Histology of Respiratory System PDF

Histology of Respiratory System For PC-I Students  Introduction Respiratory system provides for exchange of O2 and CO2 to and from the blood. Respiratory organs include the lungs and a branching system of bronchial tubes That link the sites of gas exchange with the external environm...

Histology of Respiratory System For PC-I Students  Introduction Respiratory system provides for exchange of O2 and CO2 to and from the blood. Respiratory organs include the lungs and a branching system of bronchial tubes That link the sites of gas exchange with the external environment. Anatomically, respiratory system is divided into upper and lower respiratory tracts Functionally, the system has two components: A. The conducting portion: consists of the nasal cavities, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles. B. The respiratory portion: The system where main function of gas exchange occurs. Consisting of respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. Alveoli: The cellular sites of the exchange of O2 and CO2 between inspired air and blood. Are saclike structures that make up most of lungs. 2. Transitional portion: This may be considered as a part of the conducting portion. e.g. Terminal bronchioles Structural importance of the conducting respiratory system The conducting part of respiratory system is essential to ensure a continuous supply of air by the aid of three main elements: 1. Cartilage: It is hyaline in nature but in larynx it is partly elastic and partly hyaline It is essential to support the conducting wall and prevent its luminal collapse. 2. Elastic fibers: Essential for flexibility of such part and spring back after the distension. They are situated mainly in the lamina propria especially of the bronchioles; which have the highest concentration of such fibers 3. Smooth muscle fibers: Completely surrounds the respiratory tube. They reduce the diameter of the respiratory tubules and regulate the air flow during inspiration and expiration.  The main function of the conducting part is to clean, moist and warm the air before it reaches the lung This role is performed by the following: 1. The presence of very specialized respiratory epithelium which is rich in mucous and serous glands. 2. The presence of superficial vascular network in the lamina propria 3. The presence of special hairs (Vibrissae) to remove coarse. Respiratory Epithelium Most of the conducting portion of the respiratory system are lined by pseudostratified columnar ciliated epithelium with goblet cells. When bronchi divide to bronchioles, the epithelium is changed to simple columnar then to simple cubical in the smallest terminal bronchiole.  Goblet cells are totally absent in the terminal bronchioles, but cilia continue beyond. The goblet cell disappearance to prevent the accumulation of mucous in the respiratory portion The respiratory epithelium has 5 major cells types: 1. Columnar ciliated epithelial cells: Are the most common cells. Each cell carries about 300 cilia at its apical surface. The apical localization of the mitochondria is important for energy needed for ciliary movement. The cilliary movement depends on DYNEIN protein Which helps in sliding of the cilliary microtubules. If there is a decrease in such protein the ciliary movement is affected as in KARTAGENER’S syndrome 2. Goblet cells: Is the next common cell in the respiratory epithelium. Produce mucin glycoproteins on its apical part. The nucleus is flat and located in the basal part of the cell. 3. Brush cells: They represent the rest of the columnar cells. The cell surface is characterized by numerous microvilli. There are two main types: A. Immature cells: which is the remnants of the dead goblet cells or the dying columnar cells B. Sensory receptor cells: chemosensory receptors Which exhibit nerve endings at their basal surfaces. 4. Basal short cells: Are mitotically active stem and progenitor cells that give rise to the other epithelial cell types. They are short cells located on the basal lamina and never reach the luminal surface. 5. Small granule cells: They are a member of the APUD (amine precursor uptake and decarboxylation) which perform endocrine functions. The cell contains numerous granules (100-300 nm) with dense cores. They are similar to the basal cells in their basal location. The cell acts as a regulator for mucous secretory processes. Epithelium undergoes metaplastic changes to stratified squamous epithelium NASAL CAVITY It is divided into three main regions: 1. The external vestibule: It is the dilated external part, bounded externally by the two nasal orifices (Nostrils or Nares). The nostrils are protected by thick hairs known as Vibrissae Which are essential in filteration of the air from the large particles of dust. The vestibule contains sweat and sebacous glands The nasal orifices are lined by stratified squamous keratinized, Which gradually change to the typical respiratory epithelium 2. The nasal fossae: They are two cavernous chambers separated by the osseous nasal septum. Each lateral wall of the nasal chamber is provided by bony shelflike projections known as CONCHAE (Superior, Middle and