Respiratory System Histology (University of Northern Philippines) PDF

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LC10 Respiratory System Histology COLLEGE OF MEDICINE, BATCH 2026...

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LC10 Respiratory System Histology COLLEGE OF MEDICINE, BATCH 2026 Transcribers: Flores, Gallego, Gutierrez, Labingdalawa, Malonzo Dr. Abraham Auberon Austria | Nov. 2022 Editors: Flores, Gallego, Gutierrez, Labingdalawa, Malonzo RESPIRATORY SYSTEM RESPIRATORY EPITHELIUM I. RESPIRATORY EPITHELIUM Most of the respiratory structures are lined by pseudostratified A. Conducting Portion ciliated columnar with goblet cells epithelium B. Respiratory Portion C. 5 Major Cell Types 5 MAJOR CELL TYPES 1. Ciliated Columnar Cells 1. Ciliated Columnar Cells 2. Goblet Cells The most abundant cells that extend the entire thickness of the 3. Basal Cells epithelium. The cilia sweep the surface of the epithelium and protect 4. Brush Cells 5. Small Granule Cells the lungs by removing small inhaled particles II. OLFACTORY EPITHELIUM Each with 250-300 cilia on its apical surface A. 3 Major Cell Types 1. Olfactory Neurons 2. Goblet cells 2. Supporting Cells Numerous in the more proximal airways and gradually decrease in 3. Basal Cells number toward the distal parts of the respiratory tube. These cells III. CONDUCTING PORTION contain and release mucus glycoproteins to form a protective layer on A. Nasal Cavities the epithelial surface B. Nasopharynx & Posterior Oropharynx C. Larynx 3. Basal cells D. Trachea Located close to the basal lamina without their apices reaching the E. Bronchi lumen of the epithelium. These cells serve as stem cells for continual F. Bronchioles replacement of other epithelial cells G. Terminal Bronchioles IV. RESPIRATORY PORTION Mitotically active stem and progenitor cells that give rise to the other A. Respiratory Bronchioles epithelial cell types B. Alveolar Ducts & Sacs C. Alveoli 4. Brush cells 1. Type I Alveolar Cell Less numerous than the other cells. Because their basal surfaces 2. Type II Alveolar Cell contact afferent nerve endings, it is believed that these cells function D. Alveolar Macrophage (Dust Cell) as receptor cells V. REFERENCES Columnar cell type, in which a small apical surface bears sparse blunt VI. APPENDIX microvilli Chemosensory receptors resembling gustatory cells, with similar I. RESPIRATORY EPITHELIUM signal transduction components and synaptic contact with afferent Air is moved through the lungs by a ventilating mechanism, consisting of the nerve endings on their basal surfaces thoracic cage, intercostal muscles, diaphragm, and elastic components of the lung tissue. The system can be divided anatomically into the upper and lower 5. Small granule cells (Kulchitsky cells) respiratory tracts Contain numerous membrane-bound granules and are analogous to the enteroendocrine cells of the diffuse neuroendocrine system A. CONDUCTING PORTION (DNES) consists of passageways or tubes located outside (extrapulmonary) Difficult to distinguish in routine preparations, but possess numerous and inside (intrapulmonary) of the lungs that conduct air for gaseous dense core granules 100-300 nm in diameter. exchange to and from the lungs Only about 3% of the cells in respiratory epithelium. B. RESPIRATORY PORTION consists of passageways within the lungs that not only conduct the air but also allow respiration or gaseous exchange CONDUCTING PORTION RESPIRATORY PORTION (Conducts the air toward the lung (Involve in gas exchange) tissue) Nasal cavities Respiratory bronchioles Pharynx Alveolar ducts and sacs Larynx Alveoli Trachea Figure 1. Microscopic view of the major cell types found in the Respiratory Bronchi Epithelium which is a classic example of pseudostratified ciliated columnar Terminal bronchioles epithelium Table 1. Components of respiratory system Page 1 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria II. OLFACTORY EPITHELIUM Highly specialized sensory pseudostratified epithelium Lacks goblet cells or motile cilia on its cells A specialized region of the mucous membrane covering the superior conchae at the roof of the nasal cavity. In adult humans, it is about 10 cm2 in area and up to 100μm in thickness MEDICAL APPLICATION: Anosmia or Hyposmia o Loss or reduction of the ability to smell o Caused by traumatic damage to the ethmoid bone that severs olfactory nerve axons or by damage to the olfactory epithelium caused by intranasal drug use. 3 