Content Reviewers:Rishi Desai, MD, MPH
The trachea is the large airway that extends from the larynx and divides into the two primary bronchi.
Because the trachea and bronchi share similar functions, their walls are composed of similar tissue types as well, but with a few key differences.
Let’s first look at a low power image of the trachea, stained with Hematoxylin and eosin, or H&E for short.
At this magnification, we can easily see the C-shaped cartilage ring, which is unique to the trachea.
Also, you can use the cartilage ring to determine the orientation of the image, since the closed side of the ring points anteriorly and the open side of the ring will always face posteriorly towards the esophagus.
The free ends of the ring are connected to one another by smooth muscle, called the trachealis muscle.
The smooth muscle can be differentiated from the surrounding tissue by the long appearance of their cell bodies when cut longitudinally.
Their lack of striations also indicates that the muscle is smooth muscle, not skeletal muscle.
In older individuals, portions of the tracheal cartilage can transform into bone.
For example, this cross-section of a hen’s trachea shows areas of calcification, with some hyaline cartilage still present on the right.
Looking at the epithelium at 10x magnification, it’s easier to see the different layers of the trachea.
The lumen of the airway is seen at the bottom left of this image.
So, the very thin layer lining the trachea’s respiratory epithelium.
Zooming in much further to approx 100x, the cells that make up the epithelium can be distinguished more easily.
Similar to the nasal and bronchial epithelium, the tracheal epithelium contains three main types of cells: ciliated pseudostratified columnar cells, the pale-staining mucus-secreting goblet cells, and basal cells which are found closer to the basement membrane.
The basement membrane can be seen as the pink or eosinophilic band that separates the epithelium from the underlying connective tissue.
Be careful not to confuse the layer of mucus lining the epithelium with the cilia on the surface of the columnar cells.
The cilia is much smaller and only extends about 5-7 μm from the surface.
The cilia in this image can barely be seen as the dark pink lines just under the thicker layer of mucus, which can be about 20-30 μm thick.
The proportions of these cells will vary based on their location within the trachea.
So the upper trachea will consist of more goblet and basal cells, while the lower trachea will have more ciliated columnar cells.
The layer just below the respiratory epithelium is the lamina propria, which consists of loose connective tissue and is also highly vascular, as you can see by the large number of blood vessels within this layer.