Ross & Pawlina (6th ed.), Plates 70-73, pp. 690-697
Ross & Pawlina (6th ed.), Ch 19, pgs. 664-687, 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:
The larynx is a passageway for air between the oropharynx and trachea which also functions in the production of sound. It is lined by pseudostratified columnar epithelium (see left hand side of the slide), stratified squamous epithelium (covering the vocal cords near the middle of the slide), and stratified columnar epithelium between these epithelial types. Seromucous glands are present underneath the epithelium on both sides of the slide. Identify the vocal cord as a mucoal fold covered by stratified squamous epithelium and containing abundant skeletal muscle. Notice the cartilage framework. Do you see any bone?
Examine this slide at low power to acquire an overview of the trachea (bottom of slide) and its relationship to the esophagus (top of slide). This transverse section demonstrates the C-shaped cartilage ring of the trachea and smooth muscle along the posterior (open) portion of the cartilage. Several nice examples of nerves are also present in the connective tissue surrounding the trachea. Unfortunately, the tracheal epithelium is not well preserved and mucosal cellular detail is better studied on Webslide 8 and Webslide 200.
In this longitudinal section the lumen of the trachea is at the bottom of the slide. 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.
(As a review exercise, try to identify the structure whose large lumen is located at the top of this slide.)
Use this well-preserved specimen to examine the key structural features of the trachea, including layering in the wall, epithelial types, organization of cartilage, and presence of glands. This slide shows excellent examples of the ciliated pseudostratified columnar surface epithelium, sero-mucous glands in the connective tissue, as well as many instances of plasma cells and mast cells in the lamina propria.
This section contains examples of the distal branches of the bronchial tree. Scan the slide to find bronchi and bronchioles, using the Table at the end of this lab 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 respiratorybronchioles indented with alveoli. Simple squamous alveolar ducts communicate with alveolar sacs and blind-ended alveoli. Some alveoli contain alveolarmacrophages, large rounded cells that appear to be suspended in the air space. Do not spend time examining this slide for alveolar Type I and II cells, as they are much better preserved on the next slide (Webslide 0028).
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 Clara 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 (great alveolar cells) 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. Endothelialcells, 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 mastcells containing dense pink granules are also visible. (Due to the perfusion fixation method employed here, few alveolar macrophages are present, and are best observed in Webslide 27).
The epithelium lining the trachea is typical respiratory epithelium (ciliated pseudostratified columnar) that contains numerous goblet cells. This epithelium has an unusually thick basementmembrane, 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. Slide 40 doesn't show the characteristic C-shape of the rings, but LSU slide B-20 is sectioned in a manner that is more representative of the actual anatomy of the trachea as traditionally shown in textbook figures. The bottom of Slide B-20 shows the posterior aspect of the trachea where you can see bundles of horizontally oriented smooth muscle (collectively called the "trachealis" muscle) that span the posterior ends of the C-shaped rings and can act to adjust the diameter of the trachea.
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 Clara cells) and lack cilia altogether. The smooth muscle layer is generally quite prominent in these structures as demonstrated in slide 132-2 [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 Clara 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].
III. Appearance of the lung in congestive heart failure
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.