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HISTOLOGY OF THE RESPIRATORY EPITHELIUM
Soatova Feruza Bahadirovna
Assistant at the Alfraganus University
Email address: : fsoatova51@gmail.com
https://doi.org/10.5281/zenodo.14619528
Abstract
The respiratory epithelium plays a critical role in maintaining respiratory health by
serving as a barrier and an active participant in defense mechanisms against pathogens,
allergens, and environmental irritants. This specialized pseudostratified columnar ciliated
epithelium, interspersed with goblet cells, lines much of the respiratory tract and is essential
for mucociliary clearance. Its unique structural adaptations facilitate the filtration,
humidification, and warming of inhaled air. The transition from conducting to gas-exchange
regions involves histological changes, with alveolar epithelium optimizing gas exchange
through the interplay of Type I and Type II pneumocytes. Dysfunction or damage to the
respiratory epithelium is implicated in various diseases, including asthma, cystic fibrosis, and
chronic obstructive pulmonary disease, highlighting its importance in maintaining respiratory
homeostasis. This review provides an overview of the structure, function, and
pathophysiological significance of the respiratory epithelium, emphasizing its role in health
and disease.
Keywords
: Respiratory epithelium, pseudostratified columnar epithelium, mucociliary
clearance, Type I pneumocytes, Type II pneumocytes, goblet cells.
Introduction
The respiratory system constantly interacts with the external environment as humans
breathe, filtering and clearing inhaled pathogens, allergens, and debris to maintain
homeostasis and prevent inflammation.
It is divided into two primary sections: the conducting portion and the respiratory
portion. Most of the respiratory tract, from the nasal cavity to the bronchi, is lined with
pseudostratified columnar ciliated epithelium. The bronchioles have simple columnar to
cuboidal epithelium, while the alveoli are lined with thin squamous epithelium optimized for
gas exchange.
Structure
The respiratory system consists of four main histological layers:
1.
Respiratory Mucosa:
Includes epithelium and its supporting lamina propria.
2.
Submucosa
3.
Cartilage and/or Muscular Layer
4.
Adventitia
The respiratory epithelium, primarily ciliated pseudostratified columnar epithelium,
lines most of the respiratory tract but is absent in the larynx and pharynx. Although it is a
single layer of cells, its nuclei are arranged at varying levels, creating a stratified appearance.
This unique structure serves as a barrier against pathogens and foreign particles while
facilitating the removal of debris through the mucociliary elevator.
The Conducting Portion
The conducting portion includes the nasal cavity, trachea, bronchi, and bronchioles.
These structures are lined with ciliated pseudostratified columnar epithelium containing
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goblet cells that secrete mucus. The mucus traps pathogens and debris, which are then
transported by cilia toward the throat for elimination.
In larger airways, hyaline cartilage rings provide structural support to the trachea and
bronchi, ensuring unobstructed airflow. As the airways branch, the epithelium transitions to
simple cuboidal, and non-ciliated Clara cells replace ciliated cells in distal regions.
The Gas-Exchange Portion
The gas-exchange region consists of millions of alveoli, whose walls are lined with
simple squamous epithelium to facilitate efficient diffusion of oxygen and carbon dioxide. Two
main cell types are present:
1.
Type I Pneumocytes:
Flat cells forming a thin diffusion barrier for gas exchange and
fluid transport, connected by tight junctions.
2.
Type II Pneumocytes:
Cuboidal cells that produce surfactant, reducing surface tension
and preventing alveolar collapse during exhalation. They also serve as progenitor cells,
replacing damaged Type I pneumocytes.
Function
The respiratory epithelium protects and clears the airways and lungs, ensuring efficient
functioning:
1.
Humidification and Warming:
Serous and mucous secretions humidify air, while the
extensive capillary network in the alveoli conditions and warms it.
2.
Filtration:
Mucus traps particles, and cilia move the debris toward the throat for
expulsion. Goblet cells secrete mucus to maintain moisture and trap pathogens, while ciliated
cells propel it.
