Structure of Cilia in the Respiratory System and Clinical Significance

(MD Exam - 10 Marks)

1. Introduction

• Length: 5-10 μm
• Diameter: ~0.2-0.25 μm
• Beat frequency: 10-20 Hz (coordinated metachronal rhythm)

2. Classification of Cilia

3. Ultrastructure of the Motile Cilium (Axoneme)

Figure: Complete molecular structure of a motile cilium (Histology: A Text and Atlas)

A. The 9+2 Axonemal Configuration

• 2 central singlet microtubules - enclosed by a central sheath with projections at 14-nm intervals
• 9 peripheral (outer) microtubule doublets - arranged in a ring around the central pair
• Total = 11 microtubules (the "9+2 pattern")

• Tubule A: complete microtubule with 13 tubulin protofilaments (post-translationally modified by acetylation and polyglutamylation for stability)
• Tubule B: incomplete, with only 10 tubulin protofilaments, sharing 3 protofilaments with tubule A

Figure: Ciliary axoneme cross-section with labeled structural proteins (Junqueira's Basic Histology)

B. Accessory Proteins of the Axoneme

C. Mechanism of Ciliary Movement (Power Stroke)

1. Ciliary dynein (a large ATPase) projects inner and outer dynein arms from tubule A of each doublet.
2. Dynein arms form temporary cross-bridges with the B tubule of the adjacent doublet.
3. ATP hydrolysis powers the sliding of adjacent doublets relative to each other.
4. Nexin cross-links convert sliding motion into bending of the entire cilium.
5. The result is a coordinated fast forward power stroke followed by a slower backward recovery stroke.
6. In the respiratory tract, the beat frequency averages 10-20 Hz, directed toward the pharynx.

4. The Basal Body (Kinetosome)

Structure of the Basal Body:

• Contains 9 microtubule triplets (A, B, C) - note: C microtubule is shorter, terminating at the transitional zone
• No central pair of microtubules (unlike the axoneme above)
• The C microtubule stops at the transitional zone; only A and B continue into the axoneme as the outer doublets

Basal Body-Associated Structures:

5. The Mucociliary Escalator (Mucociliary Clearance System)

Two-Layer Mucus System:

1. Sol layer (periciliary fluid / PCL): Low-viscosity aqueous layer surrounding the cilia. Maintained at ~7 μm depth - equal to ciliary height. Allows free ciliary movement.
2. Gel layer (mucus blanket): Thick, viscous mucus layer sitting on top of the sol layer, secreted by goblet cells and submucosal glands (mucins MUC5AC and MUC5B).

Mechanism of Clearance:

• Cilia beat in a coordinated metachronal wave (each cilium beats slightly after its neighbor, like a wave)
• During the power stroke: cilia extend fully and contact the gel layer, propelling mucus toward the pharynx
• During the recovery stroke: cilia bend to avoid resistance and move through the sol layer only
• Mucus (with trapped particles, bacteria, debris) is moved at ~5 mm/minute toward the oropharynx, then swallowed or expectorated
• Ciliated cells are most numerous in the trachea and proximal bronchi, decreasing distally

6. Clinical Significance

A. Primary Ciliary Dyskinesia (PCD) / Immotile Cilia Syndrome

• Dynein arm proteins (most commonly outer dynein arms - absent or shortened)
• Radial spoke proteins
• Central pair proteins
• Nexin-dynein regulatory complex proteins (>45 genes implicated)

• Complete loss of mucociliary transport
• Accumulation of mucus in the tracheobronchial tree
• Repeated bacterial infections (Pseudomonas, Haemophilus, Staphylococcus)
• Progressive airway destruction

B. Kartagener Syndrome (Classic Triad)

1. Bronchiectasis - bilateral, lower lobe predominance
2. Chronic sinusitis (+ rhinitis, nasal polyps)
3. Situs inversus totalis (complete mirror-image reversal of visceral organs)

Figure: EM cross-section of cilium in PCD - note absent dynein arms (×180,000) (Histology: A Text and Atlas)

C. Acquired Ciliary Dysfunction

D. Young Syndrome

• A milder ciliopathy caused by malformation of radial spokes and dynein arms
• Features: chronic sinobronchitis + azoospermia (obstructive)
• Unlike Kartagener, no situs inversus

7. Diagnostic Methods for Ciliary Dysfunction

8. Summary Table for Revision

Key Points for Exam

1. The 9+2 axonemal configuration is universal for all motile cilia, including respiratory, oviductal, ependymal, and sperm flagella.
2. Dynein arms are the motor unit - their absence is the hallmark of PCD on EM.
3. Nexin converts sliding into bending; radial spokes regulate amplitude.
4. The basal foot determines the direction of the beat - its misorientation causes dyskinetic cilia in Kartagener syndrome.
5. Kartagener = PCD + situs inversus (50% of PCD patients) - because nodal cilia control left-right asymmetry in the embryo.
6. Smoking is the most common acquired cause of ciliary dysfunction - leads to chronic bronchitis and COPD.
7. Electron microscopy remains the definitive diagnostic tool for PCD.
8. Mucociliary clearance is impaired in: PCD, cystic fibrosis, chronic bronchitis/COPD, asthma, bronchiectasis, and smoking.