Histology Fact Sheet: Respiratory System

The conducting portion of the airway is where air is moved, warmed and moistened. The nasal cavities, pharynx, larynx, trachea and bronchi are all part of the conducting portion of the airway.

The respiratory portion of the airway is where gas exchange occurs. The respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli are all part of the respiratory portion.

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Histology of the Pharynx

The pharynx is lined by both stratified squamous epithelium and ciliated pseudostratified epithelium with goblet cells. Different regions are lined by a different type of epithelium. Regions of the pharynx that are likely to be roughened up by food are lined by stratified squamous epithelium. Other regions of the pharynx are lined by ciliated pseudostratified epithelium with goblet cells.

The vestibule is lined by stratified squamous epithelium.

Histology of the Nasal Cavity

Olfactory mucosa lines the roof and portions of the walls of the nasal cavity. It contains several cell types: basal cells, brush cells, olfactory cells and sustentacular cells.

Basal cells are located in the basal lamina. Brush cells are involved with general sensation of the olfactory mucosa. Olfactory cells are bipolar neurons that are the receptors for smell. Sustentacular cells are supporting cells. Sustentacular cells are most numerous cell type in the olfactory epithelium.

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Histology of the Paranasal Sinuses

The paranasal sinuses are lined by ciliated pseudostratified epithelium with goblet cells.

In man, respiratory mucosa is composed of ciliated pseudostratified columnar epithelium with goblet cells. Respiratory mucosa contains several cell types: ciliated cells, goblet cells, basal cells, and brush cells. Respiratory mucosa is located lining the respiratory segment of the nasal cavity. It lines the conchae and
the paranasal sinuses.

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Histology of the Larynx

The epiglottis is part of the larynx. It is composed of elastic cartilage.

The "Adam's apple" is a nickname for part of the larynx formed by the thyroid
cartilage. The thyroid cartilage is composed of hyaline cartilage.

The larynx is composed of several cartilages. The thyroid cartilage, cricoid
cartilage, arytenoid cartilages, corniculate cartilages and cuneiform cartilages are
all composed of hyaline cartilage. The epiglottis is elastic cartilage. There is no
fibrocartilage in the larynx.

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Histology of the Trachea

The trachea branches into two bronchi which branch into smaller bronchi. The rings of the trachea are composed of hyaline cartilage.

The trachea is lined by pseudostratified squamous epithelium. Epithelium lines body cavities and surfaces. Pseudostratified squamous epithelium is"pseudostratified" because it is only one cell layer thick, yet it appears to be stratified. In reality, every cell touches the basement membrane.

Histology hint from Sarah Bellham: The prefix "pseudo" is of Greek origin and it means false or counterfeit. For example: pseudonym, pseudo-science or pseudostratified.

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Histology of the Respiratory Tract

The respiratory tract is made of branching structures, much like the branches of a
tree.

Histology of the Bronchi

The bronchi ultimately branch into smaller bronchioles.

Histology of the Bronchioles and Alveoli

Bronchioles are distinguished from bronchi in that they do not have cartilage and submucosal glands. The terminal bronchioles are the last part of the airway in which gas exchange does not occur. Terminal bronchioles lead to the respiratory bronchioles. The respiratory bronchioles are the first section of the respiratory tree that gas exchange can occur.

The alveoli duct is analogous to a thoroughfare with many cul-du-sacs branching off of it. At the end of the alveoli duct is an alveoli sac. An alveoli sac is a cluster of alveoli, much like a cluster of grapes. Alveoli are individual sacs where gas exchange occurs.

Histology slide of the lung
Histology slide courtesy of Education Interactive, Histology Photo CD

The alveoli are formed by simple squamous epithelium. Epithelium lines body cavities and surfaces. Simple squamous epithelium is "simple" because it is one cell thick. "Squamous" refers to the fact that the cells are flat.

The Clara cell is found in the terminal bronchioles.

The type I pneumocyte is a squamous epithelial cell. It covers most of the surface of the alveoli. The type II pneumocyte is also called a septal cell.  The type II pneumocyte secretes surfactant. The dust cell is also called the alveolar phagocyte.

Brush cells are occasionally, but rarely, seen in the alveolar epithelium.

The alveolar pores are the pores of Kohn. These are openings between adjacent alveoli.
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Macrophages are mononuclear phagocytes. Many tissues have resident (fixed) macrophages. Fixed macrophages are given a unique name, depending on the tissue that they are located in. Dust cells are alveolar macrophages found in the respiratory tract.

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Sources:

Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith

http://www.training.seer.cancer.gov; funded by the U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program, via contract number N01-CN-67006, with Emory University, Atlanta SEER Cancer Registry, Atlanta, Georgia, U.S.A.