3.
Oxidant Defense and Injury Response:
Basal cells repair damaged epithelium and
defend against oxidative stress.
4.
Gas Exchange:
Oxygen and carbon dioxide are exchanged in the alveoli through
diffusion between capillaries and the alveolar walls.
Microscopy
Under light microscopy, hematoxylin and eosin (H&E) staining highlights the
pseudostratified epithelium with nuclei at varying heights, creating a stratified appearance.
Goblet cells, basal cells, and cilia are distinct features. The basement membrane appears as a
thick pink line in the trachea, while cartilage and smooth muscle help distinguish bronchi
from bronchioles. Alveoli become apparent in respiratory bronchioles, with alveolar sacs
visible as clusters of smooth muscle, elastic fibers, and collagen.
The respiratory system's complex structure ensures effective filtration, air conditioning,
and gas exchange, supporting overall respiratory health.
Microscopy: Electron
Electron microscopy (EM) offers a detailed visualization of individual cell types and
ultrastructural features within respiratory tissue. At the level of the trachea, EM allows clear
identification of basal cells, goblet cells, and ciliated cells, along with their organelles and
cytoplasmic components. Ciliated epithelium with microvilli is distinctly visible, and cross-
sections of cilia reveal the characteristic 9+2 microtubule arrangement within the cytoplasm.
In the alveoli, EM highlights the ultra-thin air-blood barrier formed by Type I
pneumocytes, capillary endothelium, and the fused basal lamina. Type II pneumocytes,
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distinguishable from the delicate Type I cells, are notable for their lamellar bodies, rough
endoplasmic reticulum, Golgi apparatus, and microvilli.
Pathophysiology
Numerous diseases can impact the respiratory system, arising from defective barrier
function, genetic mutations, or inflammatory processes. Below are discussions of key
respiratory diseases to illustrate the significance of a properly functioning respiratory system.
Asthma
Asthma is a chronic inflammatory condition characterized by airway remodeling and
hyperreactivity to environmental triggers, often accompanied by excessive mucus production.
Common in both children and adults, asthma is increasingly prevalent, posing health,
economic, and environmental challenges.
The underlying cause is inflammation and edema in the airways, leading to
bronchospasms that restrict airflow. Environmental factors like dust, pollen, or pathogens can
trigger bronchoconstriction, where smooth muscle tightens, narrowing the bronchi and
bronchioles. This interaction involves the mucosal epithelium, mast cells, smooth muscle, and
the parasympathetic nervous system, causing wheezing and shortness of breath.
Cystic Fibrosis
Once fatal in infancy, cystic fibrosis (CF) now has a median life expectancy of around 40
years with advances in diagnosis and treatment. CF is an autosomal recessive disorder caused
by mutations in the CFTR gene, most commonly the phe508del mutation. The CFTR protein
regulates fluid secretion by controlling chloride and sodium transport. Mutations result in
thick mucus secretions that affect multiple organs, including the lungs, pancreas, liver, and
reproductive system.
CF often leads to chronic lung disease and recurrent infections, progressively damaging
the respiratory system. Treatment focuses on enhancing mucociliary clearance, reducing
bacterial infections, and improving quality of life.
Ciliary Dyskinesia
Ciliary movement is crucial for clearing mucus and inhaled materials from the
respiratory tract. Primary ciliary dyskinesia (PCD) is a disorder that impairs this process,
leading to chronic sinus and pulmonary infections, situs abnormalities, and infertility due to
defective sperm motility or fallopian tube function.
PCD is diagnosed using ultrastructural analysis of cilia and genetic testing for associated
mutations. The triad of chronic sinusitis, bronchiectasis, and situs inversus caused by ciliary
dysfunction is known as Kartagener syndrome.
These diseases underscore the critical roles of respiratory structures and highlight the
systemic consequences when their functions are impaired.
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