Microanatomy of the Respiratory System - Dr. Kwaku Nantwi 2024-25 Notes

Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 1 of 27 MICROANATOMY OF THE RESPIRATORY SYSTEM Lecture Learning Objectives: 1. Describe the components of the 2 major divisions of the...

Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 1 of 27 MICROANATOMY OF THE RESPIRATORY SYSTEM Lecture Learning Objectives: 1. Describe the components of the 2 major divisions of the respiratory system. 2. Describe the microanatomy and function of the nasal cavity and nasopharynx. List the 3 major components of the nasal cavity and describe the epithelium of each. For the olfactory mucosa, describe the cell types of the epithelium and their function, apical specializations, their turnover, the lamina propria, and glands at the LM (H&E) level. Describe the olfactory transduction process. For the respiratory mucosa, describe the cell types of the epithelium and their function, apical specializations, the lamina propria and glands at the LM (H&E) level. Describe the epithelium, lamina propria of the nasopharynx and note the direction of mucus flow. 3. Describe the microanatomy and function of the nasal cavity and nasopharynx. Describe the mucosa, submucosa, and wall of the larynx including the modification of the general plan at certain sites. Diagram the larynx and include the following: vocal fold, vestibular (false vocal) fold, glottis, infraglottic cavity, ventricle, vestibule, vocalis muscle, laryngeal and tracheal cartilages 4. Describe the microanatomy and function of the trachea. Describe the mucosa, submucosa, cartilagenous layer, and adventitia of the trachea. Describe the 5 types of cells found in the tracheal epithelium, the function of each, the classification of this type of epithelium and any relationship between the cells and nerve processes. Describe the histologic basis of bronchitis. Describe the anatomical orientation and form of the tracheal cartilages as well as their histological classification and association with smooth muscle. 5. Describe the microanatomy of bronchi. Compare and contrast the histological organization of the bronchi with the trachea. Compare the histological organization of extrapulmonary and intrapulmonary bronchi. Describe the microanatomy of intrapulmonary bronchi. Describe the organization of a bronchopulmonary segment. 6. Describe the microanatomy of bronchioles. Compare and contrast the histological organization of the bronchioles with the bronchi. Describe the new cell type added to bronchioles, its function, and the cell types lost in the epithelium. Describe the terminations of the bronchiolar tree. Describe a pulmonary lobule. Describe a terminal bronchiole. 7. Describe the microanatomy and function of the respiratory division of the respiratory system Describe a respiratory bronchiole. Describe the histological basis for emphysema and cystic fibrosis. Describe alveoli, alveolar ducts, and alveolar sacs. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 2 of 27 8. Describe the alveolar wall (inter-alveolar septum). Describe the cells that comprise the alveolar wall Describe the constituents of the thick and thin parts of the alveolar wall. Describe the functional morphology and recognition features in LM and TEM of type I alveolar cells, type II alveolar cells, and alveolar macrophages. Describe the components of the alveolar membrane (gas exchange barrier). Lecture Content Outline I. Divisions of respiratory system A. Conducting (upper) division B. Respiratory (lower) division II. Nasal cavity and nasopharynx A. Vestibule B. Respiratory portion C. Olfactory mucosa of the nasal cavity D. Nasopharynx III. Larynx IV. Trachea A. Mucosa B. Submucosa C. Cartilaginous layer D. Adventitia V. Bronchi A. Extrapulmonary bronchi B. Intrapulmonary bronchi VI. Bronchioles A. Characteristics B. Bronchiolar epithelium C. Lamina propria D. Muscularis mucosa E. Terminations (terminal bronchioles) VII. Respiratory division of respiratory system A. Respiratory bronchioles B. Alveolar ducts C. Alveoli D. Alveolar pores (pores of Kohn) VIII. Alveolar wall (inter-alveolar septum) A. Capillaries B. Pores of Kohn C. Alveolar lining cells D. Other cells E. Alveolar membrane (gas exchange barrier) Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 3 of 27 MICROANATOMY OF THE RESPIRATORY SYSTEM I. DIVISIONS OF RESPIRATORY SYSTEM A. Conducting (upper) division 1. Components: nasal cavity, nasopharynx, oropharynx, laryngopharynx, larynx, trachea, bronchi, and bronchioles through the level of terminal bronchioles. 