Respiratory AND olfactory Epithelium
•The nose is the first place where we encounter typical respiratory epithelium, usually abbreviated as "TRE.”.
•The true respiratory (TRE) is found throughout the length of the air passages of the respiratory tract.
•It is the characteristic lining epithelium for the bulk of the respiratory system, from the nose all the way to small bronchioles.
•Nasal Cavity (coronal section)
Function of nose:
- One function of the nose is to warm incoming air.
- To trap some (by no means all) of the dirt and bacterial particles that are inhaled.
- The blood vessels underneath the epithelium serve to transfer heat to the incoming air flow, so that the humidity of the lung can be maintained at saturation
•The serous and mucous secretions produced by the nasal glands underlying the epithelium (and the goblet cells embedded in the epithelial sheet) serve to catch and hold any dirt particles and bacteria that are inhaled.
•Keeping the surface moist provides a fluid sheet that traps things.
•The beating of the cilia being unidirectional.
•This is much less important in the nasal cavity than it is in the deeper parts of the respiratory tract, however.
•Nasal Cavity
•The walls of nasal cavity as well as the septum of the nose are made up of cartilage and bone.
•Cavity is lined by epithelium resting on lamina propria.
The nasal cavity has two regions of epithelium,
1-The olfactory epithelium
2-The respiratoryepithelium which replaces respiratory epithelium in some regions.
•True Respiratory Epithelium
•TRE is a pseudostratified, ciliated, columnar epithelium interspersed with goblet cells.
• It rests on a CT lamina propria containing a large collection of glands and blood vessels.
•Respiratory Epithelium
•Olfactory Epithelium
•This olfactory epithelium is also ciliated, but its function isn't to trap dust and microbes, as is that of the TRE.
• Olfactory epithelium is much higher than TRE, which is related to its chemoreceptive function.
• It's a pseudostratified columnar type.
•The cilia are quite easily made out at the free surface but typically there are few or no goblet cells in the olfactory areas.
•Three cell types are present, only one of which is chemoreceptive.
•Olfactory cells are really a form of bipolar neuron:
•Along with olfactory cells there are supporting cells (SC) and basal cells.
•The supporting cells and the basal cells (BC) can be thought of as quasi-glial elements.
•The axons of the olfactory cells are bundled together to form the olfactory nerve.
•The olfactory nerve fibers pass through the bone of the nose into the olfactory lobe of the brain.
•Both TRE and Olfactory Epithelium
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The Mechanism of Smell:
•The mechanism of smell is still very obscure.
• It is known that the plasma membrane of the olfactory cells is the actual site of chemoreception
•If the olfactory cells' binding sites aren't cleared of the materials bound to them, after a while they no longer transmit a signal, a phenomenon called "olfactory fatigue."
•After you've been in a barn for a few minutes you "don't smell it anymore."
Cell Types in Respiratory Epithelium
Electron microscopy reveals Six types of cells present in epithelia lining the conducting portion.
- Goblet cells
- Brush cells
- Ciliated columnar cells
- Basal (short) cells
- Small granular cells
- Clara cells
1- Ciliated columnar cells
•Most abundant cell type
•Each cell has up to 300 ciliary tubules on the apical surface.
•Beneath the apical surface of the cell there are basal bodies and numerous mitochondria.
•Basal bodies serve as the anchoring sites for ciliary axonemes
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2-Brush Cells
•Columnar cells having numerous microvilli on apical surface.
•Afferent nerve endings innervate them on their basal surface
•Considered as sensory receptors.
•Brush cells are indicated by thick arrows in the bottom part of EM photograph.
3- Goblet cells
•Second most abundant cell type.
•Apical surface is packed with membrane bound, polysaccharide rich, mucous granules.
•Function is to secrete mucous to keep surfaces moist and to trap inhaled particles.
4- Basal (Short) Cells
•They are small rounded cells that lie on the basal lamina, but do not extend to the luminal surface of the epithelium.
•They are generative cells.
•They undergo mitosis and differentiate into other cell types.
5-Small Granule Cells
•Each cell resembles a basal cell, except that it possesses numerous granules (100 – 300 nm in diameter) in the basal cytoplasm.
•These endocrine like cells act as effectors in the integration of the mucous and serous secretory processes.
6-Clara Cells
•Present in the terminal and respiratory bronchioles.
•They have dome-shaped apices, which protrude into the lumen.
•They are devoid of cilia.
•They have secretory granules in their apical region.
•Secrete glycosamino-glycan, that protect the bronchiolar lining.
•They are also a secondary source of surfactant for the broncheo-alveolar fluid