Respiratory System Histology (MED-202) Fall 2024 PDF

Respiratory System MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology: Chapter 12 Lecture Material  Learning Objectives Describe the histology of the conducting portion of the respir...

Respiratory System MED-202 Histology I Fall 2024 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology: Chapter 12 Lecture Material  Learning Objectives Describe the histology of the conducting portion of the respiratory system (nasal cavity, pharynx, larynx, trachea, bonchi). Discuss the histology of the respiratory portion of the system (intrapulmonary bronchial tree, bronchioles and alveoli).  Overview of the (Mechanical) Respiratory System Performs three principal functions: – Air conduction – Air filtration – Gas exchange Additionally is involved in: - the production of speech (larynx) - sense of smell (olfactory mucosa in the nasal cavity)  Overview of the Respiratory System Nasopharynx Conducting portion air is moistened, warmed, and filtered Respiratory portion gas exchange takes place  Overview of the Respiratory System nasal cavity Conducting portion - air is moistened, nasopharynx warmed, and filtered Contains: larynx - Nasal cavity - Naosopharynx trachea - Larynx - Trachea - Bronchioles bronchioles Respiratory portion - gas exchange takes place respiratory Contains: bronchioles - Respiratory bronchioles alveolar - Alveolar ducts ducts - Alveolar sacs - Alveoli alveoli alveolar sacs  Structure of the Respiratory System Respiratory mucosa = respiratory epithelium + basement membrane + lamina propria  Respiratory epithelium Respiratory epithelium - ciliated simple columnar epithelium lining most of the respiratory tract - moistens and protects the airways functions as a barrier to pathogens and foreign particles prevents infection and tissue injury by - the secretion of mucus - the action of mucociliary clearance Larynx or pharynx stratified squamous epithelium Upper respiratory ciliated pseudostratified epithelium Transition to simple cuboidal epithelium and finally to simple squamous epithelium in the alveolar ducts and alveoli Trachea  Respiratory epithelium - Cells Four main types: 1. Ciliated cells 2. Goblet cells - become increasingly fewer down the respiratory tree absent in the terminal bronchioles 3. Clara or Club cells - substitute goblet cells 4. Airway basal cells 5. Pulmonary neuroendocrine cells - innervated cells - make up around 0.5% of the respiratory epithelial cells  Respiratory epithelium - Cells ciliated cells goblet cells 30% basal cells 30% Contain motile cilia for mucociliary 30% Produce mucus escalator Stem cells for epithelium constitutively and in Transport chloride into lumen (osmosis) response to irritants Contain microvilli for absorption Respiratory epithelium - Cells  Respiratory epithelium – Ciliated Cells Ciliated cells - columnar epithelial bearing cilia on the apical site - the primary components in the mucociliary clearance mechanism - ~200 cilia on each cell - beat constantly at a rate of 10 - 20 times per second - the direction of their beat is targeted towards the pharynx 6  Respiratory epithelium – Goblet Cells Goblet cells - columnar epithelial cells - contain membrane-bound mucous granules and secrete mucus as part of the airway surface liquid - the mucus: - helps to maintain epithelial moisture - traps material and pathogens moving through the airway and determines how well mucociliary clearance works  Respiratory epithelium – Club Cells Club cells - also known as bronchiolar exocrine cells or Clara cells - low columnar/cuboidal cells with short microvilli - found in the small airways (bronchioles) of the lungs - protect the bronchiolar epithelium by secreting substances such as the secretory protein uteroglobin (immunomodulation) - detoxify harmful substances inhaled into Terminal bronchiole the lungs cytochrome P450 enzymes Club cells - could act as stem cells to regenerate the bronchiolar epithelium  Respiratory epithelium – Club Cells Club or Clara cells  Respiratory epithelium – Basal Cells Basal cells - small, nearly cuboidal - differentiate into the other cell types found within the epithelium - respond to injury by migrating and differentiating to restore a healthy epithelial cell layer  Respiratory epithelium – Pulmonary Neuroendocrine Cells Pulmonary neuroendocrine cells (PNECs): - found as solitary cells or as clusters (neuroepithelial bodies) - located in the respiratory epithelium of the upper and lower respiratory tract - the source of several types of lung cancer- most notably, small cell carcinoma of the lung, and bronchial carcinoid tumor - may play in hypoxia detection - possibly involved in regulating localized epithelial cell growth PNECs Club cells  Nasal Cavities Nasal cavities are divided into three regions: Nasal vestibule - dilated space inside the nostrils Respiratory region - largest part (inferior 2/3) - lined by respiratory mucosa Olfactory region - located at the apex of each cavity - lined by olfactory mucosa  Nasal Cavity: Respiratory Region Respiratory mucosa: – ciliated, pseudostratified columnar epithelium – the epithelium also contains: Goblet cells: mucus-producing Brush cells: bear short microvilli Basal cells: stem cells – covers most of the parts in the conducting system – warms, moistens and filters inspired air cilia connective tissue goblet cell Nasal Cavity: Respiratory Region Dilated Veins to warm the air  Nasal Cavity: Olfactory Region Olfactory mucosa: – ciliated, pseudostratified columnar epithelium – the epithelium also contains: olfactory receptor cells: bipolar neurons spanning the entire epithelium basal cells: stem cells supporting