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Respiratory System |
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Overview: 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:
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 0026_R: Trachea, human, H&E [DigitalScope] This transverse section demonstrates the characteristic, C-shaped cartilage rings of the trachea and smooth muscle (the trachealis muscle) along the posterior (open) portion of the cartilage seen on the left side of the specimen. 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 basement membrane is unusually thick in the trachea and appears here as a faintly stained layer of uniform thickness (about 5 µm) directly under the epithelium. Capillaries are also numerous in the connective tissue below the epithelium along with a few glands comprised of simple cuboidal or columnar cells that produce both serous and mucous secretions that coat the mucosal surface (note that this particular specimen does not happen to have many glands, but they can be readily seen in the next specimen). How do these secretions get to the lumen of the trachea and what is their function?
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.
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:
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:
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:
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:
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] 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].
Respiratory System Practice Questions 1. Which cells are primarily responsible for producing surfactant in the lungs?A) Club cells [ANSWER]
2. The trachea is supported by which of the following structures? A) Complete rings of hyaline cartilage [ANSWER]
3. The vocal cords (true vocal cords) are lined by which type of epithelium?A) Stratified squamous non-keratinized epithelium [ANSWER]
4. A 35-year-old man with a long history of smoking presents with a chronic cough and increased production of thick mucus. A biopsy of his trachea reveals a marked increase in the number of mucus-secreting cells. What cells in the respiratory epithelium are responsible for this increased mucus production? A) Club cells [ANSWER]
5. A 50-year-old man who has worked in a coal mine for 20 years presents with a chronic cough and difficulty breathing. A sputum sample shows abundant macrophages filled with carbon pigment. These macrophages are most likely from which part of the respiratory system? A) Nasal cavity [ANSWER]
Organ Review UCinn_3930 [DigitalScope] Identify as specifically as possible the organ shown in the slide AND provide at least FOUR specific or characteristic features (e.g. specific cell types and/or arrangements of cells/tissues in the organ) supporting your identification. For EACH feature listed, provide a brief explanation regarding its functional significance and/or how that feature is helpful in terms of distinguishing the organ. [ANSWER] |
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