RESPIRATORY SYSTEM
Objectives
You should be
able to recognize the following structures:
| Nasal Cavity: | Lung: | |||
| respiratory epithelium | visceral pleura | |||
| mixed glands | bronchus | |||
| cilia | bronchiole | |||
| goblet cells | respiratory bronchiole | |||
| olfactory mucosa | alveolar duct | |||
| olfactory epithelium | smooth muscle in alveolar duct wall | |||
| basal cells | pulmonary artery | |||
| Bowman's glands | pulmonary vein | |||
| olfactory nerve fibers | capillary | |||
| nasal septum | alveolus | |||
| nasal conchae | interalveolar septum | |||
| swell bodies | type I epithelial cell | |||
| type II epithelial cell | ||||
| Trachea: | alveolar macrophage | |||
| mucosa | ||||
| U-shaped cartilage rings | ||||
| mixed glands | ||||
| trachealis muscle | ||||
| Larynx: | |||
| epiglottis | |||
| elastic cartilage (H&E and elastic stains) | |||
| dorsal and ventral epithelium | |||
| true vocal cord | |||
| vocal ligament | |||
| vocalis muscle | |||
| false vocal cord | |||
* * * * *
 |
Blue Histology |
Slides
 | D-75 | Bronchial tree (H&E) | |
| D-77 | Lung monkey (H&E) | |
| D-79 | Lung (H&E) | |
| D-80 | Trachea (H&E) | |
| D-83 | Nose, monkey (H&E) | |
| D-99 | Larynx and epiglottis (H&E) | |
| D-100 | Vocal Cord (H&E) |
Optional slides
| D-76 | Bronchus with lymph node (H&E) | |
| D-78 | Lung, 5 1/2; month fetus | |
| D-91 | Hard palate | |
| D-98 | Epiglottis (elastic stain) |
Slide descriptions
D-83 Nose, monkey (H&E)
Begin the study of this slide by gross observation with the aid of the diagram of a coronal section in your printout. If you hold the slide as oriented above you can see the hard palate at the right consisting of the palate bone and the oral mucosa on its surface. The nasal septum is in the midline towards the left. The large cavities are the air passageways, and protruding into these spaces are two large masses on each side that represent the middle and inferior nasal conchae. They increase the surface area of the lateral walls of the nose. Turbinate bones support the conchae. These bones have yellow bone marrow inside which makes them look hollow. In removing this block of tissue, the maxillary sinuses were largely cut away, but their medial walls remained intact on some of the slides. The sinuses communicate with the nasal cavity by a small meatus which may or may not show up on your particular slide. Just above and below the hard palate, parts of maxillary tooth roots may be found embedded in their sockets. The mucosa of the entire nose is underlain by bone, except for the mid anterior part of the nasal septum, which is cartilage. More posteriorly this cartilage abuts directly with the bone of the bony septum (illustration). From there on down the nasal cavitay is supported entirely by cartilage.
Note at low magnification that most of the lining of the nasal cavity, including that of the sinus walls, is made up of a pseudostratified epithelium that is ciliated, and in most places contains large numbers of goblet cells. Numerous mixed (serous and mucous) glands extend into the lamina propria from the epithelium. This respiratory mucosa covers most of the respiratory passageway from the nose all of the way down through the bronchi. Study the various cell types at higher magnification. Now compare the epithelium in various parts of the nose. There is a generalization that the epithelium is taller, with more glands and goblet cells in areas that are more exposed to the passage of air. Can you confirm this? How does the mucosa lining the maxillary sinus differ from that in the passageways of the nose. Does this make sense to you?
The lamina propria is extremely richly vascularized to provide an ample supply of blood to cool or warm the inspired air as appropriate. On the conchae the blood vessels are exaggerated into large venous plexuses called swell bodies. Their dilation and engorgement controls the passage of air through each side of the nose. These are the vessels that "stop up" your nose during allergic reactions, and this can be temporarily alleviated by vasoconstrictive drugs. Swell bodies have a dense collagenous walls to define their size when swollen with blood.
Olfactory mucosa covers a relatively small, specific region of the upper part of the septum and the upper part of the lateral walls of the nasal cavity, including the superior concha (see the low power picture of slide D-83 above).. It is concerned with the sense of smell and is fundamentally nervous tissue. Olfactory epithelium is a pseudostratified columnar type taller than the adjacent respiratory epithelium. The sudden transition from respiratory to olfactory epithelium is readily apparent, even under low power, because the goblet cells and mixed glands abruptly stop at the edge of olfactory epithelium. The lamina propria underlying the olfactory epithelium does have numerous glands. These are the rather specialized serous glands (Bowman's glands) that provide a fluid to wash away old odors. One of their distinctive structural characteristics is that their ducts enlarge just under the surface epithelium. The glands clearly are serous but many people refer to their secretion as mucus, for some strange reason. Maybe these people just have not looked at these glands.
