Content Reviewers:Rishi Desai, MD, MPH
Bronchioles are air-conducting passways that are typically less than 1 mm in diameter.
The smallest bronchioles, called terminal bronchioles, are the most distal airways that are still a part of the conducting portion of the respiratory system.
The terminal bronchioles then lead to the first part of the respiratory portion, which includes the respiratory bronchioles and alveolar ducts.
These ducts are transitional airways that gradually become increasingly involved with gas exchange.
These airways then lead to the alveolar sacs and their individual alveoli.
Unlike the bronchi, the bronchioles won’t contain large cartilage plates or glands in their submucosa, but they will still contain a layer of smooth muscle as seen in this H&E slide of a bronchiole surrounded by alveoli at 10x magnification.
Increasing the magnification to 40x, you can see that the larger airways, or primary bronchioles have ciliated, pseudostratified columnar epithelium with goblet cells.
When using an H&E stain, the goblet cells will appear lighter than the surrounding epithelial cells and as their name suggests, these cells are often shaped similarly to the top portion of a goblet.
As the airway’s diameter decreases, the bronchioles gradually transition to simple ciliated columnar epithelium with Clara cells instead of goblet cells.
The terminal bronchioles will have a diameter less than 0.5 mm.
The Clara cells can be identified by their tall non-ciliated columnar appearance, as well as their dome-shaped apical ends that contain secretory granules.
If a patient has bronchial asthma, you would expect to find histological changes to the bronchiole.
In addition to sometimes finding mucus in the lumen of the bronchiole, the bronchiolar wall will be thicker due to muscle hypertrophy, inflammation, and enlarged blood vessels, which can be seen in this image from the lungs of asthma mouse models.
Moving onto the first part of the respiratory portion of the respiratory system, we have the respiratory bronchioles.
These bronchioles also have simple cuboidal epithelium with more Clara cells and some ciliated cells, but the difference is that they have thinner walls and now contain a small number of alveoli in their walls.
Each respiratory bronchiole then branches further into alveolar ducts.
Alveolar ducts are linear airways that have multiple alveoli along its airway, which can be seen in this image of the lungs at 10x magnification.
The neighboring alveoli are separated by only an interalveolar septum, made of both type I and type II pneumocytes.
These ducts are the most distal portion of the respiratory system to still contain smooth muscle.
Only a small amount can be seen in this slide on the left, with the stereotypical knob-like appearance.
The alveolar ducts then lead to the alveolar sacs, which are the outpouchings that each contain many alveoli.
The alveoli are about 200 micrometers in diameter and have very thin walls that consist of three components that can be seen in the center of this low power image: the surface epithelium, supporting tissue, and blood vessels.
The epithelium creates the continuous lining for each alveolus and is made up of type I and type II pneumocytes.
About 95% of the alveolar surface area is composed of type I pneumocytes, which are labels in blue in this high power image of alveoli.
These cells can be identified by their large flat appearance and elongated nuclei.
Their cytoplasm is very thin, which can be less than 80 nm in thickness, and the cells form tight junctions between themselves and neighboring cells.