Focus: Human Anatomy

Human Anatomy · Bone Tissue

Focus: Human Anatomy

Bone Tissue & Cartilage

Bone is a living, dynamic connective tissue: an organic framework hardened by mineral, constantly built, repaired, and resorbed by its cells. We move from cartilage, to the architecture of a whole bone, to the tissue itself, then to how bone forms, grows, heals, and fails. Dr. Sharilyn Rennie

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Part 1

Cartilage, the Supporting Framework

Cartilage is the soft supporting tissue and the template most bones are built on.

Part 1 · Cartilage

Cartilage: cells, matrix, and growth

Connective
  • Cells. Chondroblasts secrete matrix, then become chondrocytes trapped in lacunae.
  • Matrix. A firm gel of ground substance (chondroitin sulfate, water) with collagen and sometimes elastic fibers. It is avascular with no nerves, so it heals poorly.
  • Perichondrium. A connective tissue sheath carrying the blood supply and stem cells (absent on fibrocartilage and articular cartilage).
  • Growth: appositional (added to the surface from the perichondrium) and interstitial (chondrocytes divide from within).
H&E histology of hyaline cartilage with its perichondrium: a pink fibrous perichondrium sheath over the purple cartilage matrix, with chondrocytes sitting in lacunae (inset). Labeled perichondrium, hyaline cartilage, lacuna, and chondrocyte.

Part 1 · The three cartilages

Hyaline, elastic, and fibrocartilage

Connective
  • Hyaline (most common): fine collagen masked by glassy matrix. Articular surfaces, costal cartilage, trachea and bronchi, nose, epiphyseal plates, fetal skeleton.
  • Elastic: elastic fibers added, so it springs back. External ear, epiglottis.
  • Fibrocartilage: thick parallel collagen with rows of chondrocytes; resists compression and tension. Intervertebral discs, menisci, pubic symphysis.
Three histology panels comparing the cartilages: hyaline cartilage, elastic cartilage, and fibrocartilage.

Part 2

Bone as an Organ

A long bone holds compact and spongy bone, cartilage, marrow, vessels, and nerves.

Part 2 · Why bone

Functions and the shape classes

Skeletal
  • Support, protection, movement (levers for muscles).
  • Mineral reservoir (about 99% of body calcium, plus phosphate).
  • Blood cell formation in red marrow; fat storage in yellow marrow.
  • Shape classes: long, short, flat, irregular, sesamoid.

Part 2 · Long bone

Gross anatomy of a long bone

Skeletal
  • Diaphysis: shaft of compact bone around the medullary cavity.
  • Epiphyses: spongy-bone ends under a compact shell, capped by articular cartilage.
  • Metaphysis: holds the epiphyseal plate (growing) or line (done).
  • Periosteum: outer fibrous + inner osteogenic layers, locked on by perforating (Sharpey's) fibers; anchors tendons and ligaments.
  • Endosteum lines the cavity and canals; nutrient foramen admits vessels.
Articular cartilageProximal epiphysis (spongy bone)Epiphyseal lineCompact boneMedullary cavity (yellow marrow)Nutrient foramenDiaphysis (shaft)PeriosteumSharpey's fibersEndosteumDistal epiphysis

Part 3

Bone Tissue, Up Close

The matrix gives two strengths, four cells keep it alive, and the lamellae show how it is built and remodeled.

Part 3 · The matrix

Two materials, two strengths

Histology
Collagen (osteoid)organic · flexibility + tensile strengthHydroxyapatiteCa²⁺ phosphate · hardness + compressiontoo little mineralbendstoo little collagenbrittle

Part 3 · The cells

The four bone cells

Histology
BONE CELLSBuild boneBreak bone downOsteogenic cellstem cell · the one that dividesdividesOsteoblastbuilds bone (secretes osteoid)walled inOsteocytemature, in lacuna · senses stressOsteoclastgiant · resorbs bone (ruffled border)in Howship’s lacunaeBONE MATRIXbuild + resorb = remodelingbuildsresorbs

Part 3 · Compact bone

Compact bone: the osteon

Histology
  • Compact bone is built from osteons (Haversian systems).
  • Central (Haversian) canal carries a vessel and nerve; concentric lamellae ring it.
  • Osteocytes in lacunae connect by canaliculi.
  • Perforating (Volkmann's) canals run crosswise, linking canals and the periosteum.
Ground compact-bone micrograph showing osteons. Labeled Haversian (central) canal, concentric lamellae ringing it, interstitial lamellae between osteons, and osteocyte lacunae with canaliculi.