Three cell types make up olfactory epithelium. The most numerous are sustentacular cells ( = supporting cells). They extend from the basement membrane to the surface. The olfactory neurons are the critical cells of the epithelium. They are round with two processes. A dendrite goes to the surface where it sprouts several long non-motile stereocilia which bear the receptors for odoriferous molecules. These stereocilia are very aberrant cilia and are not to be confused with the long microvilli in the epididymis which unfortunately have been given this same name. The neurons also send an axon down through the basement membrane. It joins with others in the lamina propria to form an axon bundle which travel to the olfactory lobes of the brain (a very short distance). The nuclei in those axonal bundles, of course, belong to Schwann cells, characteristic of peripheral nervous tissue. Are these bundles of axons surrounded by perineural sheaths? They illustrate why small unmyelinated nerves are a classical "look-alike" of smooth muscle. Think for a moment how you would distinguish the two.
The third cell type is the basal cell. It is the stem cell, able to divide and differentiate into both sustentacular cells and olfactory neurons. Olfactory neurons are very unusual in that they can be replaced by new neurons that differentiate in an adult. This is pretty important because the cell bodies of these neurons are very exposed. Think of what happens to them in the nose of a person who smokes tobacco - or worse. Late middle-aged and elderly persons often loose the ability to smell, a condition called anosmia.
Since the full thickness of the hard palate is included on this slide, briefly note that its oral surface is lined with keratinized stratified squamous epithelium with pronounced rete pegs. Substantial patches of mucous glands underlie this mucosa. Finally, the two bones of the hard palate meet in the midline as an extraordinarily complex interdigitating suture joint. It is worth noticing if for no other reason than its beauty.
D-99, Larynx and epiglottis (H&E)
The epiglottis is shown attached to the wall of the larynx. The various other cartilages and muscles of the larynx have been bypassed in this section. The epiglottis is a flap of tissue, which closes off the upper entrance to the larynx during swallowing. You examined it earlier to see its core of flexible elastic cartilage (worth another peek now). During breathing the epiglottis stand straight up from the anterior side of the larynx. Swallowing pushes it back across the opening of the larynx so that the posterior surface is folded underneath and food passes across the anterior surface. The epithelium reflects the local pattern of abrasive stresses, so that the upper anterior surface is covered with stratified squamous nonkeratinized epithelium. The lower ventral surface is lined largely with pseudostratified columnar ciliated epithelium except along the periphery where it slaps down on the rim of the larynx during swallowing. There it can be partially converted towards a stratified squamous form. How would you describe the epithelium along the posterior side of the epiglottis on your slide (view1, view2)? The submucosa has substantial amounts of elastic fibers, blood vessels and on the posterior surface, nicely visible mixed mucous-serous glands. Where would you draw the line between the lamina propria and the submucosa?
This slide is great for looking at the various structures you expect to see in ordinary loose connective tissue, the full range of small blood vessels: macrophages which live up to their name of foam cells, mixed glands, small nerve bundles and even a parasympathetic ganglion or two.
D-100 Vocal Cord (H&E)
Hold this slide vertically, with the label at the top. This is a longitudinal (frontal) section through the lateral wall (side wall) of the larynx. It passes through the false vocal cord above and the true vocal cord below. These two ridges in the lateral wall of the larynx are separated by a cleft called the ventricle. They can be seen easily with the naked eye, and you should appreciate where they are in terms of gross anatomy.
The true vocal cord contains two main functional components, the vocalis muscle and the vocal ligament. Both appear in cross-section on this slide (illustration). Skeletal muscle is easily recognized, but the ligament is a bit trickier to identify. Ask yourself what a small ligament should look like in cross-section. Then, examine the connective tissue just under the epithelium at the angular part of the vocal cord to see if it fits this expectation. There has been some inevitable tissue shrinkage which has separated the thick round collagen fibers from each other a bit. Among them are fibroblast nuclei looking small and round because they are elongated in the direction of the fibers (illustration, higher mag.) Some of you looked at a much more massive ligament in the suture of the calvarium (on slide D-142). If that image is fuzzy in your mind try this refresher. The vocal ligament is overlain with a stratified squamous epithelium. (A patch of hyperplastic epithelium may be present on the true vocal cord. If so, or if not, do not worry about it). The transition between the stratified squamous epithelium and the surrounding typical respiratory epithelium is relatively sharp but does show some interesting transitional forms (ohhhhh, stratified columnar epithelium! ). The muscles and muscular actions which control the tension in the vocal cords are very complex. Wait for the gross anatomists to explain them.