2. Function a. The upper conducting division functions to transmit air to lower division while warming, moistening, and removing particulate material. b. To carry out the function of conduction, a combination of cartilage, elastic fibers and smooth muscle fibers provides the conducting passageways with a rigid structural support as well as flexibility. c. Warming, moistening and removing particulate material are carried out by specific structural features: thin walled blood capillaries/venules, goblet cells, extensive surface area, coordinated beating of cilia in the epithelial cells etc. B. Respiratory (lower) division 1. Components: respiratory bronchioles, alveolar sacs, alveolar ducts and alveoli. 2. Function: The lower division serves as the site of gas exchange between air and blood. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 4 of 27 II. NASAL CAVITY AND NASOPHARYNX A. Vestibule of nasal cavity - inside nostrils 1. Lined with stratified squamous epithelium, slightly keratinized. Contains some sebaceous and sweat glands and some hair follicles. 2. The underlying connective tissue blends with the perichondrium of nasal cartilage. B. Respiratory portion of nasal cavity – main area 1. Lined with pseudostratified columnar epithelium (Figure 1) a. Also called Figure 1. Respiratory epithelium. "respiratory epithelium" - characteristic of the majority of the conducting division of the respiratory system). b. Main cell types include: i. ciliated cells - beat synchronously in a posterior direction ii. goblet cells - produce mucus which traps particulate matter iii. basal cells - precursors of other cell types Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 5 of 27 iv. small granule cells - resemble basal cells, but contain secretory granules. These are neuroendocrine cells of the amine precursor uptake and decarboxylation (APUD) system. v. brush cells - a general name for cells that bear short blunt microvilli. 2. Lamina propria a. Contains both mucus and serous glands b. Contains a complex set of capillary loops, which are classified as thin-walled veins. The arrangement of the loops permits incoming air to be warmed by blood flowing through the upper part of the loops. c. Connective tissue (CT) blends with periosteum or perichondrium of nasal cavity. 3. Paranasal sinuses - continuous with the nasal cavity. Lining is slightly thinner. C. Olfactory mucosa of the nasal cavity 1. Located in the roof of each nasal cavity and extends a short distance down over the superior concha and the adjacent nasal septum. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 6 of 27 2. Epithelium is exceptionally tall pseudostratified columnar and lacks goblet cells. It contains four types of cells (Figure 2): a. Supporting (sustentacular) Figure 2. Olfactory epithelium in the olfactory mucosa. cells Figure 11-4A from Cui, 1st ed., 2011. i. Tall cylindrical cells, broad at the apex and narrow at the base. They are characterized by apical microvilli. ii. They provide metabolic and physical support for olfactory cells. b. Basal cells i. Small conical cells with dark ovoid nuclei. ii. The basal cells are mitotically active and able to regenerate olfactory function in a few weeks. c. Sensory cells - bipolar neurons distributed between the sustentacular cells. Their somas are in the middle layers of the epithelium. i. Spindle shaped cells with an apical process or dendrite which extends to the surface. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 7 of 27 ii. Termination of the apical process is in a bulb-like expansion, the olfactory vesicle or knob, with 10-20 modified cilia. iii. The proximal part of the cell extends into the lamina propria as an axon, which continues to the olfactory bulb of the brain. Axons are clustered into unmyelinated bundles by glial ensheathing cells. Figure 3. Illustration of olfactory transduction pathways. Figure 19.4 from Ross and Pawlina, 7th ed., 2016. iv. Cilia contain about 350 types of odorant receptors (OR) for different odorants. Olfactory transduction pathways mediate odorant perception (Figure 3). Odor perception is via G-protein-linked receptors that become activated through 2nd Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 8 of 27 messenger transduction. Individual cells vary in sensitivity but can detect the full range of odorants. v. The neurons are unusual in that they are continuously replaced and have only a 30-60 day life span. d. Brush cells - cells with apical microvilli which function in general sensation. The cells are presynaptic to afferent fibers. 