Quick Jump to Sections

Overview Divisions Bone Types Structure Joints Ossification Functions Clinical NEET Points
Section A

Overview of Skeletal System

Basic Facts

  • Total bones (adult): 206 bones (some individuals have sesamoid variations — up to 213)
  • Bones at birth: ~300–350 bones (many fuse during development)
  • Body weight: Skeleton contributes ~15% of total body weight
  • Hardest substance in body: Enamel (not bone); bone is 2nd hardest
  • Longest bone: Femur (thigh bone) — ~48 cm in adult male
  • Smallest bone: Stapes (stirrup bone of middle ear) — ~3 mm
  • Largest sesamoid bone: Patella (kneecap)
  • Most fractured bone: Clavicle

Key Statistics

Adult Bones
206
Standard count
Birth Bones
~300
Fuse over time
Water Content
~22%
In bone matrix
Calcium Store
99%
Body's Ca²⁺ in bone
Total Joints
360+
All types combined
Longest Bone
Femur
~48 cm (adult male)
Section B

Divisions of the Skeleton

Axial Skeleton

80 Bones
  • Skull: 22 bones (8 cranial + 14 facial)
  • Vertebral column: 26 bones (7C + 12T + 5L + 1S + 1Cx)
  • Thoracic cage: 25 bones (12 pairs ribs + sternum)
  • Hyoid bone: 1 bone (U-shaped, only bone not articulating with another)
  • Ossicles of ear: 6 bones (3 per side — malleus, incus, stapes)
  • Forms the central axis of the body

Appendicular Skeleton

126 Bones
  • Upper limb (each): 32 bones → 64 total
  • Lower limb (each): 31 bones → 62 total (including patella)
  • Pectoral girdle: Clavicle + Scapula (2 per side = 4 total)
  • Pelvic girdle: 2 hip bones (os coxae) — each = ilium + ischium + pubis
  • Enables locomotion and upper limb manipulation
  • Attaches to axial skeleton via girdles

Detailed Bone Count

RegionBonesCount
CraniumFrontal, parietal×2, temporal×2, occipital, sphenoid, ethmoid8
FaceMandible, maxilla×2, zygomatic×2, nasal×2, palatine×2, lacrimal×2, vomer, inferior nasal concha×214
VertebraeCervical×7, Thoracic×12, Lumbar×5, Sacrum (fused 5), Coccyx (fused 4)26
ThoraxSternum (manubrium+body+xiphoid), Ribs×12 pairs25
Upper Limb (×2)Clavicle, scapula, humerus, radius, ulna, carpals×8, metacarpals×5, phalanges×1464
Lower Limb (×2)Hip bone, femur, patella, tibia, fibula, tarsals×7, metatarsals×5, phalanges×1462
Ear ossiclesMalleus, incus, stapes × 2 sides6
HyoidHyoid bone1
TOTAL206
Section C

Classification of Bones

By Shape

1

Long Bones

Longer than wide. Femur, tibia, fibula, humerus, radius, ulna, metacarpals, metatarsals, phalanges. Have epiphysis, diaphysis, metaphysis.

2

Short Bones

Cube-shaped. Carpals (wrist) and tarsals (ankle). Provide stability with limited movement.

3

Flat Bones

Thin, curved. Skull bones (frontal, parietal), sternum, scapula, ribs, hip bone. Protect organs; large surface for muscle attachment.

4

Irregular Bones

Complex shapes. Vertebrae, facial bones, calcaneus. Don't fit other categories.

5

Sesamoid Bones

Develop within tendons. Patella (largest), pisiform, fabella (variable). Reduce friction in tendons.

6

Pneumatic Bones

Air-filled sinuses. Frontal, maxilla, sphenoid, ethmoid bones. Reduce skull weight; resonate voice.