The false vocal cord is not much to write home to Mom about. It is covered with typical respiratory epithelium and has a large number of mixed glands in the submucosa. Lymphocytes almost invariably infiltrate into the tissue around the ventricle between the false and true vocal cords.
D-80 Trachea (H&E)
The respiratory tree begins with the trachea. Exactly what is visible on your slide depends on whether the plane of section passes mainly through one of the U-shaped supporting cartilages, or between the two neighboring cartilages. My section, above, cut through a ring at the bottom but sliced between rings to the right. This makes it easy to see that the main masses of glands lie about as deep from the epithelium as the cartilages but are located between cartilages. The cartilages are U shaped instead of complete rings. A band of smooth muscle, the trachealis muscle, stretches between the two ends of a cartilage to form the posterior wall of the trachea. The muscle fibers insert into the dense elastic fiber bundles surrounding the tracheal cartilages and are joined to the mucous membrane by a layer of loose connective tissue.
What is the function of the cartilage? If your answer is correct, why are the rings incomplete (with a U instead of an O shape)?
The mucosa is typical of the respiratory tract. Note its very tall pseudostratified ciliated epithelium.
D-75 Bronchial tree (H&E)
This slide has sections through a branching bronchus and associated structures. Note the typical respiratory epithelium with goblet cells, plates of cartilage, smooth muscle and mucous glands in the lamina propria and submucosa of the bronchi illustration. The smooth muscle is a particularly important component. It has the form of ribbons that spiral down the bronchus and continue along the air passage ways all the way down to the alveolar sacs. These bronchi are intrapulmonary (i.e. within the lung) as shown by the presence of lung tissue around them. Do not waste much time examining this lung tissue because it has been allowed to collapse, is poorly preserved and has pathological features.
Branches of the pulmonary artery and pulmonary vein accompany the bronchi, getting smaller and smaller as the size of the bronchus decreases. You should be able to distinguish artery from vein if you keep in mind that pulmonary arteries have relatively thin walls for their size. The walls are thin because the pulmonary blood pressure is considerably lower than the systemic.
You cannot miss seeing the clusters of macrophages, black from the dust particles that they have ingested. Examine several of the clusters and note that they are located in seams of connective tissue. This is where macrophages permanently retire after they engorge themselves with indigestible particles. Many of the macrophages have taken up so many dust particles that the only detail you can see is their cell boundary. Other cells have taken up less opaque material allowing you to see that they are rounded up cells with a large pale nucleus. Note that many of these cells, probably lay in pools of fluid. This condition would be considered pathological. The presence of numerous plasma cells reinforces this supposition. On your other slides you will have to hunt to find macrophages in the alveoli.
D-77 Lung monkey (H&E)
This slide is excellent for observing the smaller parts of the respiratory tree. The lungs that one normally sees after surgery or upon autopsy are more or less collapsed and gravely distorted from their patterns in life. To prevent this, steps were taken to avoid collapse while preparing slide D-77 from a monkey. Using low magnification you are able to observe: bronchus, respiratory bronchiole, alveolar duct, alveolar sac, alveolus and interalveolar septum (illustration). Because this monkey was small you will find relatively few bronchioles. All or nearly all of the candidates will have at least a bit of cartilage to one side. In a human lung there are more sections through bronchioles than bronchi, but not here. How do these two structures relate to the lobular organization of the lung? At higher magnification, look at the epithelium, glands and smooth muscle of the bronchi.
Next, find a respiratory bronchiole (longitudinal, transverse). Note its relationship to the pulmonary artery (illustration). Typically, the two run next to one another with the alveoli extending from the portion of the wall of the bronchiole farthest from the artery.
Alveolar ducts (longitudinal, transverse) are particularly difficult to comprehend, yet they are a most important structural entity. Only they and alveolar sacs can significantly expand or contract during inspiration or expiration of air. Thus, volume change is an accommodation of these ducts. To pick out alveolar ducts it is necessary to realize that the alveoli themselves are not parts of the walls of the ducts but outpocketings from the walls. The wall itself is composed of little more than minimal bands of smooth muscle overlain by a low epithelium (illustration). Often a section through the wall shows only a single smooth muscle cell cut transversely as a bulge at the end of an alveolar wall. If the structure of the alveolar duct is not completely clear to you consult your lecture notes or textbook or talk with us. Alveolar sacs are merely the somewhat expanded tip ends of alveolar ducts. They have the same basic structure and function as the ducts - and are no big deal.