2. Lamina propria a. Contains branched tubule-alveolar glands (olfactory glands or Bowman glands) i. Produce a watery secretion to trap odorants and moisten and cleanse the surface. ii. These glands also release lysozyme and secretory IgA. iii. The glands deliver their secretions (proteinaceous) via ducts onto the surface of the epithelium. b. Large unmyelinated olfactory nerves can be seen in the lamina propria prior to passing through the cribriform plate to terminate in the olfactory bulb. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 9 of 27 D. Nasopharynx 1. Lining epithelium is mostly pseudostratified ciliated columnar with goblet cells. Cilia propel material toward oropharynx. In areas of abrasion, the epithelium may be stratified squamous. 2. Lamina propria contains glands, mostly mucous; abundant lymphatic tissue including nasopharyngeal tonsils (adenoids). 3. Muscle - pharyngeal constrictors. III. LARYNX Important role in phonation in addition to role as part of respiratory conducting system. A. Components (Figure 4): 1. Includes the ventricular (false vocal) folds 2. The true vocal folds. 3. The walls are made up of supporting cartilages, interconnecting ligaments, intrinsic and extrinsic muscles and mucosal lining. a. The larynx provides a carefully guarded passageway between the Figure 4. Frontal section of larynx. Figure 19.5a from Ross and Pawlina, 7th ed., 2016. pharynx and the trachea. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 10 of 27 b. During swallowing, movements of the cartilages close the entrance to the larynx so that food and water cannot enter. B. Mucosa: Components 1. Lined with non-keratinized stratified squamous epithelium in regions of abrasion and pseudostratified ciliated columnar (respiratory) epithelium in the rest of the areas. 2. Although there is variability, the ventricular folds are typically covered with pseudostratified columnar epithelium whereas the vocal folds are covered with stratified squamous, non-keratinized epithelium. 3. In general, respiratory mucosa may change to stratified squamous epithelium. a. This change is referred to as Squamous Metaplasia. b. It is generally reversible, but in areas of altered air flow the change may persist. c. Clinical correlation: (Chronic bronchitis, See Section V) C. Submucosa: Components 1. Contains few serous and many mucous glands. 2. The true vocal folds contain a supporting ligament of fibroelastic connective tissue and the skeletal muscle of the vocalis muscle. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 11 of 27 D. Other components of wall of larynx 1. Wall is supported by hyaline and elastic cartilages. 2. Also contains connective tissue, and skeletal muscle. IV. TRACHEA - a 10 cm conducting tube; maintained patent by cartilagenous rings. A. Mucosa - pseudostratified columnar epithelium (respiratory epithelium, Figures 5 and 6) with a distinct basement membrane and Figure 5. Light micrograph of tracheal epithelium. underlying lamina propria. From Erlandsen and Magney, 1985. 1. Ciliated columnar cells a. Most numerous cell type. b. Cilia propel materials upward toward the pharynx, thus protecting the lungs from mucus accumulation. Figure 6. Diagram of tracheal epithelium. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 12 of 27 c. Loss of cilia from irritation leads to mucus accumulation in lungs. Clinical Correlation: Chronic bronchitis i. Respiratory epithelium changes to stratified squamous epithelium, which is not as functionally efficient. ii. Loss of cilia: Removal of mucus is impaired. iii. Coughing as a compensatory means to expel accumulated mucus. iv. Over time, the number of ciliated cells decreases significantly. v. Cycle can be repeated - pathology. 2. Goblet cells a. Mucus traps inhaled dust and other particles. b. Irritation leads to increased secretion. 3. Non-ciliated columnar cells (brush cells) a. Possess apical microvilli. b. Function in general sensation. 4. Basal cells: A stem cell population, whose cells rest on the basal lamina, but do not extend to the lumen. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 13 of 27 5. Neuroendocrine (small granule) cells a. About 8% of the total cell population. b. The granules in the cells are located basally and are released into the underlying connective tissue (CT) where they provide local regulatory control. c. Some small granule cells can be visualized by staining with silver or dichromate salts. d. Similar to the enteroendocrine cells found in the GI system. e. Function is based on the selective control of smooth muscle contraction in walls of blood vessels or bronchial tree. B. Submucosa (Figure 7) 1. Loose connective tissue (but denser than lamina propria) with mixed serous and mucous glands. 