Part 3 · Lamellae

The three kinds of lamellae

Histology
  • Concentric lamellae: the rings of an osteon around its central canal.
  • Interstitial lamellae: incomplete fragments between osteons, the leftovers of old osteons partly resorbed during remodeling.
  • Circumferential lamellae: broad sheets wrapping the whole bone just deep to the periosteum (outer) and around the marrow cavity (inner); they resist twisting.
Photo of a compact-bone model: osteons built of concentric lamellae, interstitial lamellae between them, and broad outer and inner circumferential lamellae highlighted in blue.

Part 3 · Spongy bone

Spongy (cancellous) bone

Histology
  • An open lattice of trabeculae, with no osteons.
  • Trabeculae line up along the bone's lines of stress.
  • Each still has lamellae, lacunae, osteocytes, and canaliculi, but is fed by diffusion from marrow.
  • Spaces hold red marrow; between two compact plates of a flat bone it is called diploe.
Spongy (cancellous) bone: a close-up of the open lattice of bony trabeculae and the spongy interior of a long-bone epiphysis. Labeled spongy bone, trabeculae, and epiphysis.

Part 4

Formation, Growth & Repair

Bone forms two ways, lengthens through five plate zones, widens at the surface, and heals a fracture in four stages.

Part 4 · Intramembranous

Intramembranous ossification

Skeletal

Bone forms directly within a membrane of mesenchyme. It builds the flat bones of the skull and the clavicle.

1Ossification centermesenchyme → osteoblasts2Osteoid calcifiescells → osteocytes3Trabeculae + periosteumwoven, spongy bone4Remodelscompact shell, spongy core

Part 4 · Endochondral

Endochondral ossification, stage by stage

Skeletal

Bone replaces a hyaline cartilage model. This builds most bones, including all long bones.

  • 1. A cartilage model forms; the perichondrium becomes periosteum and lays a bony collar.
  • 2. Cartilage in the center calcifies and its chondrocytes die.
  • 3. Vessels invade the primary ossification center; osteoblasts build bone in the diaphysis.
  • 4. Osteoclasts hollow the medullary cavity; ossification spreads toward the ends.
  • 5. Secondary ossification centers form in the epiphyses after birth.
  • 6. Cartilage remains only as articular cartilage and the epiphyseal plate.
1Cartilage model2Bony collar forms3Primary ossificationcenter4Medullary cavityhollows out5Secondary centersin the epiphyses6Cartilage remains asplate + joint caps

Part 4 · Compare & contrast

Two kinds of ossification, compared

Skeletal

Use this to answer comparison questions, like "which bones form each way?"

OSSIFICATIONIntramembranousEndochondralStarts fromA sheet of mesenchyme(connective-tissue membrane)Starts fromA hyaline cartilage modelWhich bonesFlat skull bones, most ofthe mandible, the clavicleWhich bonesMost bones: all long bones,skull base, vertebrae, pelvisHow bone appearsOsteoblasts build bonedirectly within the membraneHow bone appearsBone gradually replacesthe cartilage modelEpiphyseal plate?NoneEpiphyseal plate?Yes, so the bone lengthensat a growth plateWhen / paceEarlier and faster;shapes the fetal skullWhen / paceSlower; builds the restthrough childhood
Quick test: a bone with an epiphyseal plate on X-ray formed by endochondral ossification. Flat skull bones and the clavicle did not.

Part 4 · Growth in length

The epiphyseal plate and its five zones

Skeletal

A bone lengthens at the plate: cartilage is added on the epiphyseal side and replaced by bone on the diaphyseal side.

  • 1. Resting (reserve): anchors the plate to the epiphysis.
  • 2. Proliferation: chondrocytes divide and stack like coins, lengthening the bone.
  • 3. Hypertrophy: chondrocytes enlarge.
  • 4. Calcification: matrix calcifies, chondrocytes die.
  • 5. Ossification: bone replaces calcified cartilage on the diaphysis side.
1 Resting cartilage2 Proliferation3 Hypertrophy4 Calcification5 Ossificationepiphysisdiaphysis

Part 4 · Width & remodeling

Growth in width, remodeling, and closure

Skeletal
  • Appositional growth: periosteal osteoblasts add bone to the surface while endosteal osteoclasts widen the medullary cavity.
  • Remodeling: lifelong resorption and rebuilding, tuned by stress (Wolff's law) and calcium-regulating hormones.
  • Closure: in late adolescence the plate ossifies into the epiphyseal line, and lengthwise growth stops.

Part 4 · Bone repair

How a fracture heals: four stages

Clinical

Bone repair is its own ordered process (extends beyond the notes).