Use oil immersion, to study the interalveolar septum. It is formed of two extremely attenuated sheets of epithelium enclosing many capillaries and very little other connective tissue. The many elongated nuclei belong largely to type I epithelial cells and endothelial cells. It is not possible for you to distinguish between these two cell types. The two other cell types are type II epithelial cells (septal cells) and macrophages. Type II cells are cuboidal and usually occur in the corners where two interalveolar walls come together (example #1, example #2). They produce surfactant. Alveolar macrophages can be found anywhere. Once laden with dust they like to retire to a comfortable patch of connective tissue and just veg (example #1, exampl#2).
One thing to get straight is the blood vasculature. Pulmonary arteries run alongside the air passageways as far down as respiratory bronchioles. Pulmonary veins, in contrast, go their separate way when the bronchi become bronchioles. Be sure that you can tell pulmonary arteries and veins apart . Obviously you will look at the smooth muscle in their walls to do so (example). Be aware, however, that the walls of pulmonary arteries are thinner than those of systemic arteries because the pulmonary blood pressure is lower.
D-79 lung human (H&E)
Slide D-79 is human lung collected at autopsy in a routine manner. It is not a pretty sight. The slide is included for comparison with the expanded monkey lung. Because humans are big, D-76 allows you to see some large bronchioles. Their longitudinal mucosal folds should be visible. Also, pulmonary arteries and pulmonary veins are adequately preserved. Otherwise it is not worthwhile to look extensively at this slide. If you just have to keep looking instead of going home, go back to slide D-78.
* * * * *
Optional slides
D-76 Bronchus with lymph node (H&E)
Slide D-76 shows another section through the branching bronchial tree, upstream from the last slide. The pulmonary vessels have not been included but the tissue is better preserved than on D-75 and has a lymph node. The smaller of the two sections through the bronchial lumen, cut more transversely, shows a scalloped appearance. Bronchi characteristically have pronounced mucosal folds that run longitudinally. Note the typical respiratory epithelium and mixed glands in the submucosa (illustration).
The lymph node contains many black macrophages but mostly restricted to its medulla, as one would expect (illustration). Elsewhere on the slide there are few "dust cells" to be seen. Right under the epithelium of the bronchi you can find a few scattered about along with some lymphocytes.
Look at the tissue remaining on the outside of the bronchi. What type is it? This section must be from the hilus of the lung where the bronchi are running in connective tissue, instead of being intrapulmonary as is the case for D-75. The small arteries along the edge are systemic instead of pulmonary (illustration); look at the thickness of their media. These vessels probably supplied blood to the connective tissue of the hilus region or to the bronchial arteries that nourish the tissue of the bronchi.
D-78 fetal lung (5 1/2 months) (H&E)
The lung develops in the fetus in much the same manner as a gland. An outpocketing from the foregut (endoderm) grows out first as the trachea and then branches into bronchi and bronchioles. Branches of the pulmonary artery follow the bronchi and bronchioles. The obvious ducts you see at low power will become bronchi. Most of the other ducts will become alveolar ducts but their alveoli have not yet formed. The epithelial cells are cuboidal. When alveoli form these cells will stretch out to be very squamous. Here and there you can see some cilia in the larger ducts. Do you see goblet cells?
Pulmonary arteries are easy to find. Pulmonary veins less so. Remember that the veins tend to lie between lobules. Using them as a guide and the shrinkage artifacts that emphasize the small amount of interlobular connective tissue see if you can find or imagine lobules.
The visceral layer of pleura is visible but not too exciting.
We really should think of the lung as a gland - which secretes CO2
D-91 Hard palate, monkey (H&E)
This slide shows a lot of stuff. You guys probably will try to waste your time on the oral side with its teeth masticatory epithelium and palatal glands. This is a mistake. The interesting side is to top side - just ask any ears nose and throat specialist. The part of the nasal cavity shown here more or less repeats what you saw in D-83. Note that over the (inferior) conchae the numerous glands are mucosal while the swell bodies are submucosal. Yes, you are right, the swell bodies are very poorly preserved but you can still figure out what layer they were in.
D-98 Epiglottis (elastic stain)
The stain on this slide emphasizes the large amount of elastica in the cartilage and throughout the epiglottis. It gives you a chance to compare the appearance of elastic cartilage under elastic stain and H & E. Use your detective skills to deduce which surface is anterior and which posterior. Has the epithelium of the anterior surface been modified in this slide as it was in the last one. How would you classify the epithelium along the posterior surface (e.g. here)?