Section D

Bone Structure & Histology

Macroscopic Structure (Long Bone)

  • Diaphysis: Shaft of long bone; compact bone surrounds medullary cavity (yellow marrow in adults)
  • Epiphysis: Ends of long bone; mainly spongy (cancellous) bone covered by articular cartilage
  • Metaphysis: Flared region between epiphysis and diaphysis; contains growth plate (epiphyseal plate) in children
  • Epiphyseal plate (growth plate): Hyaline cartilage; site of longitudinal bone growth; fuses ~18–25 years
  • Periosteum: Fibrous outer covering; contains osteoblasts; site of appositional (width) growth; bone's blood supply and nerve supply
  • Endosteum: Inner lining of medullary cavity; contains osteoblasts and osteoclasts
  • Articular cartilage: Hyaline cartilage covering epiphysis — no periosteum here

Microscopic Structure (Haversian System)

  • Osteon (Haversian system): Functional unit of compact bone; cylindrical structure
  • Haversian canal: Central canal containing blood vessels, lymphatics and nerves
  • Lamellae: Concentric rings of calcified matrix around Haversian canal
  • Lacunae: Small spaces between lamellae containing osteocytes
  • Canaliculi: Tiny channels connecting lacunae — allow nutrient exchange between osteocytes
  • Volkmann's canals (perforating canals): Run perpendicular to Haversian canals; connect adjacent osteons and periosteum
  • Interstitial lamellae: Remnants of old osteons between active osteons

Bone Cells

CellOriginFunctionKey Feature
OsteoblastsMesenchymeBone formation — secrete osteoid (collagen + proteoglycans)Alkaline phosphatase marker; line periosteum/endosteum
OsteocytesOsteoblasts (trapped)Maintain bone matrix; mechanosensing; calcium regulationIn lacunae; long processes in canaliculi
OsteoclastsMonocyte/macrophage lineageBone resorption — dissolve mineral matrixMultinucleated; ruffled border; in Howship's lacunae; TRAP positive
Osteoprogenitor cellsMesenchymal stem cellsDifferentiate into osteoblastsMitotically active; in periosteum

Bone Matrix Composition: ~65% inorganic (mainly hydroxyapatite Ca₁₀(PO₄)₆(OH)₂) + ~35% organic (type I collagen = osteoid). Collagen gives flexibility; mineral gives hardness.

Section E

Joints (Arthrology)

Fibrous Joints

Synarthrosis
  • No movement (synarthrosis) or minimal
  • Joined by fibrous connective tissue
  • Sutures: Skull bones (coronal, sagittal, lambdoid sutures)
  • Syndesmosis: Bones joined by ligament — more movement (inferior tibiofibular joint)
  • Gomphosis: Peg-in-socket — teeth in alveolar sockets (periodontal ligament)
  • No synovial cavity

Cartilaginous Joints

Amphiarthrosis
  • Limited movement (amphiarthrosis)
  • Primary (synchondrosis): Hyaline cartilage — 1st sternocostal joint, epiphyseal plates. Temporary.
  • Secondary (symphysis): Fibrocartilage — pubic symphysis, intervertebral discs, manubriosternal joint. Permanent.
  • No synovial cavity
  • Pubic symphysis: fibrocartilaginous disc — widens during childbirth

Synovial Joints

Diarthrosis — Most Mobile
  • Freely movable (diarthrosis) — most joints of limbs
  • Features: joint cavity, synovial membrane, synovial fluid, articular cartilage, fibrous capsule, ligaments
  • Plane: Intercarpal, intertarsal
  • Hinge: Elbow, knee (modified), ankle, interphalangeal
  • Pivot: Atlantoaxial, proximal/distal radioulnar
  • Condyloid: Radiocarpal (wrist), MCP joints
  • Saddle: 1st carpometacarpal (thumb) — most mobile
  • Ball & socket: Shoulder (most mobile), hip (most stable)