  • 1. Fracture hematoma. Torn vessels bleed; a clot forms and nearby bone cells die.
  • 2. Fibrocartilaginous (soft) callus. Fibroblasts and chondroblasts bridge the gap with collagen and cartilage.
  • 3. Bony (hard) callus. Osteoblasts replace the soft callus with spongy bone over weeks.
  • 4. Bone remodeling. Osteoclasts and osteoblasts reshape the callus into compact bone, restoring the bone's form.
1Fracture hematomaTorn vessels bleed;a clot fills the gap2Soft callusFibrocartilage bridgesthe broken ends3Hard callusOsteoblasts lay downspongy bone4RemodelingCompact bone is restored,near-normal shape

Part 4 · Fracture types

Naming fractures

Clinical
  • Closed (simple) vs open (compound): skin intact vs broken.
  • Complete vs incomplete: through the whole bone vs partial (greenstick, common in children).
  • Transverse (straight across), oblique (angled), spiral (twisting force).
  • Comminuted (three or more fragments), compression (crushed, as in vertebrae), impacted (ends driven together), depressed (pushed inward, as in the skull).
  • Displaced vs nondisplaced: ends out of line vs aligned.
Transversestraight acrossObliqueangledSpiraltwisting forceComminuted3+ fragmentsGreenstickpartial (children)Impactedends driven togetherCompressioncrushed (vertebrae)Open (compound)bone breaks skin

Part 5

Check Your Understanding

From recall to reasoning, then the key takeaways.

Part 5 · Recall (DOK 1)

Name the zone

Practice

Depth of Knowledge 1 · Recall

Which zone of the epiphyseal plate lengthens the bone as chondrocytes divide and stack?

Part 5 · Skill / concept (DOK 2)

Order the repair

Practice

Depth of Knowledge 2 · Skill / concept

Put fracture healing in order.

Part 5 · Strategic reasoning (DOK 3)

Reason it through

Practice

Depth of Knowledge 3 · Strategic reasoning

A child with bowed legs has soft, poorly mineralized bone but normal collagen. Which deficiency best explains this?

Wrap-up

Key takeaways

Summary
  • Cartilage is hyaline, elastic, or fibrocartilage; it grows by apposition and interstitially.
  • A long bone = diaphysis + epiphyses, wrapped by periosteum (anchored by Sharpey's fibers) and lined by endosteum.
  • Matrix = osteoid (flexibility) + hydroxyapatite (hardness); four cells build, maintain, and carve it.
  • Compact bone = osteons with concentric, interstitial, and circumferential lamellae; spongy bone = trabeculae.
  • Bone forms by intramembranous or endochondral ossification, lengthens through the five plate zones, and widens by apposition.
  • A fracture heals: hematoma, soft callus, hard callus, remodeling.
  • Bone fails through osteopenia/osteoporosis, rickets/osteomalacia (vitamin D), osteomyelitis (infection), and osteosarcoma (cancer).

Part 6

When Bone Fails

Common metabolic, infectious, and neoplastic bone disorders (beyond the Week 2 notes).

Part 6 · Metabolic

Osteopenia, osteoporosis, and soft bone

Clinical
  • Osteopenia: below-normal bone mass, a warning stage before osteoporosis.
  • Osteoporosis: resorption outpaces deposition, so bone becomes porous and fragile; fractures of the hip, vertebrae, and wrist rise. Risk climbs with age and after menopause (falling estrogen).
  • Rickets (children) / osteomalacia (adults): vitamin D deficiency (or low calcium/phosphate) leaves matrix poorly mineralized, so bones soften and bend (bowed legs in children).

Part 6 · Infection & tumor

Osteomyelitis and osteosarcoma

Clinical
  • Osteomyelitis: infection of bone and marrow, usually bacterial (often Staphylococcus aureus). Bacteria reach bone three ways: hematogenous spread (through the bloodstream, the most common route in children, lodging in the metaphysis), contiguous spread (from an adjacent infection: skin or soft tissue, sinus, a tooth, or a prosthetic joint), and direct inoculation (trauma, an open fracture, a puncture wound, or surgery).
  • Osteosarcoma: the most common primary malignant bone tumor; often strikes adolescents and tends to arise near the knee (distal femur or proximal tibia).
Osteomyelitisbone infection: pus, swelling,dead bone (sequestrum)Osteosarcomabone-forming tumor: sunburstspicules, near the knee

Dr. Sharilyn Rennie · Focus: Human Anatomy · The Skeletal System: Bone Tissue & Cartilage. Need a PDF? Use Print / Save PDF above, or download the accessible PDF from Canvas.

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