columnar cells, similar to neuroglia no goblet cells – presence of Bowman’s glands – produce a serous secretion that serves as trap and solvent for odoriferous substances – abundant nerve fibres in the underlying connective tissue Respiratory Vs Olfactory Mucosa Respiratory Olfactory  Pharynx Pharynx - divided into the - nasopharynx - oropharynx - laryngopharynx - non-keratinized stratified squamous epithelium - must be kept moist to prevent them from drying out - connective tissue with - elastic fibers  Larynx Larynx - The passageway for air between the oropharynx and trachea - complex tubular region of the respiratory tract Vocal Folds (Vocal Chords): − control the flow of air through the larynx and produce sound by vibration – lined with non-keratinized stratified squamous epithelium – contains the vocalis muscle (underlying skeletal muscle)  Trachea Trachea - a short flexible tube part of the conducting system of the airways - its wall is composed of four definable layers: – Mucosa – Submucosa – Cartilaginous layer – Adventitia Mucosa Submucosa Trachea Cartilaginous layer Adventitia  Trachea Mucosa: – ciliated pseudostratified columnar epithelium – elastic-fiber-rich lamina propria Submucosa: - areolar connective tissue (loose connective tissue) Cartilaginous layer: - C-shaped hyaline cartilage (high in collagen) forms the supportive rings of the trachea − trachealis muscle (smooth muscle) joins Mucosa the rings posteriorly Adventitia: - connective tissue Esophagus binds trachea to adjacent structures Trachealis muscle Adventitia Hyaline Submucosa cartilage  Trachea epithelium Lamina propria Hyaline cartilage Trachealis muscle Trachealis muscle  Trachea Tracheal epithelium: – Ciliated cells – Mucous cells (goblet cells) – clear in an H&E staining – Brush cells – special type of microvilli-bearing cells – Small granule cells – neuroendocrine cells secreting various hormones (eg. serotonin, calcitonin, bombesin) – not well understood function. (may function in reflexes) – Basal cells  Bronchi Segmental trachea bronchi carina Bronchi - the trachea bifurcates into two primary bronchi - within the tracheal bifurcation is a keel-shaped cartilage known as the carina - the primary bronchi divide into segmental bronchi - cartilage appears as chips and not semi- Primary bronchi circular as in the trachea The wall of the bronchi are composed of: – Mucosa- respiratory mucosa – Muscularis – Submucosa – Cartilage layer – Adventitia Bronchioles Narrowest airways of the lung (0.5mm across) - deliver air to alveoli - no cartilage or glands - simple ciliated cuboidal epithelium  Bronchioles Goblet cells are replaced by Clara cells Clara cells – non-ciliated – secretory cells embedded in the epithelium layer – secrete a surface-active agent a lipoprotein, that prevents luminal adhesion if the wall of the airway collapses during expiration – produce the secretory protein CC16 Associated with Chronic Obstructive Pulmonary Disease (COPD) and asthma  Respiratory Bronchioles Involved in both air conduction and gas exchange. Lined with simple cuboidal epithelium Gives rise to alveolar ducts  Alveoli Sites of gas exchange between air and blood Each alveolus is surrounded by a network of capillaries that bring blood to close proximity to the inhaled air The surface of all the alveoli is roughly the size of a tennis court Alveolar ducts: elongated airways Alveolar sacs: Clusters of alveoli Alveolar sac Capillaries Alveolar duct Respiratory bronchiole Alveolus Gas exchange between alveoli and capillaries Pulmonary Circulation  Alveoli Alveolar epithelium is composed of type I and II alveolar cells (pneumocytes). Type I: – Flat squamous cells – Line 95% of the surface of alveoli – Do not divide Type II: – Cuboidal epithelial cells – Cover only 5% of the alveoli surface – Synthesize surfactant decreases alveoli surface tension preventing lung collapse – Have stem cell like properties and replace type I cells after injury – SARS-CoV-2  Blood - air barrier Four layers in the blood-air barrier a. capillary endothelium (continuous, no fenestrations) b. fused basal lamina c. alveolar epithelium (type I pneumocyte) d. surfactant Note the proximity of the red blood cell to the inhaled air within the alveoli.  Blood - air barrier Blood–air barrier - is extremely thin (~600 nm-2μm; in some places merely 200 nm) allows sufficient oxygen diffusion - yet it is extremely strong type IV collagen in between the endothelial and epithelial cells - it prevents: a. air bubbles from forming in the blood b. blood entering the alveoli  Clinical Correlates Bronchitis − an inflammation of the main airways of the lungs (bronchi) − often caused by a respiratory infection − acute or chronic 1. Acute Bronchitis − temporary − usually lasts for a few weeks − usually caused by a viral infection  Clinical Correlates 2. Chronic bronchitis − characterized pathologically by a. Mucus hyper-secretion – increased goblet cells b. Damaged cilia c. Chronic inflammation of the bronchi and bronchioles d. The airway epithelium often shows squamous metaplasia (SM), which may result from increased proliferation of airway epithelial cells. − can result in Chronic obstructive pulmonary disease (COPD) include SM - benign transformation of an epithelium to a squamous morphology. Clinical Correlates  Summary 1. Overview of the respiratory system, conducting vs respiratory. 2. Structural and cellular organisation of the respiratory mucosa. 3. Mucosa organisation in different parts of the respiratory system. 4. Cellular organisation of the respiratory portion with emphasis of alveolar organisation and function. 5. Blood-air barrier 6. Clinical correlates Thank you!