• Cylindrical outer structure composed of a helical array of polymerized heterodimers of α- and β-tubulin.
• Each dimer has 2 GTP bound.
• Incorporated into flagella, cilia, mitotic spindles.
• Grows slowly, collapses quickly.
• Also involved in slow axoplasmic transport in neurons.
• Molecular motor proteins—transport cellular cargo toward opposite ends of microtubule tracks.
  • Dynein—retrograde to microtubule (+ → −).
  • Kinesin—anterograde to microtubule (− → +).
• Negative end Near Nucleus. Positive end Points to Periphery
• Drugs that act on microtubules (Microtubules Get Constructed Very Poorly):
  • Mebendazole (antihelminthic)
  • Griseofulvin (antifungal)
  • Colchicine (antigout)
  • Vincristine/Vinblastine (anticancer)
  • Paclitaxel (anticancer)
• Cilia structure
  • 9 doublet + 2 singlet arrangement of microtubules (A).
  • Basal body (base of cilium below cell membrane) consists of 9 microtubule triplets B with no central microtubules.
  • Axonemal dynein—ATPase that links peripheral 9 doublets and causes bending of cilium by differential sliding of doublets.
  • Gap junctions enable coordinated ciliary movement.
• Kartagener syndrome (1° ciliary dyskinesia)
  • immotile cilia due to a dynein arm defect.
  • Autosomal recessive.
  • Results in ↓ male and female fertility due to immotile sperm (compared to cystic fibrosis, which has an absent vas deferens) and dysfunctional fallopian tube cilia, respectively;
  • ↑ risk of ectopic pregnancy.
  • Can cause impaired mucociliary clearance, bronchiectasis, digital clubbing recurrent sinusitis, nasal polyps, chronic ear infections, conductive hearing loss, and situs inversus (eg, dextrocardia on CXR (C)).
  • (Kartagener’s restaurant: take-out only, there’s no dynein “dine-in”).
  • Diagnosis: Low nasal nitric oxide levels. Bronchoscopy and electron microscopic visualization of ciliary abnormalities. Genetic testing.