2. The submucosa is separated from the mucosa by a band of elastic tissue. 3. Lymphatic tissue is present in the submucosa as well as in the lamina propria. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 14 of 27 Figure 7. Diagram of cross-section of trachea, showing layers of tracheal wall. C. Cartilagenous layer (Figure 7) 1. Patency of the tube is maintained by about 20 C-shaped hyaline cartilages, which have the open part of the C located posteriorly, where it is connected by a bundle of smooth muscle (trachealis muscle). 2. Adjacent cartilaginous rings are connected by fibro-elastic connective tissue which blends with their perichondrium. D. Adventitia (Figure 7) 1. Layer external to the cartilages. 2. Components: connective tissue, blood vessels and nerves. V. BRONCHI - 10 generations of branching. The trachea divides into two branches, the left bronchus and the right bronchus. A. Extrapulmonary bronchi 1. Two branches: right and left primary bronchi Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 15 of 27 2. Branches lie outside the lungs, i.e., they are not confined within the lung lobes 3. They differ from the trachea only in diameter. B. Intrapulmonary bronchi 1. Right lung has 3 lobes and each lobe is served by secondary bronchi; left has 2. 2. The secondary bronchi divide into tertiary segmental bronchi, each of which supplies a bronchopulmonary segment. a. There are 10 broncho-pulmonary segments in the right lung. b. 8-10 segments on the left c. Each segment is separated from the others by a layer of connective tissue. d. Each broncho-pulmonary segment is a discrete anatomical and functional unit. 3. Histologic layers of intrapulmonary bronchi (Figures 8 and 9) are similar to those of the trachea and extrapulmonary bronchi except for: a. Epithelium is initially pseudostratified ciliated columnar with goblet cells, but it changes gradually to ciliated columnar with goblet cells. i. Ciliated cells transport material upward and out of the lungs. (Goblet cell mucus is Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 16 of 27 typically protective, but excess secretion may occlude airways.) ii. Basal cells and endocrine cells are also present. Figure 8. Diagram of cross-section of intrapulmonary bronchus, showing layers of bronchial wall. b. Interlacing bundles of smooth muscle (muscularis mucosae) are added to the mucosa at the border with the submucosa; thus, the layers are: i. Mucosa (with muscularis Figure 9. Cross-section through an intrapulmonary bronchus. mucosae) ii. Submucosa (with glands) iii. Cartilagenous layer iv. Adventitia Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 17 of 27 c. C-shaped cartilages are replaced by irregular plates of hyaline cartilage of decreasing size. Figure 10. Left: Diagram of a cross-section of a bronchiole. Right: Diagram of a cross-section of a respiratory bronchiole. Wall layers are shown. VI. BRONCHIOLES Air passageways of 1 mm or less in diameter; exhibit 10 more generations of branching A. Characterized by relatively thin walls with no cartilage and no submucosal glands, and relatively more smooth muscle (Figure 10, right half). B. Bronchiolar epithelium – simple columnar or simple cuboidal. 1. Ciliated cells continue throughout. 2. Goblet cells may be present in larger bronchioles but are lost in subsequent branchings and are replaced by club cells. 3. Club cells or nonciliated bronchiolar cells (formerly called Clara cells). Note: The cells were originally named “Clara cells” after Dr. Max Clara. However, it has more recently been uncovered that Max Clara was a Nazi histologist who conducted research on the corpses of executed prisoners. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 18 of 27 Therefore, there has been an initiative to rename these cells “club cells”. You will still need to be aware of the name Clara cells, only because you will see this name if you read past literature, and it is unfortunately still used in some books and articles. However, when referring to these cells in the future, please call them "club cells". a. Dome-shaped apical margin. b. Have appearance of typical protein-secretory cells (rER, Golgi, secretory granules). c. Functions of club cells: i. Secrete one of the components of surfactant which prevents luminal adhesion. ii. Synthesis of club cell protein (CC16), which is a measurable pulmonary marker. iii. Club cells are also implicated in Cl- transport (water following) into the lumen which aids in keeping luminal surface moist. Lack of Cl- transport is a Figure 11. Schematic diagram of pathology in cystic fibrosis. Figure F19.3.1 from Ross factor in cystic and Pawlina, 7th ed., 2016. fibrosis (Figure 11). Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 19 of 27 iv. They act as reserve cells; they are able to divide, differentiate and replace other damaged cells. v. They contain enzymes/enzyme systems which can detoxify noxious substances. 4. Other cells: a. Small granule cells b. Basal cells c. Brush cells C. Lamina propria contains elastic fibers. D. Muscularis mucosa increases and is surrounded by CT of adventitia. E. Terminations (Figure 12) 1. Pulmonary lobule a. A “lobular” bronchiole supplies air to a unit of the Figure 12. Structure of a pulmonary lobule. Modified lung from Figure 13-23 in Histology and Cell Biology (Kierszenbaum and Tres, 4th ed., 2016). parenchyma called a pulmonary lobule (sometimes called a secondary pulmonary lobule). Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 20 of 27 b. The tissue in the lobule is surrounded by an interlobular septum, which is connective tissue containing blood vessels and lymphatic vessels. c. The lobules may be visible on the surface of the lung as faintly outlined polygonal areas (~2 cm diameter). d. The lobular bronchiole eventually branches into 3-5 terminal bronchioles. 2. Terminal bronchiole a. Smallest branch of the conducting division. b. Terminal bronchioles further divide into 3 generations of respiratory bronchioles and their associated alveolar ducts. VII. RESPIRATORY DIVISION OF RESPIRATORY SYSTEM A. Respiratory bronchiole - smallest functional unit of lung. This is the first site of gas exchange. 1. Form in groups of two or more by division of terminal bronchioles (Figure 13). Figure 13. Site of the ramification of a terminal bronchiole (right) that continues into two mrespiratory bronchioles. Figure 314 from Sobotta and Hammersen, 1980 Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 21 of 27 2. Pulmonary acinus (Figure 14) a. A respiratory bronchiole defines a pulmonary acinus (sometimes called a primary pulmonary lobule or a respiratory bronchiolar unit). b. It includes the lung parenchyma supplied with air by a single respiratory Figure 14. Structure of a pulmonary acinus. Modified bronchiole. from Figure 13-8 in Histology and Cell Biology (Kierszenbaum and Tres, 4th ed., 2016). 3. Lining epithelium is simple cuboidal epithelium with occasional ciliated cells but mostly club cells. 4. A few alveoli appear as diverticula/outpouching from the bronchiole. 5. The lamina propria contains elastic fibers, which are vulnerable to destruction by elastases and proteases from neutrophils. a. Resulting loss of elasticity gives rise to emphysema, which is characterized by enlarged inter-alveolar spaces (Figure 15). Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 22 of 27 Figure 15. Lung tissue from a patient with emphysema. Figure 11-16B from Cui, 2011. b. Large inter-alveolar airspaces significantly decrease the area available for gas exchange. Efficiency of gas exchange is thus compromised. 6. A circumferential layer of smooth muscle surrounds the connective tissue. B. Alveolar ducts 1. Thin walled tubes lined with bronchiolar cells. The external wall is comprised of fibroelastic connective tissue with intermittent bundles of smooth muscle. 2. Alveolar ducts terminate by branching into 2-4 chambers from which single alveoli or clusters of alveoli (alveolar sacs) branch. C. Alveoli are the terminal air spaces of the respiratory system and the site of gas exchange between air and blood. 1. Close packing of alveoli results in the alveoli being separated only by thin interalveolar septae of delicate connective tissue with collagen, elastic and reticular fibers. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 23 of 27 2. Capillaries are a major structural element of the interalveolar septae. D. Alveolar pores (pores of Kohn) allow air to pass between alveoli. VIII. ALVEOLAR WALL (INTER-ALVEOLAR SEPTUM) A. Capillaries and their endothelial lining cells form the scaffolding of the inter-alveolar septae. The capillaries are enmeshed in elastin and fine collagen fibers. B. Pores that allow for equalization of pressure between alveoli and provide collateral air circulation should a bronchiole be obstructed. C. Alveolar lining cells 1. Squamous alveolar cells (Type I alveolar cells, Type I pneumocytes) - form lining of alveolus. a. Cover 95% of the surface. b. Function: Freely permeable to gases, forming the major site of gas exchange. 