CorpusHuman Anatomy
Cells & TissuesOrganelle

Cilia

Cilium (pl. cilia)

Also known as: cilium, motile cilia, primary cilium, axoneme, kinocilium, mucociliary clearance

Hair-like, microtubule-based projections of the cell surface: motile cilia (9+2 axoneme) beat in coordinated waves to move fluid, whereas single non-motile primary cilia (9+0) act as sensory antennae.

Smell and Taste — Chemoreceptors The Eukaryotic Cell

Overview

Axoneme9 + 2 (motile); 9 + 0 (primary)
Length × width≈5–10 µm × 0.25 µm
Beat frequency (airway)≈10–20 Hz
Cilia per airway cell≈200–300

Each motile cilium is ≈0.2–0.25 µm wide and ≈5–10 µm long, covered by plasma membrane, and contains an axoneme: nine peripheral microtubule doublets surrounding two central single microtubules (9 + 2). Each doublet bears inner and outer dynein arms; radial spokes link the doublets to the central pair and nexin links join adjacent doublets. Axonemal dynein, powered by ATP, tries to slide neighbouring doublets past each other; because they are cross-linked, sliding is converted into bending. The axoneme grows from a basal body (a modified centriole, 9 × 3 triplets) anchored by striated rootlets.

Motile cilia beat with a fast, stiff effective (power) stroke and a slower, flexible recovery stroke, at ≈10–20 Hz, coordinated into metachronal waves. A ciliated airway cell carries ≈200–300 cilia. In the airways they propel the mucus blanket toward the pharynx (mucociliary escalator); in the uterine tube they help move the oocyte; ependymal cilia move CSF.

Almost every cell has a single non-motile primary cilium (9 + 0, no dynein arms) that concentrates receptors and channels and acts as a signalling antenna (Hedgehog signalling, flow sensing in renal tubules via polycystins). Nodal cilia in the embryo are an exception — 9 + 0 yet motile — and their leftward flow establishes left–right asymmetry. Modified cilia form the outer segments of rods and cones, the olfactory cilia and the kinocilium of inner-ear hair cells.

Location

Motile: respiratory epithelium (nasal cavity, paranasal sinuses, trachea, bronchi), uterine tubes, efferent ductules of testis, ependyma. Primary cilia: nearly all cells.

Parts & subdivisions

  1. Axoneme
    9 + 2 microtubules (motile) or 9 + 0 (primary).
  2. Dynein arms
    ATPase motors on the A-tubule of each doublet; generate sliding.
  3. Basal body
    9 × 3 triplet centriole anchoring the cilium.

Anatomy

Histology

Visible in H&E as a fine fringe on the apical surface of ciliated cells, anchored by a dark line of basal bodies. Distinguish from microvilli (much shorter, actin core, form a brush/striated border) and stereocilia (long, non-motile microvilli of the epididymis and inner-ear hair cells).

Function

  • Move mucus, fluid or cells along epithelial surfaces.
  • Sense chemical and mechanical signals (primary cilia).
  • Establish left–right body asymmetry (nodal cilia).

Clinical significance

Primary ciliary dyskinesia / Kartagener syndrome

Autosomal recessive defects in axonemal proteins, most often the outer dynein arms (DNAH5, DNAI1): neonatal respiratory distress, chronic wet cough, rhinosinusitis, otitis media, bronchiectasis, male infertility (immotile sperm), reduced female fertility (ectopic pregnancy risk). About half have situs inversus (Kartagener triad: situs inversus, sinusitis, bronchiectasis). Low nasal nitric oxide is a screening test.

Primary-cilium ciliopathies

Autosomal dominant polycystic kidney disease (PKD1/PKD2 encode polycystin-1/2 in primary cilia), Bardet–Biedl syndrome (obesity, retinal dystrophy, polydactyly, renal disease), nephronophthisis, Joubert syndrome.

Smoking

Cigarette smoke paralyses and destroys cilia and induces squamous metaplasia, impairing mucociliary clearance.

Test yourself

A man with recurrent sinusitis, bronchiectasis and infertility has dextrocardia. Electron microscopy of his respiratory cilia would most likely show:
  1. Absent dynein arms on the outer doublets
  2. Absent central pair (9+0 arrangement)
  3. Missing actin cores
  4. Abnormal keratin filaments

Answer: Absent dynein arms on the outer doublets

Kartagener syndrome is a form of primary ciliary dyskinesia, most commonly due to defective outer dynein arms. Immotile cilia impair mucociliary clearance and sperm motility, and failure of nodal cilia leads to situs inversus in ≈50 % of patients.

The trachea is lined by:
  1. Simple columnar epithelium with microvilli
  2. Stratified squamous non-keratinised epithelium
  3. Pseudostratified ciliated columnar epithelium with goblet cells
  4. Simple cuboidal epithelium

Answer: Pseudostratified ciliated columnar epithelium with goblet cells

"Respiratory epithelium": all cells touch the basement membrane but not all reach the lumen, so nuclei lie at different levels. Cilia and goblet-cell mucus form the mucociliary escalator.

More cells & tissues questions

Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.