Anatomy chapters
Bone and cartilage
The three types of cartilage, the cells and structure of bone, how bone forms and grows, and the diseases that follow when these processes go wrong.
- Cartilage has no blood vessels; it is fed by diffusion from the perichondrium.
- Three cartilages: hyaline (most common), elastic (ear, epiglottis) and fibrocartilage (discs, symphysis).
- Bone cells: osteoblasts build, osteocytes maintain, osteoclasts break down.
On this page
Overview
Cartilage and bone are specialised connective tissues that support the body.
Cartilage is firm but flexible. It has no blood vessels or nerves. Bone is hard because its matrix is mineralised, and it is richly supplied with blood.
Cartilage
Cartilage cells are chondrocytes. They sit in small spaces (lacunae) in the matrix, often in groups of two to four (isogenous groups) formed by division of one cell.
Most cartilage is covered by perichondrium, a layer of dense connective tissue that carries blood vessels. Nutrients diffuse from it into the cartilage.
Cartilage has no blood vessels, so it heals poorly.
Types of cartilage
| Type | Fibres | Perichondrium | Sites |
|---|---|---|---|
| Hyaline | Type II collagen (not visible on light microscopy) | Present (except articular cartilage) | Articular surfaces, costal cartilages, nose, larynx (thyroid, cricoid), trachea and bronchi, growth plates, fetal skeleton |
| Elastic | Type II collagen + many elastic fibres | Present | External ear, external auditory canal, auditory tube, epiglottis |
| Fibrocartilage | Thick bundles of type I collagen | Absent | Intervertebral discs (annulus), pubic symphysis, menisci of the knee, articular discs |
Articular cartilage is hyaline but has no perichondrium. It is fed by synovial fluid.
The epiglottis is elastic cartilage, but the thyroid and cricoid cartilages are hyaline. Hyaline cartilage can calcify with age; elastic cartilage does not.
Growth of cartilage
- Appositional growth: new cartilage added on the surface by cells of the perichondrium.
- Interstitial growth: chondrocytes inside the cartilage divide and lay down matrix.
Bone
Bone matrix has an organic part (mainly type I collagen) that gives toughness, and an inorganic part (calcium phosphate as hydroxyapatite crystals) that gives hardness.
Cells of bone
| Cell | Origin | Job |
|---|---|---|
| Osteoprogenitor cells | Mesenchyme | Stem cells that become osteoblasts |
| Osteoblasts | Osteoprogenitor cells | Lay down new matrix (osteoid) and start mineralisation; rich in alkaline phosphatase |
| Osteocytes | Osteoblasts trapped in matrix | Maintain bone; linked by fine processes in canaliculi |
| Osteoclasts | Monocyte-macrophage line | Large, multinucleated cells that resorb bone in pits called Howship's lacunae |
Osteoclasts come from the blood-forming (monocyte-macrophage) line, not from osteoblasts. Their ruffled border releases acid and enzymes.
Compact and spongy bone
Compact bone forms the outer shell. Its unit is the osteon (Haversian system):
- A central Haversian canal with vessels and nerves.
- Rings of bone (lamellae) around it.
- Osteocytes in lacunae between lamellae, connected by canaliculi.
Volkmann's canals run across the bone and connect Haversian canals with each other and with the surface.
Spongy (cancellous) bone fills the ends of long bones and the inside of flat bones. It is a mesh of thin plates (trabeculae) with red marrow in between. It has no osteons.
Woven and lamellar bone
- Woven bone: collagen in random directions. Found in the fetus and in fracture repair. Weaker.
- Lamellar bone: collagen in orderly layers. The mature adult bone.
Periosteum and endosteum
The periosteum covers the outer surface (not the joint surfaces). Its inner layer has bone-forming cells. It is attached to bone by Sharpey's fibres. The endosteum lines the inner surfaces.
Ossification
There are two ways bone forms (see types of bones and joints):
- Intramembranous: directly in mesenchyme. Skull vault, most of the face, clavicle.
- Endochondral: a hyaline cartilage model is replaced by bone. Most other bones.
Zones of the growth plate
From the epiphysis towards the shaft:
| Zone | What happens |
|---|---|
| Resting (reserve) | Small, quiet chondrocytes |
| Proliferation | Chondrocytes divide and line up in columns |
| Hypertrophy | Cells swell |
| Calcification | Matrix calcifies and the cells die |
| Ossification | Vessels and osteoblasts invade and lay down bone |
Achondroplasia is a defect in the zone of proliferation (a mutation in the FGFR3 gene), so long bones stay short.
Clinical relevance
Rickets and osteomalacia
How it happens: lack of vitamin D or calcium, so osteoid is not mineralised.
What you see: in children (rickets), bowed legs, widened wrists and a "rachitic rosary" of swollen costochondral junctions. In adults (osteomalacia), bone pain and fractures.
Osteoporosis
How it happens: bone is lost faster than it is made, for example after menopause.
What you see: thin, normally mineralised bone that fractures easily: wrist, hip and vertebral bodies.
Fracture healing
How it happens: a blood clot forms, then a soft cartilage callus, then a hard woven-bone callus, which is finally remodelled into lamellar bone.
What you see: healing over weeks; poor blood supply or movement slows it.
Osteoarthritis
How it happens: articular cartilage wears away; it cannot regrow well because it has no blood supply.
What you see: painful, stiff joints with narrowed joint space.
Expect: the type of cartilage at a given site (epiglottis, ear, discs, trachea), which cartilages lack perichondrium, the origin of osteoclasts, the parts of an osteon, Volkmann's canals, and the zone of the growth plate affected in achondroplasia.
Quick revision
- Hyaline: commonest; joints, trachea, costal cartilage, growth plates.
- Elastic: ear and epiglottis. Fibrocartilage: discs, symphysis, menisci.
- No perichondrium: articular cartilage and fibrocartilage.
- Osteoblast builds, osteocyte maintains, osteoclast resorbs.
- Osteoclasts come from the monocyte-macrophage line.
- Osteon: Haversian canal, lamellae, osteocytes, canaliculi; Volkmann's canals join them.
- Achondroplasia: zone of proliferation.
Written by The House Job from standard histology references (Junqueira's Basic Histology).
FAQ
What type of cartilage is the epiglottis?
Elastic cartilage.
Which types of cartilage have no perichondrium?
Fibrocartilage and the hyaline cartilage covering joint surfaces.
Where do osteoclasts come from?
From the monocyte-macrophage cells of the bone marrow.
What is a Haversian system?
The structural unit of compact bone: a central canal with blood vessels surrounded by rings of bone and osteocytes.