Respiratory epithelium, or airway epithelium, is a type of ciliated columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways. It is not present in the larynx and pharynx. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by the secretion of mucus and the action of mucociliary clearance.
Diagram of the trachea showing respiratory epithelium at the top
Illustration depicting the respiratory epithelium.
|Anatomical terms of microanatomy|
The respiratory epithelium lining the upper respiratory airways is classified as ciliated pseudostratified columnar epithelium. This designation is due to the arrangement of the multiple cell types composing the respiratory epithelium. While all cells make contact with the basement membrane and are, therefore, a single layer of cells, the nuclei are not aligned in the same plane. Hence, it appears as though several layers of cells are present and the epithelium is called pseudostratified. The respiratory mucosa transitions to simple cuboidal epithelium and finally to simple squamous epithelium in the alveolar ducts and alveoli.
The cells in the respiratory epithelium are of four main types: a) ciliated cells, b) goblet cells, and c) club cells, and d) basal cells. The ciliated cells are columnar epithelial cells with specialized ciliary modifications. Between the ciliated cells are numerous microvilli, attached as tufts to tuft cells (also known as brush cells). Their function is unknown but their locations particularly at the alveoli suggest a clearance role.
The respiratory epithelium functions to moisten and protect the airways. It acts as a physical barrier to pathogens, as well as the removal of pathogens in the mechanism of mucociliary clearance.
The ciliated cells are the primary components in the mucociliary clearance mechanism. Each epithelial cell has around 200 cilia that beat constantly at a rate of between 10 and 20 times per second. The direction of their beat is targeted towards the pharynx, either upwards from the lower respiratory tract or downwards from the nasal structures.
Goblet cells, so named because they are shaped like a wine goblet, are columnar epithelial cells that contain membrane-bound mucous granules and secrete mucus as part of the epithelial lining fluid (ELF), the composition of which is tightly regulated; the mucus helps maintain epithelial moisture and traps particulate material and pathogens moving through the airway. and determines how well mucociliary clearance works. The basal cells are small, nearly cuboidal cells thought to have some ability to differentiate into other cells types found within the epithelium. For example, these basal cells respond to injury of the airway epithelium, migrating to cover a site denuded of differentiated epithelial cells, and subsequently differentiating to restore a healthy epithelial cell layer.
Certain parts of the respiratory tract, such as the oropharynx, are also subject to the abrasive swallowing of food. To prevent the destruction of the respiratory epithelium in these areas, it changes to stratified squamous epithelium, which is better suited to the constant sloughing and abrasion. The squamous layer of the oropharynx is continuous with the esophagus.
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