Histology of the Upper Respiratory Tract

Histology of the Upper Respiratory Tract ➢ Introduction - - - - The respiratory system is divided into two parts, one that conducts the air through the respiratory tract and links the lungs to the external environment, the conducting zone, which is formed mainly by cartilage. The second part exchanges oxygen and carbon dioxide with the blood, and is called the respiratory zone. The main function of the lung is to exchange gases. This function occurs in the respiratory zone (i.e. respiratory bronchioles, alveolar sacs). However, the conducting zone is essential for warming, cleansing and moisturizing the inspired air. The respiratory epithelium changes gradually. It starts as pseudostratified ciliated columnar, then gradually changes into simple ciliated columnar, simple ciliated cuboidal, simple non-ciliated cuboidal (clara cells) and finally simple squamous epithelium. This transition is important for the function of the respiratory tract; the simple squamous epithelium for example is very thin which is perfect for gas exchange, unlike the cuboidal cells which are thick. The transitional area is considered weak, and susceptible to malignant changes. ➢ The Blood-Air Barrier Each alveolus is surrounded by capillaries and other alveoli, between them is a thin adherent membrane which is the blood-air barrier, also known as the respiratory membrane. This barrier consists of the wall of capillaries (endothelium), wall of alveolar cells (squamous epithelium) and fused basal lamina in between. Elastic fibers and connective tissue also exist between alveoli, these are important for inflation and deflation. ➢ Conducting Zone: - Nose, nasopharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles. - The bronchi are divided into primary bronchi, which are extrapulmonary, and secondary and tertiary bronchi, which are intrapulmonary. - Primary bronchi are the right and left main bronchi. - Secondary bronchi are intrapulmonary, and each enters one lobe (lobar bronchi). So, there are three secondary bronchi on the right, and two on the left. - Bronchioles can be conducting or respiratory. Large and terminal bronchioles are conducting, while the respiratory bronchioles are respiratory. - Function of the Conducting Zone: Cleansing, warming and moisturizing the inspired air. ➢ Respiratory Zone: - Respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. - These are the main sites of gas exchange. 1|Page - The respiratory bronchioles are called the site of transition (will be explained later on). - This area is rich in elastic and reticular fibers. These add to the elasticity of lungtissue to allow inflation and deflation. ➢ The Layers of the Respiratory Wall - The mucosa, which is composed of three layers: 1- Epithelium: mostly pseudostratified ciliated columnar epithelium with goblet cells. 2- Lamina propria: loose connective tissue,contains A Step Forward The structure always fits the function. Ex: Stratified epithelium fits protection, simple squamous epithelium fits diffusion, and ciliated epithelium fits sites where ciliary movement is needed..etc. Bear this in mind as we go through the lecture. seromucous glands, might also contain lymphocytes 3- Muscularis mucosa: contains smooth muscle cells. - Submucosa: connective tissue rich in seromucous glands that secrete serous fluid and mucus that are important in conditioning and protection. - Supportive layer: hyaline cartilage and smooth muscle. - Adventitia: a covering of connective tissue. - These layers vary along the respiratory tract; as we go distally cartilage, goblet cells and glands decrease in amount, while the smooth muscles and elastic fibers increase for inflation and deflation of the lungs. So the lamina propria in the wall of upper parts contains more seromucous glands than the more distal parts. There are no glands in the respiratory bronchioles. - In the trachea, cartilage is in the form of C-shaped rings. These rings are not complete because the esophagus is posterior to the trachea. So, cartilage is replaced posteriorly with the trachealis muscle (this is a smooth muscle). - In the right and left main bronchi there are plates of cartilage in the shape of rings that gradually decrease in the tertiary bronchi and disappear in the bronchioles where they are replaced by smooth muscle. - Mucous traps pathogens and foreign bodies, serous fluid moisturizes air, while the cilia move from inside to outside (in one direction only) to propel anything trying to pass in, out of the respiratory tract. 2|Page ➢ Gas Exchange - As mentioned before, each alveolus is surrounded by a huge network of capillaries. - From the pulmonary trunk, blood reaches alveolar arterioles and capillaries rich in CO2, at the respiratory membrane, exchange occurs, where O2 diffuses from alveoli into capillaries and venules, and CO2 diffuses from capillaries into alveoli. Later the pulmonary vein carries the oxygenated blood to the left atrium. ➢ Respiratory Epithelium - Under the microscope, pseudostratified ciliated columnar cells and goblet cells are the main types of cells seen, however, there are other 5 types of cells seen in respiratory epithelium. - The respiratory basement membrane is very thick. All cells rest on the basement membrane but not all of them reach the surface. 3|Page 1. Ciliated Columnar Cells - The most abundant type. - Each cell has 250-300 cilia on its surface. - These cilia are inserted into the basal bodies in the apical part of the cell. - The apical part of these cells is abundant in mitochondria, to provide the energy needed for ciliary movement. - Cilia are composed of proteins. These are important for ciliary movement. - Example of which is the dynein. - Nicotine prevents the formation of dynein, which leads to improper movement of cilia. - The protein dynein is essential for movement of cilia. - Nicotin prevents formation of dynein which impairs ciliary movement. - The flagellum of the spermatozoon is a modified cilium, that’s why smoking predisposes males for infertility, in addition to increased risk of chronic respiratory infections due to loss of ciliary movement. - This is known as immotile cilia syndrome/ Kartagner syndrome. 