Respiratory System

Wheater's, Ch 12: Respiratory System
Junquiera, Ch 17: Respiratory System


The goal of this lab is to examine the organization of the conducting and respiratory portions of the respiratory system.  In your slides you should be able to find excellent examples of epithelial transitions along the tract. Attention should be paid to the key structural features summarized in the table below: 

Secretory Cell
Sm. Muscle
Glands in C. T.
Trachea Pseudost.Columnar Ciliated Goblet C-shaped At opening of C-shaped cartilage Serous/Mucous ~2 cm --
Bronchi Pseudost. Columnar Ciliated Goblet Pieces Yes (encircles lumen) Serous/Mucous ~1.5 - 0.05 cm Enters lung
Bronchioles to Terminal Bronchioles Simple Columnar Ciliated Club cells No Yes No 0.5 - 0.2 mm No Alveoli in Walls
Respiratory Bronchioles Simple Cuboidal (some cilia) Club cells No Yes No ~0.15 mm Alveoli in Walls
Alveolar Duct Mostly
Simple Squamous
No No Yes/No No ~0.1 mm --
Alveoli Simple Squamous (Type I and II cells) Type II Cell No No No <0.5 mm --










I. Slide Descriptions

Webslide UVa_077: Larynx and trachea, coronal section, H&E [DigitalScope]

The larynx is a passageway for air between the oropharynx and trachea which also functions in the production of sound. The epithelial lining varies by location: the vestibular folds (or "false" vocal folds) are lined by a mixture of stratified squamous to stratified columnar epithelium whereas the true vocal folds are typically covered by stratified squamous non-keratinized epithelium that sometimes can keratinize in response to repeated injury. Inferior to the vocal folds, the epithelium goes back to stratified columnar before finally transitioning to the pseudostratified columnar epithelium of the trachea, which anatomically begins at the 1st tracheal ring of cartolage just inferior to the cricoid cartilage.

Seromucous glands are present underneath the epithelium superior and inferior to the vocal folds on both sides of the slide. Identify the vocal cord as a mucosal fold covered by stratified squamous epithelium and containing abundant skeletal muscle of the vocalis muscle. Notice the cartilage framework (thyroid and cricoid cartilages and tracheal rings), some of which contains sites of ossification that occurs with aging.




Webslide LSU_B-20:  Trachea, H&E [DigitalScope]

Observe the layering of the trachea, identifying mucosa, underlying connective tissue, cartilage, and adventitia.  Note that the pseudostratified columnar surface epithelium contains ciliated, goblet, and basal cells.  The basal lamina is unusually thick in the trachea, and appears here as a faintly stained layer of uniform thickness (about 4 µm) directly under the epithelium.  In the connective tissue are simple cuboidal or columnar glands that produce both serous and mucous secretions that coat the mucosal surface.  How do these secretions get to the lumen of the trachea and what is their function?  Capillaries are also numerous below the epithelium.

You may wish to also look at Webslide 0026_R [DigitalScope], which is also of the trachea. In this slide, the epithelium is VERY well-prepared such that one can beter see the ciliated and goblet cells. The C-shaped cartilage and trachealis muscle is also well-demonstrated, but the glands are not as numerous in this sample and are better appreciated in the other trachea slides for this lab.



Webslide UCinn_3788:  Esophagus & Trachea, mouse 1.5µm, AF-TB [DigitalScope]

Examine this slide at low power to acquire an overview of the trachea (right side of slide) and its relationship to the esophagus (left side of slide). This transverse section demonstrates the C-shaped cartilage rings of the trachea, smooth muscle along the posterior (open) portion of the cartilage, and abundant seromucous glands.  Several nice examples of nerves and blood vessels are also present in the connective tissue of the mediastinum surrounding the trachea. 

You may wish to also look at Webslide 0026_R [DigitalScope], which is also of the trachea. In this slide, the epithelium is VERY well-prepared such that one can beter see the ciliated and goblet cells. The C-shaped cartilage and trachealis muscle is also well-demonstrated, but the glands are not as numerous in this sample and are better appreciated in the other trachea slides for this lab.