2. Great alveolar cells (Type II alveolar cells, Type II pneumocytes) (Figure 16) a. Intermingled with small, squamous alveolar cells and joined with them by continuous system of zonulae occludens. Figure 16. Diagram of a type II (great) alveolar cell, showing the production of surfactant. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 24 of 27 b. Type II cells are equal in number to Type I cells, but cover only 5% of the surface. c. Function: Produce surfactant. i. Contain multilamellar bodies (cytosomes) which are rich in phospholipids and are discharged into the alveolar lumen where they function as part of the surfactant system. ii. The cells incorporate large amounts of choline as they synthesize phosphatidyl- choline. This major component (40%) of surfactant reduces surface tension and is essential for maintaining patency of the alveoli. iii. Surfactant also contains cholesterol (50%) and 4 proteins (SP-A, -B, -C, and -D) which stabilize the surfactant and contribute to immune response. iv. Surfactant deficiency in premature infants leads to collapse of alveoli (atelectasis) and is complicated by seepage of fibrin and other proteins from the capillaries into air spaces, forming a hyaline membrane. d. Type II alveolar cells are able to proliferate to both Type I and Type II cells. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 25 of 27 D. Other cells 1. Alveolar macrophages or dust cells a. Highly phagocytic, very motile cells, found both within the inter-alveolar septum and migrating along the luminal surface of the airway. b. Functions: i. Continuously produce lysosomal enzymes. ii. Many become multinucleate giant cells. iii. Usually repopulate from monocytes. c. Can migrate up the respiratory tree, to ultimately be swallowed or may re-enter the CT of the inter- alveolar septum. 2. Fibroblasts: a few in the inter-alveolar septum produce collagen fibers and elastic fibers. E. Alveolar Membrane - barrier for gas exchange (Figure 17) 1. Gas exchange occurs across the thinnest portion of the barrier which includes: a. Surfactant b. Type I alveolar cell c. Basal lamina of Type I alveolar cell d. Basal lamina of capillary endothelial cell (the two basal laminae are often fused) e. Capillary endothelial cell Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 26 of 27 2. Fluid exchange occurs across portions of the alveolar membrane where cells are thicker and the basal laminae are not fused. Figure 17. TEM of the alveolar wall. The wall is composed of epithelium, capillaries, and connective tissue elements. The alveolar epithelium consists of two types of cells: the squamous alveolar epithelial cell, or type I cell, extends thin cytoplasmic processes over the capillary wall, and the great alveolar epithelial cell, or type II cell, is characterized by the presence of lamellar bodies containing surfactant. In many locations the basal lamina of the epithelial cells fuses into one layer with the respective basal lamina of the capillaries. Abbreviations: Alu, alveolar lumen; Am: gas exchange barrier; Abm, fused basal laminae of epithelial and endothelial cells; Ed, endothelial cell; Fc, fibroblast; R, erythrocyte; Cap, capillary; Sc, squamous or type I alveolar cell; Gc, great or type II alveolar cell; Lb, lamellar bodies; Cf, collagenous fibers; Ef, elastic fibers. Microanatomy of the Respiratory System Dr. Kwaku Nantwi Page 27 of 27 References Cui, D., Daley, W., Fratkin, J.D., Haines, D.E., and Lynch, J.C. Atlas of Histology: With Functional and Clinical Correlations, 1st ed., Lippincott, Williams and Wilkins: Baltimore, 2011, Ch. 11, pp. 199-219. Erlandsen, S.L. and Magney, J.E., Human Histology: A Microfiche Atlas, University of Minnesota Press: Minneapolis, 1985. Kierszenbaum, A.L. and Tres, L.L., Histology and Cell Biology: An Introduction to Pathology, 4th ed., Elsevier-Saunders: Philadelphia, 2016, Ch. 13. Ross, M.H. and Pawlina, W., Histology: A Text and Atlas, 7th ed., Lippincott, Williams, & Wilkins, WoltersKluwer Health: Philadelphia, 2016, Ch. 19, pp. 662-697. Young, B. and Heath, J.W., Wheater’s Functional Histology: A Text and Colour Atlas, 4th ed., Churchill Livingstone: Edinburgh, 2000, Ch. 12, pp. 222-236. KN/: 9/22/21