Synovial Fluid

  • Produced by: Synovial membrane (type B synoviocytes / fibroblast-like)
  • Composition: Dialysate of blood plasma + hyaluronic acid + lubricin
  • Functions: Lubrication, nutrient supply to avascular articular cartilage, shock absorption
  • Normal volume: ~0.5–4 mL in knee joint
  • Normal appearance: Clear, straw-coloured, viscous (due to hyaluronic acid)
  • Aspiration (arthrocentesis): Used to diagnose gout (urate crystals), pseudogout, infection, haemarthrosis
Section F

Ossification (Bone Formation)

Types of Ossification

Intramembranous Ossification

  • Bone forms directly from mesenchyme
  • No cartilage template
  • Flat bones of skull (frontal, parietal, occipital squama)
  • Clavicle (partly), mandible, maxilla
  • Fontanelles are membrane-covered gaps between ossifying bones
  • Wormian (sutural) bones form here

Endochondral Ossification

  • Bone replaces a hyaline cartilage model
  • All long bones and short bones
  • Most bones of skull base
  • Primary ossification centre: diaphysis (forms in utero)
  • Secondary ossification centre: epiphysis (forms postnatally)
  • Growth at epiphyseal (growth) plate until ~25 years

NEET Alert: Clavicle is special — ossifies by BOTH intramembranous AND endochondral ossification (the only long bone to have intramembranous component).

Key Ossification Timings

Bone/CentreAppearsFusesNote
Clavicle (1° centre)5th–6th week IU—First bone to ossify
Femur (1° centre)7th week IU—Longest bone
Distal femur epiphysis36th week IU18–20 yrsSeen on X-ray of newborn — confirms term baby
Medial epicondyle (elbow)~5 years~17 yearsLast to fuse at elbow — important trauma landmark
Femoral head epiphysis~6 months~17 yearsAbsent at birth — helps diagnose CDH
Iliac crestPuberty~25 yearsUsed for bone age assessment
Section G

Functions of Skeletal System

Six Major Functions

Support

  • Structural framework for body
  • Supports soft tissues and organs
  • Maintains body posture (vertebral column)
  • Cradles and supports abdominal viscera (pelvis)

Protection

  • Skull — protects brain
  • Vertebral column — protects spinal cord
  • Thoracic cage — protects heart, lungs, great vessels
  • Pelvis — protects pelvic organs (bladder, reproductive organs)

Movement

  • Bones act as levers for muscle action
  • Joints are fulcrums
  • 3 classes of levers in the body
  • Sesamoid bones redirect tendon pull

Haematopoiesis

  • Red bone marrow — produces all blood cells
  • Active sites in adults: sternum, vertebrae, ribs, iliac crest, proximal femur/humerus
  • In children: all bones have red marrow
  • Yellow marrow (fat) can revert to red in anaemia

Mineral Storage

  • 99% of body's calcium stored in bone
  • 85% of body's phosphorus in bone
  • Also stores Mg²⁺, Na⁺, K⁺
  • Regulated by PTH (resorption) and calcitonin (deposition)

Endocrine Function

  • Osteocalcin (from osteoblasts) — regulates insulin sensitivity, male fertility, energy expenditure
  • FGF-23 (from osteocytes) — regulates phosphate reabsorption in kidney
  • Bone as an endocrine organ is a recent discovery
Section H