inferior). The inferior and the middle conchae are lined by respiratory epithelium. The roof of the nasal cavities and the superior conchae are covered with specialized olfactory epithelium. The lamina propria of the conchae is rich in large venous plexuses known as swell bodies , which are very important in warming the inspired air. The dilation of swell bodies in nasal allergic diseases, decreasing the capacity of the nasal cavity leading to a sense of nasal block 3. Paranasal sinuses: They include bony cavities in the frontal, maxillary, ethmoid and sphenoid bones. They are lined by thin respiratory epithelium with few goblet cells. The lamina propria is continuous with the underlying periosteum. The mucous content of such cavities is drained through the nasal cavity The olfactory epithelium Specialized region of the mucous membrane covering the superior conchae at the roof of the nasal cavity. Consists olfactory chemoreceptors for the sense of smell  Pseudostratified columnar epithelium has three major cell types: 1. Olfactory sensory cell: Are bipolar neurons present through out the epithelium. 2. Sustentacular (supporting) cells: Are columnar, with broad, cylindrical apexes containing the nuclei and narrower bases. Consists abundant ion channels that help to maintain a microenvironment conducive to olfactory function and survival 3. Basal supporting cells: Are small, spherical or cone shaped cells near the basal lamina. Are stem cells for the other two types, Replacing the olfactory neurons every 2 to 3 months and support cells less frequently. The lamina propria of the olfactory epithelium possesses large serous glands, called olfactory glands (of Bowman). Which produce fluid surrounding olfactory cilia and facilitating the access of new odoriferous substances. NASOPHARYNX It is the first portion of the pharynx at its contact with soft palate or continuous caudally with the oropharynx. It is lined with respiratory epithelium. And its mucosa contains pharyngeal tonsil Connect to middle ear cavity through the auditory tubes. The nasal cavities open posteriorly into the nasopharnx. The larynx: Is an irregular cartilage tube that connects the nasopharynx with the trachea. The laryngeal cartilages reinforced mainly by; Hyaline cartilage (in the thyroid, cricoid and most of arytenoid cartilage) But smaller elastic cartilages: (in the epiglottis cuneiform cartilage, Tip of the arytenoids cartilages). All of which are connected by ligaments.  The larynx and its cartilages perform the following functions: 1. It acts as a valve that prevents the entrance of food or fluid to the trachea. 2. Maintains an opened airway 3. Produces tones for phonation EPIGLOTTIS A flattened structure projecting from the upper rim of the larynx, Serves to prevent swallowed food or fluid entering that passage. It is one of the elastic laryngeal cartilages. It has two surfaces: 1. Laryngeal surface - lined by respiratory epithelium 2. Lingual surface -lined by stratified squamous epithelium  Mixed mucous and serous glands are seen under the mucosa of the epiglottis. Two pairs of folding are found below the epiglottis and extend through the cavity of larynx: 1. Upper pair (False vocal cords): immovable vestibular folds Is covered by respiratory epithelium overlying seromucous glands and lymphoid nodules. 2. Lower pair (True vocal cords): Have important for phonation or sound production. Covered by stratified squamous epithelium that protects the mucosa from abrasion and desiccation from rapid air movement. Deep to the mucosa of each vocal fold are large bundles of striated fibers that comprise the vocalis muscle. During phonation the vocalis muscles draw the paired vocal folds together (adduction).  Narrowing the intervening luminal space, called the rima glottidis Air expelled from lungs causes the adducted vocal folds to vibrate and produce. The pith and other qualities of the sound are altered by:  Changing the tension on the vocal folds.  The width of the rima glottidis,  The volume of air expelled  Speech is produced when sounds made in the larynx are modified by: Movements of the pharynx, tongue, and lips. The larynx is larger in males than in females after puberty, Causing men`s voices to be typically deeper than women`s voices. TRACHEA 10-12 cm long in adults. Made up of 16-20 C-shaped hyaline cartilage, which reinforces the wall and keeps the tracheal lumen. The wall is formed of the following layers: 1. Mucosa: a. Epithelium: Pseudostratified columnar ciliated epithelium b. Lamina propria: Loose connective tissue rich in blood vessels, lymphocytes and elastic fibers  2. Submucosa: – Loose connective tissue, contains blood vessels, nerves and lymphatics – Scattered mucous and serous secreting glands are also seen. 3. Supporting layer (Fibro-cartilagenous coat): 4. External fibrous layer (Adventitia): – Made up of connective tissue containing blood vessels and nerves Trachealis Muscle: A bundle of smooth muscle on the posterior surface of the trachea against esophagus, A sheet of fibroelastic tissue attached to the perichondrium. Relaxes during swallowing to facilitate the passage of food by allowing the esophagus to bulge into lumen of the trachea. Strongly contracts in the cough reflex to narrow the tracheal lumen. And provide for increased velocity of the expelled air and better loosening of material in the air passage. Bronchial Tree and Lung The trachea divides into two primary bronchi that enter each lung at hilum, along with arteies, veins, and lymphatic vessels. The primary bronchi giving rise to three secondary (lobar) bronchi in the right lung and two in the left lung. Each of which supplies a pulmonary lobe.  