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Markings on a Bone

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1. Projections (Prominences)

Processes (Large Named Projections)

Knob-like and Rounded Projections

Bumps and Ridges for Muscle/Tendon Attachment

Blade-like and Pointed Structures

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2. Depressions (Concavities)

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3. Openings (Foramina and Passageways)

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Quick Reference Summary Table

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Why Bone Markings Matter

• Muscle/tendon attachment: Tuberosities, tubercles, crests, and lines are roughened to increase the surface area gripping tendons and ligaments (e.g., the tibial tuberosity anchors the patellar tendon - attachment of the quadriceps femoris).
• Articulation: Condyles, heads, facets, and trochleae form the smooth surfaces of joints, covered by hyaline cartilage in life.
• Neurovascular passageways: Foramina and canals guide nerves and blood vessels to their targets without injury from movement (e.g., the nutrient foramen admits the nutrient artery that supplies compact bone).
• Palpable landmarks: Many markings (iliac crest, medial epicondyle, mastoid process, tibial tuberosity, acromion) are used clinically to locate underlying structures, plan injections, and assess injuries.
• Forensic/anthropological identification: Patterns of markings reflect musculature, activity level, sex, and age in skeletal remains.

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Bones and Cartilages: A Complete Overview

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1. The Skeleton

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2. Functions of Bones

1. Support - provides a rigid framework for the body; supports soft tissues
2. Protection - shields vital organs (brain in cranium, heart/lungs in thorax, spinal cord in vertebral column)
3. Movement - bones act as levers, multiplying forces generated by skeletal muscle contractions and converting them into bodily movement
4. Hematopoiesis - bone marrow (in medullary cavities and cancellous bone) is the site of blood cell production
5. Mineral reservoir - stores 99% of the body's calcium and large amounts of phosphate in hydroxyapatite crystals; releases or absorbs these ions to maintain homeostasis
6. Energy storage - yellow marrow (fat) in the medullary cavity stores lipid energy reserves

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3. Bone Tissue and Its Types

Bone as a Connective Tissue

• Calcified extracellular matrix (organic + inorganic)
• Three major cell types: osteoblasts, osteocytes, osteoclasts

The Three Bone Cell Types

• Originate from mesenchymal stem cells
• Active cuboidal cells arranged in a single layer on bone surfaces
• Synthesize and secrete osteoid: the organic matrix (type I collagen, proteoglycans, osteonectin, osteocalcin)
• Direct mineralization of osteoid by hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]
• After completing secretion, become osteocytes, bone lining cells, or undergo apoptosis

• Most abundant bone cell
• Former osteoblasts trapped in calcified matrix inside lacunae
• Extend long cytoplasmic processes through narrow canaliculi connecting lacunae to each other and to blood vessels
• Form a "lacunar-canalicular network" - the bone's mechanostat
• Maintain calcified matrix; their death triggers matrix resorption
• Act as mechanosensors: detect mechanical load and signal osteoblasts/osteoclasts accordingly

• Large, multinucleated (up to 50 nuclei), motile cells
• Derived from fusion of bone marrow monocytes
• Form a ruffled border on bone surface, creating a sealed resorption lacuna (Howship's lacuna)
• Secrete H⁺ ions (carbonic anhydrase II) and lysosomal enzymes (cathepsin K, collagenase) to dissolve bone matrix
• Development requires M-CSF and RANKL from osteoblasts

Bone Matrix

Periosteum and Endosteum

• Periosteum: two-layered connective tissue sheath covering all bone surfaces except articular cartilage
  • Outer (fibrous) layer: dense collagen, fibroblasts; Sharpey's fibers anchor it to bone
  • Inner (cellular/osteogenic) layer: osteoprogenitor cells, osteoblasts, blood vessels
• Endosteum: thin connective tissue layer lining medullary cavity and trabecular surfaces; contains osteoprogenitor cells and osteoblasts

Types of Bone Tissue (Junqueira's, p. 364-365)

By Histological Organization

By Gross Appearance

• Dense outer shell of bones
• Structural unit = osteon (Haversian system): cylindrical unit ~1 cm long, 250-350 μm diameter
• Each osteon = 5-20 concentric lamellae around a central Haversian canal (contains blood vessels, nerves)
• Volkmann's canals: transverse/oblique channels connecting Haversian canals to each other and to periosteal vessels
• Outer and inner surfaces have circumferential lamellae; spaces between osteons filled by interstitial lamellae

• Lattice of thin, interconnecting bony struts called trabeculae
• Spaces between trabeculae contain bone marrow
• No osteons; trabeculae nourished by diffusion from adjacent marrow
• Located in epiphyses of long bones, short/flat/irregular bones