MAJOR CELL TYPES 1. Olfactory Neurons Sensory bipolar neurons that are distributed between the more apical supportive cells and the basal cells present throughout the epithelium. Figure 2. Cell types of olfactory epithelium. (ON) Olfactory neurons, (S) Nuclei form an irregular row near the middle of this thick epithelium Supporting cells, (B) Basal cells. The apical (luminal) pole of each olfactory cell is its dendrite end and has a knoblike swelling with about a dozen basal bodies, from which III. CONDUCTING PORTION long cilia project into the overlying aqueous layer Cleans and humidifies inspired air and provides conduits for air Cilia have non motile axonemes and collectively provide a large movement to and from alveoli. surface for transmembrane chemoreceptors To ensure an uninterrupted supply of air, a combination of cartilage, The receptors respond to odoriferous substances by generating an collagen and elastic fibers, and smooth muscle provides this portion action potential along the axons extending from the basal ends of with rigid structural support and the necessary flexibility and these neurons extensibility. They form the olfactory nerve (cranial nerve I) and eventually synapse with neurons in the olfactory bulb of the brain A. NASAL CAVITIES axons leave the epithelium and unite in the lamina propria as very Divided based on the epithelium covering the nasal cavities small nerves that then pass to the brain through foramina in the cribriform plate of the ethmoid bone 1. Vestibules of the nasal cavities Best-known neurons to be replaced regularly because of regenerative - Epithelium: Stratified squamous, keratinized to non-keratinized activity of the epithelial stem cells from which they arise (as you go inside) - Glands: Sebaceous and sweat glands 2. Supporting Cells - Musculoskeletal Support: Hyaline cartilage Columnar, with narrow bases and broad, cylindrical apexes containing - Features and Functions: Vibrissae (stiff hairs) and moisture both the nuclei and extending microvilli into the fluid layer. filter and humidify air Express abundant ion channels that help maintain a - Outermost part microenvironment conducive to olfactory function and survival. 2. Most areas of nasal cavities 3. Basal cells - Epithelium: Respiratory Small, spherical or cone-shaped cells near the basal lamina. - Glands: Seromucous glands These are the stem cells for the other two types, replacing the - Musculoskeletal Support: Bone and hyaline cartilage olfactory neurons every 2-3 months and support cells less frequently - Features and Functions: Rich vasculature and glands; warm, humidify, and clean air 3. Superior areas of nasal cavities - Epithelium: Olfactory, with bipolar neurons - Glands: Serous (Bowman) glands - Musculoskeletal Support: Bone (ethmoid) - Features and Functions: Solubilize and detect odorant molecules in air - Location of olfactory nerve B. NASOPHARYNX AND POSTERIOR OROPHARYNX - Epithelium: Respiratory and stratified squamous Figure 2. Olfactory mucosa: details of a transitional area. Olfactory epithelium - Glands: Seromucous glands can easily be differentiated with respiratory epithelium since it doesn’t have - Musculoskeletal Support: Bone and skeletal muscle cilia. - Features and Functions: Conduct air to larynx; pharyngeal and palatine tonsils Page 2 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria - Nasopharynx (first part of the pharynx) is continuous caudally with the oropharynx (throat), the posterior part of the oral cavity leading to the larynx and esophagus C. LARYNX Epithelium: Respiratory and stratified squamous Glands: Mucous glands, smaller seromucous glands Musculoskeletal Support: Elastic and hyaline cartilage, ligaments, skeletal muscle Features and Functions: Site for phonation Figure 5. Tracheal Wall (sectional view). Stain: H&E. Medium magnification Closer look at the tracheal wall. The left side is the lumen that contains the respiratory epithelium and on the right side are those structures that give support to the trachea. E. BRONCHI Epithelium: Respiratory Musculoskeletal Support: Prominent spiral bands of smooth muscle; irregular hyaline cartilage plates Features and Functions: Repeated branching; conduct air deeper into lungs Bronchi have a smaller amount of Hyaline cartilage compared to the trachea. As a general rule, as you go distally to the respiratory tract, the running epithelium usually becomes smaller; along with the decrease of cartilage and numbers of goblet