone and sex hormones regulate bone growth.

Bone Remodeling

• Occurs throughout life in response to blood calcium levels and mechanical stress.

• Calcium regulation:

  • Parathyroid hormone (PTH): Released when blood calcium is low, activates osteoclasts to release calcium.

  • Hypercalcemia: High blood calcium prompts osteoblasts to store calcium in bone.

Bone Fractures and Repair

Types of Fractures

• Closed (simple) fracture: Bone breaks but does not penetrate skin.

• Open (compound) fracture: Broken bone penetrates skin.

Fracture Treatment

• Reduction: Realignment of bone (closed: manual; open: surgical with pins/wires).

• Immobilization: Prevents movement during healing (6-8 weeks typical).

Fracture Repair Process

1. Hematoma formation (blood-filled swelling).

2. Fibrocartilage callus forms (cartilage and collagen stabilize break).

3. Bony callus replaces fibrocartilage (osteoblasts/clasts migrate in).

4. Bone remodeling occurs in response to mechanical stress.

Axial Skeleton

Skull

• Cranium: Encloses brain (frontal, occipital, ethmoid, sphenoid, parietal (2), temporal (2)).

• Facial bones: Maxillae (2), palatine (2), lacrimal (2), zygomatic (2), nasal (2), vomer, inferior nasal conchae (2), mandible.

• Sutures: Immovable joints between skull bones.

• Mandible: Only freely movable skull bone.

Paranasal Sinuses

• Hollow portions of bones around nasal cavity.

• Functions: Lighten skull, amplify voice.

Hyoid Bone

• Does not articulate with other bones.

• Movable base for tongue; aids swallowing and speech.

Vertebral Column

• Axial support from skull to pelvis.

• Separated by intervertebral discs.

• Regions:

  • 7 cervical (neck)

  • 12 thoracic (chest)

  • 5 lumbar (lower back)

  • Sacrum (5 fused)

  • Coccyx (3-5 fused)

Spinal Curvatures

• Primary: Thoracic and sacral (C-shaped in newborns).

• Secondary: Cervical and lumbar (S-shaped in adults).

Common Vertebra Features

• Body (centrum)

• Vertebral arch (pedicle, lamina)

• Vertebral foramen

• Transverse and spinous processes

• Articular processes (superior/inferior)

Thoracic Cage

• Protects thoracic organs.

• Consists of sternum, ribs, thoracic vertebrae.

• Rib types:

  • True ribs (1-7)

  • False ribs (8-12)

  • Floating ribs (11-12)

Appendicular Skeleton

Pectoral (Shoulder) Girdle

• Attaches upper limb to axial skeleton.

• Composed of clavicle and scapula.

• Allows exceptional mobility.

Upper Limbs

• Humerus: Arm bone; head articulates with scapula, distal end with forearm.

• Forearm: Ulna (medial), radius (lateral).

• Hand:

  • Carpals (8 wrist bones)

  • Metacarpals (5 palm bones)

  • Phalanges (14 per hand; 3 per finger, 2 in thumb)

Pelvic Girdle

• Formed by two coxal bones (ilium, ischium, pubis).

• Pelvis = coxal bones + sacrum + coccyx.

• Supports upper body weight; protects reproductive organs, bladder, part of large intestine.

Female vs. Male Pelvis Table

Lower Limbs

• Femur: Thigh bone; strongest in body.

• Lower leg: Tibia (medial, shinbone), fibula (lateral, thin).

• Foot:

  • Tarsals (7, including calcaneus and talus)

  • Metatarsals (5)

  • Phalanges (14 per foot)

  • Arches: Two longitudinal, one transverse

Joints (Articulations)

Functions of Joints

• Hold bones together securely.

• Allow for mobility.

Joint Classification

• Functional:

  • Synarthroses: Immovable

  • Amphiarthroses: Slightly movable

  • Diarthroses: Freely movable

• Structural:

  • Fibrous: Generally immovable

  • Cartilaginous: Immovable or slightly movable

  • Synovial: Freely movable

Fibrous Joints

• Bones united by fibrous tissue.

• Types:

  • Sutures (immobile)

  • Syndesmoses (slightly movable, e.g., distal tibia/fibula)

  • Gomphoses (immobile, e.g., teeth in sockets)

Cartilaginous Joints

• Bones connected by cartilage.

• Types:

  • Synchondrosis (immobile, e.g., epiphyseal plates)

  • Symphysis (slightly movable, e.g., pubic symphysis, intervertebral joints)

Synovial Joints

• Articulating bones separated by joint cavity filled with synovial fluid.

• Features:

  1. Articular cartilage

  2. Articular capsule

  3. Joint cavity

  4. Reinforcing ligaments

• Bursae: Fluid-filled sacs reducing friction.

• Tendon sheath: Elongated bursa wrapping around tendon.

Summary Table: Joint Classes

Types of Synovial Joints (by Shape)

• Plane joint: Flat surfaces, gliding movements (e.g., intercarpal joints).

• Hinge joint: Cylindrical end fits into trough-shaped surface, movement in one plane (e.g., elbow).

• Pivot joint: Rounded end fits into ring, rotation (e.g., proximal radioulnar joint).

• Condylar joint: Oval surface fits into depression, movement in two planes (e.g., knuckle).

• Saddle joint: Each surface has convex and concave areas, movement in two planes (e.g., thumb).

• Ball-and-socket joint: Spherical head fits into socket, movement in all axes (e.g., shoulder, hip).

Developmental Aspects of the Skeleton

Fetal and Infant Skeleton

• Fetal skeleton is mostly hyaline cartilage and fibrous membranes.

• Ossification centers develop as fetus grows.

• Fontanels: Soft spots in infant skull, allow compression during birth and brain growth; ossify by age 2.

Growth and Proportions

• Skull is three-fourths adult size by age 2; near adult size by age 8-9.

• Face grows out from skull between ages 6-11.

• At birth, head and trunk are proportionally larger than limbs.

• During puberty, female pelvis broadens; male skeleton becomes more robust.

• Epiphyseal plates ossify by end of adolescence.

Aging and Skeletal Health

• Osteoporosis: Bone-thinning disease, common in women over 65 and men over 70.

• Leads to fragile bones and increased fracture risk.

• Vertebral collapse causes kyphosis ("dowager's hump").

• Estrogen helps maintain bone density in females.

Key Terms and Concepts

• Ossification: Process of bone formation.

• Hematopoiesis: Blood cell formation in red marrow.

• Epiphyseal plate: Growth plate in long bones.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-destroying cells.

• Parathyroid hormone (PTH): Regulates blood calcium levels.

• Fontanels: Soft spots in infant skull.

• Osteoporosis: Disease causing bone fragility.

Important Equations

• Calcium Regulation Equation:

Additional info: Some tables and bone marking details were inferred from standard anatomical knowledge to supplement fragmented content.