Webslide 0315_T (Courtesy of U. of Mich.):  Lung, H&E [DigitalScope]

At low and high magnification, note the well preserved visceral pleura. This section also contains examples of the distal branches of the bronchial tree.  Scan the slide to find bronchi and bronchioles, using the table above to remind you of the key structural differences.  Find regions where you can trace the tract from terminal bronchioles to alveoli.  Note that as the bronchioles decrease in diameter the epithelium becomes shorter and the smooth muscle layer thins.  Also observe respiratory bronchioles indented with alveoli.  Simple squamous alveolar ducts communicate with alveolar sacs and blind-ended alveoli.  Do not spend time examining this slide for alveolar Type I and II cells, as they are much better preserved on Webslide 0028



Webslide 0028_T:  Lung, monkey, 2 µm, T.B. [DigitalScope]

First, survey this important slide at low power to identify bronchi, bronchioles, and alveolar regions for study at high power.  Then, utilizing high power objective settings, carefully study the mucosal and submucosal anatomy of a bronchus. The submucosa shows smooth muscle and seromucous glands adjacent to irregular cartilaginous plates.  Next, identify a bronchiole and note:

  1. simple columnar ciliated epithelium--cells shorter than in the bronchi and trachea,
  2. domed secretory, non-ciliated exocrine cells (also known as "club cells"),
  3. smooth muscle in the submucosa, and
  4. absence of seromucous glands and cartilage.

See if you can identify respiratory bronchioles and alveolar ducts, noting the progression from a low simple columnar ciliated epithelium to a simple squamous epithelium.

Use high power to study a respiratory portion of the lung with reasonably uniform distension of alveoli.  Measure the sizes of some of the polygonal-shaped alveoli.  At high power you should be able to identify two types of cells lining the alveolar air spaces, Type I and Type II epithelial cells.  Type I cells are squamous with flattened nuclei and markedly attenuated cytoplasms.  Type II cells (aka "great pneumocytes") are more rounded, project into the alveolar lumens, and contain vacuolated cytoplasms. These vacuoles represent remnants of the multilamellar bodies associated with surfactant biosynthesis and storage. 

The alveoli contain several other cell types:  Endothelial cells, simple squamous epithelial cells with dense flattened nuclei, line the extensive capillary network throughout the alveolar septa.  How can you distinguish endothelial cells from Type I cells?  Interstitial cells resembling fibroblasts are occasionally seen in the alveolar septa, but are more commonly located at junctions of alveoli.  Occasional mast cells containing dense pink granules are also visible.  (Due to the perfusion fixation method employed here, few alveolar macrophages are present). 


II. Pathology Correlates

A. Appearance of the lung in congestive heart failure

UMich Histopathology slide 42 [DigitalScope]

While components of the alveolus may be difficult to see in normal lung tissue, pathological changes that occur in the lung as the result of congestive heart failure shown here [example] exaggerate many of these features making them a bit easier to see:

  • Poor left ventricular output causes blood pressure to rise in the left atrium, leading to dilation of the alveolar capillaries.
  • Type II pneumocytes with large round nuclei and clear cytoplasm are much more numerous and can be easily seen in the alveolar walls because they proliferate in an attempt to repair damaged alveoli.
  • Alveolar macrophages characteristic of this condition (also known as “heart failure” cells) become laden with brownish-black hemosiderin pigment resulting from the breakdown of erythrocytes leaking from the engorged capillaries.



B. Appearance of the airways in asthma

Slide 268 [DigitalScope] (large airways)

Slide 68alt [DigitalScope] (small airways and lung)

Asthma is a chronic inflammatory disorder of the airways that causes recurrent episodes of wheezing, breathlessness, chest tightness, and cough, particularly at night and/or early in the morning. These slides show changes in the airways that occur in the setting of chronic asthma. Slide 268 contains bronchi from an individual with long-standing asthma in which the following changes can be appreciated. Starting in the lumen of one of the bronchi [example], note:

  • very thick mucus layer on the surface of the epithelium and/or mucus "plugs" that fill the lumen (due to hypersecretion from goblet cells and seromucous glands)
  • goblet cell hyperplasia (response to cytokines produced in the inflammatory response)
  • basement membrane thickening (due to epithelial cell proliferation)
  • marked increase in eosinophils and fibrosis in underlying connective tissue [example]
  • bronchial smooth muscle hypertrophy and hyperplasia [example]
  • increase in submucosal glands [example]

Slide 68 shows changes in the smaller airways (bronchioles) and the lung interstitium.