cells. The mucosa of the larger bronchi is structurally similar to the tracheal mucosa except for the organization of cartilage and smooth muscle. In the primary bronchi, most cartilage rings completely encircle the lumen, but as the bronchial diameter decreases, cartilage rings are gradually replaced with smaller isolated plates of hyaline cartilage. Figure 3. Larynx (frontal section) Stain: H&E. Low magnification F. BRONCHIOLES D. TRACHEA Epithelium: Simple ciliated cuboidal to columnar, with exocrine club Epithelium: Respiratory cells Glands: Mainly mucous glands, some serous or mixed glands o As you go distally, the thickness of the lining of the Musculoskeletal Support: C-shaped rings of hyaline cartilage, with epithelium becomes smaller/thinner. smooth (trachealis) muscle in posterior opening of each o There is also an absence of goblet cells. Features and Functions: Conduct air to primary bronchi entering lungs Musculoskeletal Support: Prominent circular layer of smooth muscle; 10-12 cm long in adults no cartilage The lining of the trachea is composed of pseudostratified ciliated Features and Functions: Conduct air; important in columnar epithelium with the presence of large C-shaped cartilage bronchoconstriction and bronchodilation (hyaline) as support. At the posterior portion, there is a presence of Trachealis muscle (a smooth muscle) that joins the free ends of the rings. Contraction of the Trachealis reduces the tracheal diameter and thereby assists in raising intrathoracic pressure during coughing. Figure 6. Intrapulmonary bronchus (transverse section. Stain: H&E. Low magnification. Intrapulmonary bronchus - Compared to the trachea, the cartilage here is much smaller, and there is still the presence of smooth muscles Figure 4. Trachea (panoramic view, transverse section) Stain: H&E, Low lining the respiratory epithelium. magnification Page 3 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria Figure 9. A terminal bronchiole has a mucosa with non-ciliated cuboidal or low Figure 7. A large bronchiole has the characteristically folded respiratory columnar epithelium (E), surrounded by only one or two layers of smooth muscle epithelium (E) and prominent smooth muscle (arrows), but it is supported only (SM) embedded in connective tissue (CT). Alveoli (A) are seen in the surrounding by fibrous connective tissue (CT). Bronchioles – scanty amount of cartilage and lung tissue. (X300; PT) the epithelium are already thinned out become simple cuboidal to columnar. G. TERMINAL BRONCHIOLES Epithelium: Simple cuboidal, ciliated cells and club cells (Clara Cells) o Clara Cells - cuboidal non-ciliated cells that has 3 functions; 1. They produce one of the components of surfactant. 2. They act as stem cells 3. They contain enzyme systems which can detoxify noxious substances. Musculoskeletal Support: Thin, incomplete circular layer of smooth muscle; no cartilage Features and Functions: Conduct air to respiratory portions of lungs; exocrine club cells with several protective and surfactant functions Terminal bronchioles branch into respiratory bronchioles, which then branch further into alveolar ducts and individual alveoli. Figure 10. Most of the epithelium consists of exocrine club cells (C) with bulging domes of apical cytoplasm contain granules, as shown here in a plastic section. IV. RESPIRATORY PORTION A. RESPIRATORY BRONCHIOLES Epithelium: Simple cuboidal, ciliated cells and club cells, with scattered alveoli o Respiratory bronchioles are similar in most respects to terminal bronchioles except for the presence of scattered alveoli along their length. Musculoskeletal Support: Fewer smooth muscle fibers, mostly around Figure 8. Terminal bronchiole (transverse section). Stain: H&E. Low alveolar openings magnification. Features and Functions: Conduct air deeper, with some gas exchange, and protective and surfactant functions of club cells Page 4 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria Musculoskeletal support: None (but with a network of elastic and B. ALVEOLAR DUCTS AND SACS (found in the more distal parts) reticular fibers) Epithelium: Simple cuboidal between many alveoli Features and functions: Sites of all gas exchange; surfactant from Musculoskeletal Support: (Few) Bands of smooth muscle around type II pneumocytes; dust cells alveolar openings Features and Functions: Conduct air, with much gas exchange Saclike invaginations, 200 um in diameter. Along with the airways, they are responsible