The lobar bronchi again divide, forming tertiary (segmental) bronchi. Each of tertiary bronchi supplies bronchopulmonary segment. 10%-12% of each lung with its own connective tissue capsule, and blood vessel. The existence of such lung segments facilitate the specific surgical resection of diseased lung tissue without affecting nearby healthy tissue.  The tertiary bronchi give rise to smaller and smaller bronchi, whose terminal branches are called bronchioles. Each bronchioles enters a pulmonary lobule, where it branches to form 5-7 terminal bronchioles. THE INTRAPULMONARY BRONCHUS The wall of intrapulmonary bronchus is formed of the following layers: 1. Mucosa, which is folded and formed of: Epithelium, which is of the same respiratory type Lamina propria formed of loose connective tissue rich in blood vessels, lymphocytes and elastic fibers 2. Complete layer: of smooth muscle fibers which encircle the whole lumen. 3. Layer of alternated: hyaline cartilage plates with mucous and serous glands. The ducts of these glands open into the bronchial lumen. Some lymphatic follicles may be present BRONCHIOLES Are intralobular air pathway of about 5 mm in diameter. The bronchiolar wall is free of cartilages and glands. Are lined with respiratory epithelium which turns to columnar ciliated epithelium in the terminal bronchioles. The terminal bronchioles contain non ciliated cell known as Clara cells. Clara cells (Exocrine bronchiolar cells): Dome shaped apical ends with secretory granules. Have various functions 1. Secretion of surfactant: lipoproteins and mucins in the fluid layer on the epithelial surface. 2. Detoxifications of inhaled xenobiotic compounds by enzymes of the SER. 3. Secretion of antimicrobial peptides and cytokines for local immune defense. 4. In a stem cell subpopulation, injury-induced mitosis for replacement of the other bronchiolar cell types.  Clara cells are characterized by: – Long cisternae of rER – Absence of sER – Scattered elongated mitochondria – Small accumulation of glycogen RESPIRATORY BRONCHIOLES Each terminal bronchiole is subdivided into two or more respiratory bronchioles that include saclike alveoli. Their wall is interrupted by the openings of many alveoli. The respiratory bronchioles lined by ciliated cuboidal cells as well as Clara cells. At the junction between the bronchiole and the alveoli the cuboidal cell continue as simple squamous of pneumocyte type l ALVEOLAR DUCTS AND SACS Distal end of respiratory bronchioles branch into tubes called alveolar ducts. With the increased number of alveoli which open through the bronchiolar wall, the epithelial cells gradually decrease in number Thus the alveolar duct formed and open through the atria to the alveolar sacs Alveolar sacs Larger clusters of alveoli. Forms of alveolar ducts distally and occur occasionally along their length. The lamina propria is consisting a network of elastic and reticular fibers that encircles the alveolar opening. Capillaries surrounds each alveolus. Alveoli Are saclike evaginations, each about 200µm in diameter, from the respiratory bronchioles, alveolar duct, and alveolar sacs. Adult lung has about 200mil alveoli with a total internal surface area of 75m². It resembles a small rounded pouch open on one side to alveolar duct or alveolar sac. Site where CO2 and O2 exchanges with the blood that enhance diffusion between the external and internal environment. THE ALVEOLI AND THE INTERALVEOLAR SEPTUM The alveoli Are lined by alveolar cell or pneumocyte type l and alveolar cell or pneumocyte type ll The distance between the two adjacent alveoli is called the interalveolar septum. The interaveolar septa are vascularized with the richest capillary networks in the body. Capillary endothelial cells are extremely thin but continuous and not fenestrated. Type I alveolar cells (Type I pneumocytes) Are extremely attenuated cells that line the alveolar surfaces. Maintain the alveolar side of the blood-air barrier and cover about 95% of alveolar surface. Pinocytotic vesicles of type I play a role in the turnover of surfactant And removal of small particulate contaminants from the outer surface. All type I epithelial cells have occluding junctions that prevent the leakage of the tissue fluid into the alveolar air space. Type II alveolar cells (Type II pneumocytes or septal cells) Cuboidal cells that bulge into the air space, interspersed among the type I alveolar cells. Bound to type I alveolar cells with occluding junctions and desmosomes. Divided to replace their own population after injury And to provide progenitor cells for the type I cell population. Have rounded nuclei and nucleoli Type II vesicles are lamellar bodies.  