Histology Lecture (8) Epithelium (2) PDF

Histology lecture (8) Epithelium (2) Histology The epithelial cells specialized for reception of external sensory stimuli. 1. Olfactory epithelium. 2. Retina (Rods & Cons) 3. Organ of Corti (Hearing) 4. Taste buds Taste buds are onion-shape...

Histology lecture (8) Epithelium (2) Histology The epithelial cells specialized for reception of external sensory stimuli. 1. Olfactory epithelium. 2. Retina (Rods & Cons) 3. Organ of Corti (Hearing) 4. Taste buds Taste buds are onion-shaped structures, each one containing 50–100 cells.  Shape: Tall cells with oval central nucleus The apical surface shows microvilli (receptors project) through an opening called the taste pore. Their bases are surrounded by afferent sensory nerve axons. Molecules (tastants) dissolved in saliva contact the microvilli through the pore and interact with cell surface taste receptors.  Function: taste sensations  Shape: Tall cells.  Function: Support the sensory cells.  Shape: They are pyramidal cells rest on the basement membrane between the taste & supporting tall cells.  Function: act as a stem cells which divide and give rise to the other two types. Histology  Membrane associated structures that join adjacent cells 1. Occludens (tight) junctions 2. Adherens junctions 3. Gap junctions A  A type of intercellular connection in which the outer surfaces of adjacent cell membranes fuse to form a seal between intercellular & luminal fluids.  Anastomosing ridges project into the intercellular space from each membrane.  These ridges fuse with each other forming sealing strands that adhere tightly across the intercellular space 1- Zonula occludens: Fusion ( ) two adjacent membranes completely encircles the cell. e.g. in the epithelium of small intestine. 2- Fascia occludens: Fusion ( ) adjacent membranes is patchy. e.g. between the endothelium of blood capillaries.  Form tight seal that prevent flow of materials between the luminal and intercellular fluids Histology b A type of intercellular connections in which the adjacent membranes but they are joined by intercellular binding material.  The intercellular space is about 15 nm.  This space is filled with an electron dense binding substance (cadherin)  Inside the cell, actin filaments join up the adhesion junctions.  A type of adherens junctions in the form of a spot between the cell membrane of the adjacent cells.  Structure: by EM it is formed of: 1. Intercellular space of about 20 nm filled with an electron dense binding substance (cadherin). 2. Two cell membranes on either side. 3. Thickened electron dense plaques on the cytoplasmic sides of the membranes. 4. Intermediate filaments (tonofilaments) inserted in each plaque and loop back again into cytoplasm.  Function: It holds and prevents separation of the epithelial cell e.g. between skin keratinocytes and between the cardiac muscle fibers  A half desmosome joining the base of epithelial cells to the basement membrane. Histology C  Type of cell connection in which the cytoplasm of the two adjacent cells is joined by a gap across the intercellular space.  The two cells membranes are separated by about 2nm.  Cylindrical structures composed of integral proteins of the cell membranes are arranged side by side in the junction.  Each cylinder forms a hydrophilic channel directly linking the cytoplasm on both sides. 1. Permits direct passage of ions between the cells (between osteocytes). 2. Permits the wave of depolarization to spread in the proper sequence e.g., between the cardiac muscle fibers 3. Conduction of contraction wave in the smooth muscles of intestine. Histology Histology Epithelial cells are called polarized cells because they have apical, basal & lateral surfaces The 3 surfaces have specific structural modifications for adaptation of specific function 1. Microvilli 2. Stereocilia 3. Cilia 4. Flagella Short finger like projection. long microvilli & non motile. Epithelia cells lining intestine & Epithelia cells lining epididymis & proximal convoluted tubules of inner ear receptors kidney. Increase surface area for absorption Absorption e.g. epididymis Hearing & equilibrium e.g. hair cells of inner ear Histology They are multiple invaginations of the basal cell membrane epithelia cells lining the striated ducts of salivary glands & proximal convoluted tubules of the kidney ion transport It appears as a thin PAS positive membrane between the base of epithelial cells and the underlying connective tissue. secreted by the epithelial cells. formed of glycoprotein, proteoglycan, type IV collagen, & reticular fibers 1. Act as a support of epithelial cells. 2. Acts as a flirtation barrier e.g. in the kidney. 3. Acts as a diffusion barrier regulating exchange of macromolecules between the connective tissue and epithelium.