for the spongy structure MEDICAL APPLICATION of the lungs. Obstruction of the air supply in bronchi due to excess mucus or to Each alveolus resembles a small rounded pouch open on one side to aspirated material can lead to collapse of pulmonary lobules as an alveolar duct or alveolar sac. circulating blood absorbs gases from the affected alveoli. This Air in these structures exchanges O2 and CO2 with the blood in condition, called atelectasis, is normally reversible when the blockage surrounding capillaries, through thin specialized alveolar walls that is relieved but, if persistent, can cause fibrosis and loss of respiratory enhance diffusion between the external and internal environments. function. 1. Type I Alveolar Cells (Type I pneumocytes) Overs 97% of all alveolar cells Are extremely thin simple squamous cells that line the alveoli and are the main sites for gaseous exchange Are in very close contact with the endothelial lining of capillaries, forming a very thin blood–air barrier for gaseous exchange Organelles here are also grouped around the nucleus, reducing the thickness of the remaining cytoplasm at the blood-air barrier to as little as 25 nm. They can be recognized in the alveoli or in the connective tissue septa by their phagocytosed particulate or carbon particles. All Type I epithelial cells have tight junctions that prevent the leakage of tissue fluid into the alveolar air space. 2. Type II Alveolar cells (Type II pneumocytes or septal cells) Fewer in number and cuboidal in shape that bulge into the air space, interspersed among the type I alveolar cells and bound to them with tight junctions and desmosomes. They appear singly or in groups(groups of two or three) adjacent to the squamous type I alveolar cells within Figure 11. Respiratory bronchiole, alveolar duct, and lung alveoli. Stain: H&E. the alveoli Low magnification. Note the lining that’s becoming thinner and thinner and the These cells synthesize and secrete a phospholipid-rich product called smooth muscles that are getting fewer and fewer. pulmonary surfactant o When it is released into the alveolus, the surfactant spreads as a thin layer over the surfaces of type I alveolar cells, lowering the alveolar surface tension which helps prevent alveolar collapse at exhalation and allows alveoli to be inflated with less inspiratory force, easing the work of breathing. Critical components of the surfactant layer produced by type II alveolar cells include the phospholipid dipalmitoylphosphatidylcholine (DPPC), cholesterol, and four surfactant proteins. Can function as stem cells for type I squamous alveolar cell replacement in the alveoli during lung injury Cell nuclei are rounded and may have nucleoli, and their cytoplasm is typically lightly stained with many vesicles Surfactant also has some bactericidal effects and induces immune responses in the alveoli to counteract potentially dangerous inhaled pathogens, fungi, viruses, and bacteria Lack of adequate surfactant is a major cause of respiratory distress in Figure 12. Higher magnification shows the relationship of the many rounded, premature neonates thin-walled alveoli (A) to alveolar ducts (AD). Alveolar ducts end in two or more clusters of alveoli called alveolar sacs (AS). (X140; H&E) MEDICAL APPLICATION Infant respiratory distress syndrome, the leading cause of death in C. ALVEOLI premature babies, is due to incomplete differentiation of type II Epithelium: Types I and II alveolar cells (pneumocytes) alveolar cells and a resulting deficit of surfactant and difficulty in expanding the alveoli in breathing. Treatment involves insertion of an Page 5 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria endotracheal tube to provide both continuous positive airway SAMPLE QUESTIONS pressure (CPAP) and exogenous surfactant. 