Skeletal System: Bone Structure, Types, and Classification

Complete tutorial on the skeletal system - bone structure (compact vs. cancellous), classification by shape, axial vs. appendicular skeleton, bone cells, and growth.

Skeletal System 4 min read
This content is for informational purposes only. Always consult a healthcare professional.

The skeletal system provides the structural framework for the body, protects vital organs, enables movement, stores minerals, and produces blood cells.

Overview

Functions of the Skeleton

Function Description
Support Framework for the body, attachment for soft tissues
Protection Encloses vital organs (skull → brain, rib cage → heart/lungs)
Movement Levers for muscles, joints allow articulation
Mineral storage Calcium (99%), phosphorus (85%), magnesium
Hematopoiesis Blood cell production (bone marrow)
Energy storage Yellow marrow (adipose tissue)

Bone Classification by Shape

Type Description Examples
Long Longer than wide, shaft + two ends Femur, humerus, tibia, radius, ulna, phalanges
Short Cube-shaped, length ≈ width Carpals, tarsals
Flat Thin, flat, often curved Skull bones, sternum, ribs, scapulae
Irregular Complex shape Vertebrae, sacrum, mandible, pelvic bones
Sesamoid Embedded in tendon Patella, pisiform
Sutural (Wormian) Within skull sutures Variable

Number of Bones

Age Number
Newborn ~270
Adult 206 (varies slightly)

Bone Structure

Macroscopic Structure

Long bone anatomy:

Region Description Composition
Diaphysis Shaft (long tubular portion) Compact bone (thick cortical layer)
Epiphysis Expanded ends Cancellous (trabecular) bone with compact shell
Metaphysis Between diaphysis and epiphysis Growth plate (in children)
Periosteum Outer connective tissue membrane Fibrous layer + osteogenic layer
Endosteum Inner lining of medullary cavity Osteoprogenitor cells, osteoclasts
Medullary cavity Central cavity Bone marrow (red/yellow)

Microscopic Bone Structure

Compact (cortical) bone:

  • Dense, solid bone forming the outer wall
  • 80% of total bone mass
  • Organized into osteons (Haversian systems)

Osteon (Haversian system):

  • Central canal (Haversian canal): Contains blood vessels, nerves
  • Concentric lamellae: Rings of mineralized matrix
  • Lacunae: Spaces for osteocytes
  • Canaliculi: Small channels connecting lacunae
  • Interstitial lamellae: Remnants of old osteons
  • Circumferential lamellae: Outer/inner layers

Cancellous (trabecular) bone:

  • Spongy, lattice-like structure
  • 20% of total bone mass
  • No osteons; arranged as trabeculae
  • Trabeculae aligned along stress lines
  • Spaces contain bone marrow

Bone Cells

Cell Origin Function Location
Osteoprogenitor Mesenchymal stem cell Differentiate into osteoblasts Periosteum, endosteum
Osteoblast Osteoprogenitor Bone formation (osteoid deposition) Bone surface
Osteocyte Osteoblast (mature) Maintain bone matrix, mechanosensing Lacunae
Osteoclast Hematopoietic stem cell (monocyte lineage) Bone resorption (breakdown) Bone surface (Howship lacunae)

Bone Matrix

Component Percentage Function
Inorganic (mineral) 65% Hydroxyapatite [Ca10(PO4)6(OH)2] - hardness, compressive strength
Organic (osteoid) 35% Collagen type I (90%) - tensile strength, flexibility; Proteoglycans, glycoproteins

Bone Development and Growth

Ossification Types

Type Description Examples
Intramembranous Direct ossification from mesenchymal membrane Skull flat bones, clavicle, mandible
Endochondral Ossification via cartilage model Most bones (long bones, vertebrae, ribs)

Long Bone Growth

  • Epiphyseal (growth) plate: Cartilage plate between epiphysis and metaphysis
  • Zones of growth plate:
    1. Resting (reserve) zone
    2. Proliferative zone (chondrocyte division)
    3. Hypertrophic zone (cell enlargement)
    4. Calcification zone (matrix mineralization)
    5. Ossification zone (bone replaces cartilage)

Growth plate closure:

  • Girls: ~18 years
  • Boys: ~21 years

Bone Remodeling

Continuous process of bone resorption and formation throughout life:

  • Resorption: Osteoclasts break down bone (days)
  • Formation: Osteoblasts deposit new bone (months)
  • Balance: Net loss with aging (osteoporosis)
  • Turnover rate: ~10% per year (entire skeleton replaced every 10 years)

Axial vs. Appendicular Skeleton

Division Bones Percentage
Axial skeleton 80 ~39%
Appendicular skeleton 126 ~61%

Axial Skeleton (80 bones)

Region Bones Count
Skull Cranial, facial, ear ossicles, hyoid 29
Vertebral column Vertebrae, sacrum, coccyx 26
Thoracic cage Ribs (24), sternum (1) 25

Appendicular Skeleton (126 bones)

Region Bones Count (bilateral)
Upper limbs Clavicle, scapula, humerus, radius, ulna, carpals (8), metacarpals (5), phalanges (14) 32 × 2 = 64
Lower limbs Hip bone, femur, patella, tibia, fibula, tarsals (7), metatarsals (5), phalanges (14) 31 × 2 = 62

Joints (Articulations)

Classification by Structure

Type Description Examples
Fibrous Dense connective tissue Sutures (skull), syndesmosis (tibia/fibula), gomphosis (tooth)
Cartilaginous Cartilage Synchondrosis (growth plate), symphysis (pubic, intervertebral disc)
Synovial Joint cavity with synovial fluid Most joints (knee, shoulder, hip, elbow)

Synovial Joint Types

Type Movement Examples
Ball and socket Multi-axial Shoulder, hip
Hinge Uniaxial (flexion-extension) Elbow, knee, ankle
Pivot Uniaxial (rotation) Atlantoaxial (C1-C2), radioulnar
Condylar (ellipsoid) Biaxial Wrist (radiocarpal), metacarpophalangeal
Saddle Biaxial Carpometacarpal (thumb)
Gliding (plane) Sliding Intercarpal, intertarsal, facet joints
Spheroidal Multi-axial Hip, shoulder

Joint Movements

Movement Description
Flexion Decreasing angle between bones
Extension Increasing angle between bones
Abduction Movement away from midline
Adduction Movement toward midline
Rotation Movement around longitudinal axis
Circumduction Circular movement (combination)
Pronation Radius rotates over ulna (palm down)
Supination Radius and ulna parallel (palm up)
Inversion Sole of foot turns inward
Eversion Sole of foot turns outward
Dorsiflexion Toes up
Plantarflexion Toes down
Protraction Forward movement
Retraction Backward movement
Elevation Upward movement
Depression Downward movement

Mineral Homeostasis

Calcium Regulation

Hormone Effect on Bone Effect on Calcium
Parathyroid hormone (PTH) Resorption (osteoclast activation) ↑ Blood Ca²⁺
Calcitonin (thyroid C cells) Decreased resorption ↓ Blood Ca²⁺
Vitamin D (calcitriol) Mineralization, absorption ↑ Blood Ca²⁺

Bone as an Endocrine Organ

Bone produces:

  • Osteocalcin: Regulates glucose metabolism, insulin sensitivity
  • FGF-23: Regulates phosphate excretion (kidney)