Produce pulmonary surfactant. The surfactant film:  Lowers surface tension at the air-epithelium interface  Helps prevent alveolar collapse at exhalation  And allows to be inflated with less inspiratory force, easing the work of breathing. Component of the surfactant layer  Phospholipid dipalmitoylphosphatidylcholine (DPPC)  Cholestrol  Four surfactant proteins Surfactant proteins A (SP-A): a very aboundant hydrophilic glycoprotein SP- D: important for innate immune protection within lungs SP-B and SP-C: are required for the maturation of DPPC and its proper orientation in the surfactant film inside the alveolus. The septum is formed of: Thin alveolar epithelium Basement membrane of the alveolar epithelium Connective tissue (interstitium) of the septum containing:  Elastic and reticular fibers.  Fibroblasts, leucocytes and macrophages. Alveolar macrophages also, called dust cells are found in alveoli and in the interaveolar septum Cells of the interalveolar septum 1. Capillary endothelial cells (30%) 2. Pneumocytes type I (8%) 3. Pneumocytes type II (Septal cells) (16%) 4. Interstitial cells (Fibroblast & mast cells) (36%) 5. Alveolar macrophages (dust cells) (10%)  Air in the aveoli is separated from capillary blood by three components collectively called the respiratory membrane or blood-air barrier. A. Two or three highly attenuated, thin cells lining the alveolus. B. The fused basal laminae of these cells and of the capillary endothelial cells, and C. The thin endothelial cells of the capillary. Alveolar pores Penetrate the interalveolar septa Connect neighboring alveoli that open to different bronchioles. Equalizes air pressure in these alveoli Permit collateral circulation of air when a bronchiole is obstructed.  O2 from the alveolar air diffuses through the blood-air barrier into the capillary blood and binds hemoglobin in erythrocytes. CO2 diffuses into the alveolar air from the pulmonary blood. Most CO2 arrives in the lung as part of HCO2 inside erythrocytes. Regeneration in the Alveolar Lining Inhalation of toxic gases can kill types I and II cells lining pulmonary alveoli. Death of the first cells results in increased mitotic activity in the remaining type II cells, Progeny of which become both cell types. The normal turnover rate of type II cells is 1% per day And results in continuous renewal of both alveolar cells.  The pulmonary barrier is formed of: 1. Cytoplasm of the epithelial cells 2. Basal lamina of alveolar epithelial cells 3. Basal lamina of the endothelial cells 4. Cytoplasm of the endothelial cells Lung Vasculature and Nerves Blood circulation in the lungs includes both the pulmonary circulation and bronchial circulation, carrying O2 depleted blood for gas exchange, and systemic, nutrient rich blood respectively. With in the lung, the pulmonary artery branches and accompanies the bronchial tree, With its branches sharing the adventitia of the bronchi and bronchioles. At the level of the alveolar duct, the branches form the dense capillary networks in the interalveolar septa that contact the alveoli.  Venules arising from the capillary networks are found in the lung parenchyma, supported by connective tissue. The lymphatic vessels originate in the connective tissue of bronchioles. They follow the bronchioles, bronchi, and pulmonary vessels and all drain into lymph in the region of hilum. Both parasympathetic and sympathetic autonomic fibers innervate the lungs and control reflexes smooth muscle contractions which determine the diameters of the airways. Pleural Membranes The lung`s outer surface and the internal wall of the thoracic cavity are covered by a serous membrane called pleura. The membrane attached to lung tissue is called the visceral pleura and the membrane lining the thoracic walls is the parietal pleura. The two layers are continuous at the hilum and are both composed of simple aquamous mesothelial cells. Pleural cavity Lies between the parietal and visceral layers. Entirely lined with mesothelial cells that produce a thin film of serous fluid. That acts as a lubricant, facilitating the smooth sliding of one surface over the other during respiratory movement. Respiratory movement During inhalation;  Contraction of the intercostal muscles elevates the ribs  Contraction of the diaphragm; Lowers the bottom of the thoracic cavity, increasing its diameter And resulting in pulmonary expansion. The bronchi and bronchioles increase in diameter and length during inhalation. The respiratory portion also enlarges, mainly as a result of expansion of the alveolar ducts. During exhalation; The lungs retract passively because of muscle relaxation. And the elastic fiber`s return to the unstretched condition. Foetal Lung Glandular period: – up to 17th fetal week. – Bronchi grow and branch. – Alveoli are not present Canalicular period: – up to 25th foetal week. – Bronchi and bronchioli begin to form terminal sacs (developing primitive alveoli). – Lung tissue is vascularised. Alveolar period (terminal sac period). – It begins shortly before birth but the first mature alveoli appear only after birth. – The late alveolar period is marked by the development of mature alveoli from the terminal sacs THANK YOU