1. Respiratory tract is anatomically divided into which structures? D. ALVEOLAR MACROPHAGE & DUST CELL Are blood monocytes that have entered the pulmonary connective 2. Small granule cells (or Kulchitsky cells) are difficult to distinguish in tissue septa and alveoli. routine preparations but possess numerous dense core granules 100- The primary function of these macrophages is to clean the alveoli of 300 nm in diameter. Like enteroendocrine cells of the gut, they are invading microorganisms and inhaled particulate matter by part of what system? phagocytosis. These cells are seen either in the individual alveoli or in the thin 3. T/F. The Trachea is fully wrapped by hyaline cartilage to support it alveolar septa. from collapsing. They can be recognized in the alveoli or in the connective tissue septa by the contents of their cytoplasm, which normally contains 4. T/F. The Larynx is a conducting portion of the Respiratory System with numerous phagocytosed particulate or carbon particles. pseudostratified ciliated columnar epithelium. Active alveolar macrophages can often be distinguished from type II pneumocytes because they are slightly darker due to their content of 5. Which of the following is true of pulmonary surfactant? dust and carbon from air and complexed iron (hemosiderin) from a. Secreted by type I pneumocytes erythrocytes b. Forms layer rich in phospholipid overlying a thin aqueous phase c. Prevents alveolar collapse by increasing surface tension MEDICAL APPLICATION d. Does not affect bacterial survival In congestive heart failure, the lungs become congested with blood, e. Is secreted by goblet cells and erythrocytes pass into the alveoli, where they are phagocytized by alveolar macrophages. In such cases, these macrophages are 6. After 35 weeks of gestation, a 5-lb 5-oz girl is born to a 30-year-old called heart failure cells when present in the lung and sputum; they gravid 2, para 2 (G2P2) woman. The infant has rapid and labored are identified by a positive histochemical reaction for iron pigment breathing, which is viewed as transient tachypnea of the newborn. (hemosiderin). The infant’s 1- and 5-min APGAR scores are 8 and 9, respectively. She has respiratory distress, with a normal pulse and no heart murmurs. She is transported to the neonatal intensive care unit with worsening tachypnea. In this infant which of the following is likely to be involved? a. Failure of the type I pneumocytes to form complete blood-air barriers b. Absence of elastic fibers from the bronchiolar walls and interalveolar septa c. Failure of type II pneumocytes to complete differentiation and become fully functional d. Failure of type II pneumocyte progenitors to proliferate adequately during gestation e. Inadequate development of the parietal and visceral pleura 7. The following structures are only lined with simple cuboidal Figure 13. Alveolar walls and alveolar cells. Stain: H&E. High magnification epithelium except: x205 a. Terminal bronchioles b. Respiratory bronchioles 3 Fates of Filled Macrophages: c. Bronchioles 1. Most migrate into bronchioles where they move up the d. Alveolar ducts and sacs mucociliary apparatus for removal into the esophagus. 2. The others exit the lungs in the lymphatic drainage. 8. Bronchioles have cuboidal non-ciliated Clara cells in their epithelium. 3. Some remain in the interalveolar septa connective tissue for a Terminal bronchioles, on the other hand, have exocrine club cells in year. their lining. a. Both statements are true. Type I cells are very thin simple squamous epithelium. b. Both statements are false. Type II cells are cuboidal and fewer in numbers than type 1 c. Only the first statement is true. We also have Alveolar macrophages in which the color is dependent on what d. Only the second statement is true. particulate they ingested. 9. It cleans and humidifies inspired air and provides conduits for air movement to and from alveoli. V. REFERENCES a. Conducting portion Eroschenko, V. (2013). DiFiore's Atlas of Histology with Functional Correlations b. Olfactory epithelium (12th ed.). Lippincott Williams & Wilkins. c. Respiratory Portion Mescher, A. L. (2016). Junqueira’s Basic Histology Text and Atlas (15th ed.). d. Respiratory Epithelium Mcgraw Hill Education. Page 6 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria 10. What type of cells expresses abundant ion channels that help maintain a microenvironment conducive to olfactory function and survival? a. Basal cells b. Supporting cells c. Olfactory cells d. Goblet cells Answers: 1. Upper and Lower Respiratory Tract 2. Diffuse neuroendocrine system (DNES) 3. F. Hyaline cartilage is C-shaped and connected by a smooth muscle “trachealis” posteriorly. 4. F. The larynx has respiratory and stratified squamous epithelium. 5. B 6. C 7. C 8. B. Bronchioles have exocrine club cells while Terminal Bronchioles have ciliated Clara cells. 9. A 10. B Page 7 of 8 [HISTOLOGY] 1.10 RESPIRATORY SYSTEM HISTOLOGY – Dr. Abraham Auberon Austria VI. APPENDICES Table 2. Histologic features of the upper respiratory tract, larynx, and trachea. Table 3. Features of airways within the lungs. Page 8 of 8