2. Goblet cells - present between ciliated columnar cells - secrete mucus that traps pathogens, and foreign bodies. 4|Page 3. Brush cells - numerous microvilli on their apical surfaces. - These cells work as sensory receptors and they have afferent nerve endings at their basal surfaces. 4. Basal cells - These are small round cells. - Mitotically active, generative stem cells that can differentiate into any of the other cell types. - Basal cells lies at the base and does not reach the surface. 5. Diffuse Nueroendocrine cells (DNES) - Also called Kulchitsky cells, or small granule cells. - Regulate the local secretion of serous and mucous glands in the respiratory system. Note: under the light microscope we mostly see pseudostratified ciliated columnar with goblet cells, however the other types of cells are seen with electron microscope. ➢ Nasal Cavity The nasal cavity is divided into three parts: 1. Vestibule: - The most anterior and dilated part of the nasal cavity. - Lined by modified skin, that initially contains sebaceous and sweat glands, but it then loses its keratinization nature and undergoes a transition into typical respiratory epithelium as it goes into the nasal cavity. - It contains thick hairs (vibrissae). 2. Respiratory Area: - Contains the conchae and meatuses. - Covered by pseudostratified ciliated columnar epithelium with goblet cells. - The sub-epithelial connective tissue is rich in blood supply, and this is important for warming the air. 3. Olfactory Area: - Present in the roof and at the upper part of the nasal cavity. - Covered by olfactory mucosa. - Olfactory mucosa contains olfactory epithelium, lamina propria (corium) and Bowman’s glands. - Olfactory epithelium: Pseudostratified epithelium with 3 types of cells: 1- Olfactory cells (olfactory neurons). - These are bipolar cells present throughout the epithelium. - Their nuclei are below the level of the nuclei of supporting cells. - Bipolar cells: its apical part is its dendrite and has about a dozen basal bodies. - From these basal bodies, long non-motile cilia emerge. These cilia respond to odoriferous substances by generating action potentials that would be transmitted through the axons into the olfactory nerve. Then, they go to the brain to be interpreted. 5|Page - These axons unite in the lamina propria (see the figure), they pass through the foramina in the cribriform plate where they form the olfactory nerve which eventually synapses with other neurons in the olfactory bulb. 2- Supporting (sustentacular) cells: - columnar cells, with broad apices and narrower bases. - May be protective or nutritive. 3- Basal cells: - stem cells for other cells. - Bowman’s glands are present in the lamina propria (secrets watery mucous Facilitating the access of new odoriferous substances. 6|Page ➢ Paranasal sinuses Paranasal sinuses - Small cavities in the bones of the skull. - Characterized by thin mucosa (thinner respiratory epithelium with fewer goblet cells). - Few goblet cells. - Contains small glands. - The lamina propria contains only a few small glands and is continuous with the underlying periosteum. ➢ The Bronchial Tree - Remember: The respiratory epithelium changes gradually. It starts as pseudostratified ciliated columnar, then gradually changes into simple ciliated columnar, simple ciliated cuboidal, simple non-ciliated cuboidal (clara cells at the terminal bronchioles) and finally simple squamous epithelium. - The respiratory bronchioles are considered to be transitional area, the lining changes from columnar, cuboidal, non-ciliated until it becomes simple squamous. The hyaline cartilage also disappears at this area. - The bronchial tree consists of: trachea  left and right main bronchus secondary (lobar) bronchus segmental bronchus(10 segments on each side)  conducting bronchioles -including terminal bronchioles-  respiratory bronchioles that open into alveolar ducts. The alveolar ducts end in alveolar sacs. - As we move distally through the tree the branches increase and the diameter decreases. - The alveolar sacs and ducts are surrounded by alveoli, each alveolus is composed of alveolar cells. There are two types of alveolar cells: type 1 pneumocytes, which are simple squamous epithelial cells, and type 2 pneumocytes, these are simple cuboidal cells and produce surfactants which are important in lowering surface tension and at birth. - Only the first two are extrapulmonary. The rest structures (starting from the secondary bronchus (or the lobar bronchi) are intrapulmonary structures – inside the lungs-. ➢ The Trachea: - Layers that are found are: mucosa, sub mucosahyaline cartilage- C shaped-, Trachealis muscle, which is a smooth muscle found posteriorly. - Sero-mucous glands are found in lamina propria and sometimes in sub-mucosa. - Trachea ends at the level of T4 (angle of Louis/ sterna angle) then divides into right and left main bronchi. 7|Page - The Bronchus Extrapulmonary (primary bronchus) : Resembles trachea in structure in the form of cartilaginous circular plates instead of c-shaped Intrapulmonary (secondary and tertiary) : They have complete muscular layer Irregular cartilaginous plates The difference between trachea and bronchi As we move distally from extra to intra pulmonary the cartilaginous plates become irregular and incomplete, elastic fibers and smooth muscles increase (spiral smooth muscles start to appear at the level of bronchi giving the bronchial mucosa a folded appearance), the lumen becomes narrower and the number of lymphocytes increases. 8|Page Lymphatic nodules in the respiratory tract: - They are called “BALT”. - Lymphocytes are found along the whole respiratory tract as scattered cells but they accumulate forming nodules in certain areas. - Nodules are mainly found in the bronchus. They become scattered once again when reaching the bronchioles. *Scattered cells nodules in the bronchus only scattered cells from the bronchioles till the end of the tract.* - Like the smooth muscles and elastic fibers, BALT also becomes more abundant as bronchi become smaller.  Bronchioles will be explained in the next lecture! 9|Page