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4. Classification of Bones

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5. Markings on a Bone

Projections

Depressions

Openings

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6. Gross Anatomy of an Adult Typical Long Bone

         Proximal epiphysis
              |
         Articular cartilage (hyaline)
         Epiphyseal line (fusion scar)
              |
         Metaphysis (flared region)
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┌────────────────────────────────┐
│          DIAPHYSIS             │
│  ┌──────────────────────────┐  │
│  │   Periosteum (outer)     │  │
│  │   Compact bone (cortex)  │  │
│  │   Medullary cavity       │  │
│  │   (yellow marrow)        │  │
│  │   Endosteum (inner)      │  │
│  └──────────────────────────┘  │
└────────────────────────────────┘
              |
         Metaphysis
         Epiphyseal line
              |
         Distal epiphysis
         Articular cartilage

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7. Ossification (Osteogenesis)

A. Intramembranous Ossification

• Bone forms directly from condensed mesenchyme (no cartilage template)
• Forms: most skull and jaw bones, clavicle, scapula
• Sequence:
  1. Osteoprogenitor cells in condensed mesenchyme proliferate around capillaries
  2. Differentiate into osteoblasts → secrete osteoid → calcifies as woven bone with osteocytes in lacunae
  3. Adjacent ossification centers fuse
  4. Woven bone remodels to compact bone; central region remains cancellous with marrow

B. Endochondral Ossification

• Bone replaces a preformed hyaline cartilage model of the bone
• Forms: most bones of the body (all long bones, short bones, base of skull)
• Sequence (Junqueira's, p. 377):

Epiphyseal (Growth) Plate Zones (Proximal → Distal)

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8. Diaphysis and Epiphysis

Diaphysis

• The cylindrical shaft of a long bone
• Wall = thick compact bone (cortex)
• Central medullary cavity with yellow (adipose) marrow in adults
• Contains the nutrient foramen for the nutrient artery
• Primary ossification center forms here first (in utero)
• Functions: mechanical strength (resists bending/torsion), lever arm for muscles

Epiphysis

• The expanded ends of a long bone
• Shell of compact bone enclosing a cancellous network
• In adults: filled with red bone marrow (hematopoietic)
• Covered by articular cartilage at joint surfaces
• Contains secondary ossification centers (form around birth)
• Apophysis: a secondary ossification center that is not articular (e.g., greater trochanter); serves as tendon/ligament attachment

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9. Bone Growth and Remodeling

Bone Growth

• Occurs exclusively at the epiphyseal plate (growth plate)
• Chondrocytes proliferate on the epiphyseal side; ossification occurs on the diaphyseal side
• The plate is maintained as long as new cartilage is added at the same rate it is replaced by bone
• Stops when the plate is completely replaced by bone = epiphyseal closure (~18-20 years)

• New bone deposited beneath periosteum (periosteal osteoblasts)
• Simultaneous resorption on endosteal surface enlarges medullary cavity
• Net effect: bone widens without greatly increasing weight

Bone Remodeling (Junqueira's, p. 387)

• Rate in children can be 200× faster than adults
• Osteoclasts create cutting cones, tunneling through old bone
• Osteoblasts follow and fill the tunnel with new osteons
• Purpose:
  • Adapts bone to changing mechanical stresses
  • Repairs microdamage
  • Maintains calcium homeostasis
  • Replaces old, fatigued bone with new

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10. Blood Supply of Long Bones

• Arterial flow in mature bone = centrifugal (inside → outside): high-pressure nutrient system dominates
• Venous flow = centripetal (outside → inside)
• When nutrient artery is disrupted (fracture), periosteal system dominates → flow becomes centripetal
• Bone receives 5-10% of cardiac output

• Scaphoid (wrist), talus, femoral head, odontoid process

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11. Bone Injury and Repair

Types of Fractures (Pye's Surgical Handicraft)

Bone Fracture Healing (Junqueira's, p. 387-389)

• Good blood supply at fracture site
• Periosteum intact
• Adequate mechanical stability (not overly rigid; some micromotion stimulates callus)
• Young age; cancellous > cortical bone heals faster
• Adequate nutrition (Ca²⁺, vitamin C for collagen, vitamin D)

• Infection, poor vascularity, excessive mobility, large bone gaps, systemic disease (osteoporosis, diabetes)
• Intramedullary reaming devascularizes inner 50-80% of cortex

• Non-union: fracture fails to heal (fibrous tissue bridge persists = pseudarthrosis)
• Malunion: fracture heals in incorrect position

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Summary Table: Key Concepts

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