Bronchioles and alveoli histology

Definitions & Key takeaways

The bronchioles are the smallest air passages in the lungs and they end in tiny sacs called alveoli. The wall of each bronchiole has a layer of smooth muscle that can contract to narrow the airway. Distally are the smallest bronchioles called terminal bronchioles, which give rise to the respiratory bronchioles that end up with alveolar ducts. Distal to the alveolar ducts are alveolar sacs and their individual alveoli. The alveoli are where most of the gas exchange takes place between the lungs and the bloodstream. Each alveolus is a microscopic sac lined with a single layer of flattened epithelial cells. There are tiny blood vessels (capillaries) surrounding each alveolus, and it is through these capillaries that oxygen and carbon dioxide are exchanged. Within the alveoli are clusters of cells called type I and type II pneumocytes. The type I cells make up about 90% of the epithelial layer and their primary function is to keep the alveoli from collapsing. The type II cells are much less numerous and their primary function is to produce surfactant. Surfactant is a substance that reduces the surface tension of the fluid that covers the alveoli. This allows the alveoli to stay inflated and prevents them from collapsing.

Chapters:

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Introduction0:00–1:58

Bronchioles are air-conducting passageways 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 portion of the airways that have a respiratory function, called respiratory bronchioles.
These bronchioles then lead to the alveolar ducts, which are transitional airways that gradually become increasingly involved in gas exchange and lead to the alveolar sacs and their individual alveoli.If we compare images from a bronchus and bronchiole at low magnification, we can see that the larger bronchus has large supporting cartilage plates that aren’t seen in the smaller bronchiole.
The bronchi also contain sero-mucous glands that are not present in bronchioles, but the bronchioles will still have a surrounding layer of smooth muscle present.
The larger proximal bronchioles, similar to the one in this image, are called the primary bronchioles. If we zoom in to 40x, we can see that the primary bronchioles have ciliated, pseudostratified columnar epithelium with goblet cells.
When using a Hematoxylin and Eosin 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.
Moving distally through the airways, the bronchioles' diameter gradually decreases and the terminal bronchioles will have a diameter less than 0.5 mm.
The epithelium also transitions from ciliated pseudostratified columnar epithelium to an epithelium that consists mostly of ciliated simple columnar and cuboidal cells with exocrine club or Clara cells instead of goblet cells.
The Clara cells can be identified by their tall columnar and non-ciliated appearance, as well as their dome-shaped apical ends that contain secretory granules.If a patient has bronchial asthma, there will be histological changes to the bronchioles.

Bronchial asthma1:58–2:35

Mucus can sometimes be found in the lumen of the bronchioles, which can lead to the formation of “Curschmann’s spirals,” which are spiral-shaped mucus plugs that can be found in the sputum obtained from a tracheal and broncho-alveolar lavage.
They originate from small bronchioles and are associated with chronic lesions that produce excessive mucus. Another change that can be seen as a result of bronchial asthma includes a thicker bronchiolar wall due to hypertrophy of the surrounding smooth muscle, inflammation, mucous gland hyperplasia, and enlarged blood vessels.Moving onto the first part of the respiratory portion of the respiratory system, we have the respiratory bronchioles.

Bronchioles2:35–3:01

These bronchioles also have a mix of simple cuboidal and columnar epithelium with even more Clara cells. The bronchiolar walls are thinner compared to terminal bronchioles, but the main difference is that they now contain a small number of alveoli as well.
Each respiratory bronchiole then branches further into alveolar ducts.Alveolar ducts are linear airways that lead to many alveoli that line the duct, with occasional regions that are lined with simple cuboidal epithelium.

Alveolar ducts3:01–3:18

The alveolar ducts are also the most distal portion of the respiratory system to still contain smooth muscle around the epithelium.The alveoli have diameters about 200 micrometers wide and have small capillaries that allow red blood cells to be in close proximity with inspired air.
In this image, there’s also a large blood vessel on the right side. The alveolar surface epithelium creates the continuous lining for each alveolus and is made up of type I and type II pneumocytes.

Alveoli3:18–5:10

These cells also form the interalveolar septum that separates neighboring alveoli from each other. About 95% of the alveolar surface area is composed of type I pneumocytes, which also forms the alveolar part of the air-blood barrier.
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.The type II pneumocytes are cuboidal cells and only make up about 5% of the alveolar surface area.
These cells are usually located near the intersections of the septums and also from tight junctions with neighboring cells, similar to type 1 pneumocytes.
Type II pneumocytes are capable of synthesizing pulmonary surfactant and dividing in order to regenerate both types of pneumocytes.
As a result, hyperplasia of type II pneumocytes is an important marker for alveolar injury and repair within the lungs.Alveolar macrophages are present in both the connective tissue of the interalveolar septum as well as the lumen of the alveoli, as seen in this image.
The alveolar macrophages act as the main phagocytes of the alveolar surface. As a result, smokers, coal miners, stonemasons, and people living in urban areas will often have alveolar macrophages that are filled with carbon particles called anthracotic pigments and birefringent silica particles when slides are examined under polarized light.
In both of these example images, the individuals had a long history of cigarette smoking.Alright, as a quick recap. The larger primary bronchioles have an epithelium made of ciliated pseudostratified columnar cells with a small number of goblet cells.
The goblet cells can be identified by their lighter appearance and their goblet-shaped cells. Distally, the airway’s diameter decreases and the epithelium transitions to ciliated simple columnar and cuboidal cells with Clara cells.

Review5:10–6:07

The Clara cells can be identified by their tall non-ciliated columnar appearance and their dome-shaped apical end. The respiratory bronchioles have a similar composition as the terminal bronchioles except they have a small number of single alveolus in their wall.
The alveolar surface epithelium is made of type I and type II pneumocytes. Type I pneumocytes are large flat cells with elongated nuclei that compose about 95% of the surface area.
Type II pneumocytes are cuboidal cells located near the intersections of the alveolar septums and will be hyperplastic when there’s alveolar injury.
their wall The alveolar surface epithelium is made of type one and type I pneumocyte type one pneumocyte are large flat cells with an elongated nuclei that compose about 95% of the surface area Type I I pneumocyte are cuboidal cells located near the intersections of the alveolar septum and will be hyperplastic when there is alveolar injury