Question

The lung airway epithelium regulates the thickness and the pH of the airway surface liquid.

Describe the airway epithelium and highlight the main molecular mechanism involved in regulating the thickness of the airway surface liquid. Where possible, indicate how these processes are altered in cystic fibrosis.

Answer 1

The Airway epithelium (or, Respiratory epithelium) is basically a type of ciliated pseudostratified columnar epithelium which lines the respiratory tract in the form of respiratory mucosa. It functions in moisturizing and protecting the airway passage, by forming a thick layer of airway surface liquid. It also acts in frontline defense against different respiratory viruses.

Airway surface liquid is a watery thin layered solution which lies in between the airway epithelium and the gas present in the lumen. It is required in maintaining healthy function of the airway epithelium. In the lung airway, three mechanisms- mucus transport by beating of cilia, mucus water evaporation, and airway epithelium/submucosal glands, are of significant importance in controlling airway mucus volume. In proximal generations of bronchial region, the airway mucus volume is mainly controlled by water evaporation and replenishment from the mucus layer by the epithelium; while in the distal generations of bronchial region, the airway mucus volume is controlled mainly by mucociliary transport and by liquid absorption by the epithelium.

Cystic fibrosis is a recurrent (occuring often) respiratory disease prone with infections in respiratory tract, inflammation and even damages of lungs. The cystic fibrosis impairs the homeostatic mechanisms which are controlled by the volume and composition of Airway Surface Liquid (such as, mucociliary clearance, ciliary function maintenance, and airway antimicrobial properties). This impairment leads to occurence of acidic and dehydrated Airway Surface Liquid. In cystic fibrosis, abnormal Cystic fibrosis transmembrane conductance regulator protein (CFTR) causes defective anion transport. This abnormal CFTR mediates transport of bicarbonate which profounds an unlikely acidic environment, that hampers the antimicrobial function, causing alterance of properties of mucus and its clearance.