Clinical Anatomy

Important Clinical Conditions

  • Fractures:
    • Colles' fracture: distal radius fracture (fall on outstretched hand) — "dinner fork" deformity; commonly post-menopausal women
    • Pott's fracture: bimalleolar ankle fracture — eversion injury
    • Monteggia fracture: ulna fracture + radial head dislocation
    • Galeazzi fracture: radius fracture + distal radioulnar joint dislocation
    • Boxer's fracture: 5th metacarpal neck fracture
    • March fracture: stress fracture of 2nd metatarsal
  • Osteoporosis: Reduced bone mass (BMD T-score ≤ −2.5). Post-menopausal (oestrogen ↓ → osteoclast activity ↑). Vertebral crush fractures, hip fractures. DEXA scan for diagnosis.
  • Osteomalacia / Rickets: Defective mineralisation due to Vit D/Ca deficiency. Rickets in children (growth plate abnormality — bow legs, rachitic rosary). Osteomalacia in adults (bone pain, proximal myopathy, Looser zones on X-ray).
  • Paget's Disease: Excessive, disorganised bone remodelling. Elevated ALP. "Tam o' shanter" skull, bowing of tibia ("sabre tibia"), deafness (VIII nerve compression). Risk of osteosarcoma.
  • Osteosarcoma: Most common primary malignant bone tumour in children/young adults. Distal femur/proximal tibia. Codman's triangle + sunburst pattern on X-ray.
  • Ewing's Sarcoma: 2nd most common primary bone tumour; diaphysis of long bones; "onion skin" periosteal reaction on X-ray. EWSR1-FLI1 translocation t(11;22).
  • Congenital Dislocation of Hip (CDH): Developmental dysplasia of hip (DDH). Absent femoral head ossification centre at birth. Barlow + Ortolani tests. Trendelenburg gait if missed.
  • Gout: Urate crystal deposition in joints (especially 1st MTP — podagra). Birefringent needle-shaped crystals under polarised light. Tophi in chronic cases.

Clinical Procedures

  • Bone marrow aspiration & biopsy: Posterior superior iliac spine (PSIS) most common site; also sternum for aspiration only
  • Intraosseous (IO) access: Proximal tibia (children), distal tibia/humerus (adults) — emergency vascular access
  • Bone age assessment: X-ray of left hand and wrist — compare to Greulich & Pyle atlas
  • DEXA scan: Dual-energy X-ray absorptiometry — measures bone mineral density (BMD) at hip and lumbar spine
  • Joint aspiration (arthrocentesis): Knee most commonly — diagnose gout, pseudogout, septic arthritis, haemarthrosis
Section I

NEET High-Yield Points

Must-Know for Exams

  • 206 bones in adult; ~300 at birth; smallest = stapes; longest = femur; largest sesamoid = patella
  • Axial skeleton = 80 bones; Appendicular skeleton = 126 bones
  • First bone to ossify: Clavicle (5th–6th week IU); also has intramembranous + endochondral ossification
  • Distal femur epiphysis appears at 36 weeks IU — confirms full-term baby at birth
  • Hyoid bone = only bone in body that doesn't articulate with any other bone
  • Most common fracture: Clavicle; Most mobile joint: shoulder; Most stable joint: hip
  • Haversian canal runs longitudinally; Volkmann's canal runs perpendicularly (connects Haversian systems)
  • Osteoclasts are multinucleated, derived from monocytes, TRAP-positive, work in Howship's lacunae
  • Colles' fracture = distal radius; dinner fork deformity; fall on outstretched hand (FOOSH)
  • Medial epicondyle = last to fuse at elbow (~17 yrs); always remember "CRITOE" for elbow ossification order
  • Bone matrix: 65% inorganic (hydroxyapatite) + 35% organic (Type I collagen)
  • Wormian bones: Intramembranous islands in sutures — seen in osteogenesis imperfecta, hypothyroidism, Down syndrome
  • Osteocalcin from osteoblasts = bone's endocrine function (regulates insulin, testosterone)
  • Saddle joint (1st CMC) has greatest range of movement among non-ball-and-socket joints

Memory Tricks

Elbow Ossification — CRITOE

  • Capitulum — 1 year
  • Radial head — 3 years
  • Internal (medial) epicondyle — 5 years
  • Trochlea — 7 years
  • Olecranon — 9 years
  • External (lateral) epicondyle — 11 years

Wrist Carpals — "Some Lovers Try Positions That They Can't Handle"

Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate (proximal → distal, lateral → medial).

Fracture-Dislocation Mnemonics

Monteggia = "M" for Medial side ulna + radial head (MUGR). Galeazzi = "Go" = radius + distal radioulnar joint dislocation. Remember: M before G alphabetically = Monteggia = Ulna (U before G).