In a bronchiole [example] there are features that are obstructive in nature:

  • very thick mucus layer on the surface of the epithelium and/or mucus "plugs" that fill the lumen (due to hypersecretion from goblet cells)
  • metaplastic changes to the epithelium such that it contains goblet cells --usually bronchiole contain only a few goblets and will have club cells instead.
  • basement membrane thickening
  • smooth muscle hypertrophy and hyperplasia
  • chronic lymphocytic inflammation and thickening of the adventitia

In the lungs, there is marked thickening and fibrosis seen in the alveoli [example] that can lead to restrictive changes in the lungs (i.e. they do not expand as well). You will learn more about restrictive versus obstructive lung diseases, and it is fair to say that both may be present but one tends to predominate. In the case of ASTHMA, the typical overall presentation is OBSTRUCTIVE in nature (i.e. the patient cannot expel air very well due to occlusion of the airways).




Extra Slides


I. Trachea

UMich Slide 40 (trachea, H&E) [DigitalScope]

The epithelium lining the trachea is typical respiratory epithelium (ciliated pseudostratified columnar) that contains numerous goblet cells. This epithelium has an unusually thick basement membrane, which you can see as a narrow pink-staining region immediately basal to the epithelium. This epithelium plus its underlying layer of loose connective tissue (the lamina propria) make up the tracheal mucosa. The layer under the mucosa is the submucosa wherein you’ll find numerous seromucous glands. The mucosa is roughly separated from the submucosa by a layer of longitudinal elastic fibers --the trachea in this sample is cut in cross section, so the elastic fibers will also be in cross section and can be seen here as eosinophilic, glass-like dots [example]. Outside the submucosa is hyaline cartilage which helps to keep the lumen of the trachea from collapsing and beyond that is connective tissue of the adventitia that blends with tissue of the mediastinum.



II. The respiratory tree: from Bronchi to Alveoli

UMich Slide 129_20x (lung, H&E) [DigitalScope]
UMich Slide 130_20x (lung, H&E) [DigitalScope]
UMich Slide 130-1_40x (lung, H&E) [DigitalScope]
UMich Slide 130-2_40x (lung, H&E) [DigitalScope]

A. Bronchi: The trachea bifurcates into two primary bronchi, which enter the lung and then branch several times to give rise to smaller secondary and tertiary bronchi [example]. Bronchi differ from the trachea in having plates rather than rings of cartilage, and in having a layer of smooth muscle between the lamina propria and submucosa. In smaller branches, the amount of cartilage decreases, whereas the amount of smooth muscle increases. Also, the number of glands and goblet cells decreases. Don’t worry about trying to distinguish among primary, secondary, and tertiary bronchi, but you should be able to distiguish bronchi in general from the trachea and bronchioles (discussed below).

B. Bronchioles: Bronchioles [example] are smaller branches of the bronchi, and are distinguished from them by the absence of cartilage and glands. In larger bronchioles, the epithelium is still ciliated, but is now usually simple columnar, whereas in the smallest bronchioles, the epithelium will be simple cuboidal (mostly exocrine cells) and lack cilia altogether. The smooth muscle layer is generally quite prominent in these structures as demonstrated in slide 130-2 [example] where the bronchiole was cut in a grazing longitudinal section allowing you to see the circularly arranged bundles of smooth muscle in the bronchiolar wall. As mentioned above, the smallest conducting bronchioles consist of a simple cuboidal (or perhaps “low columnar”) epithelium of mostly exocrine cells, a few ciliated cells, and NO goblet cells, and are called terminal bronchioles [example].

C. Respiratory Bronchioles: You might see short, transitional regions of bronchioles which have alveoli in their walls. These bronchioles with alveoli in their walls are called respiratory bronchioles [example] . They characteristically exhibit a progressive reduction in height of the epithelium and in the amount of smooth muscle between the openings of adjacent alveoli.

D. Alveolar ducts: The walls of alveolar ducts [example] are so interrupted by alveoli and alveolar sacs (clusters of alveoli) that all that can be seen of the wall proper is small knobs of smooth muscle, collagen and elastic fibers. You can spot the knobs, but shouldn’t try to distinguish the constituents, which are covered by a squamous epithelium too thin to see with the light microscope.

E. Alveoli: The walls of these structures are covered on both sides by squamous epithelium (too thin to see) of Type I pneumocytes lining adjacent alveolar lumens. Within the walls is an extensive capillary network. You may see the space within these capillaries, or they may be filled with RBCs. The Type II pneumocytes [example], which secrete surfactant, have large, rounded nuclei and vacuolated cytoplasm and are often difficult to identify in the light microscope (the “vacuoles” are actually granules of phospolipids that, unfortunately, are often extracted during tissue processing). In the lumen of some alveoli, you will see macrophages, called alveolar phagocytes or "dust cells" [example].


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