Epithelial Tissue Lecture PDF

Epithelial Tissue Topic Outline Tissues Epithelial Tissue Characteristic Features of Epithelial Cells Morphological Types of Epithelial Tissue Covering or Lining Epithelia Secretory Epithelia and Glands Transport Across Epithelia Medical Application Tissues Is the aggregation of...

Epithelial Tissue Topic Outline Tissues Epithelial Tissue Characteristic Features of Epithelial Cells Morphological Types of Epithelial Tissue Covering or Lining Epithelia Secretory Epithelia and Glands Transport Across Epithelia Medical Application Tissues Is the aggregation of cells leading to a structure with specialized functions. Each tissue is an assemblage of similarly specialized cells performing a specific function There are four main tissue types in the body: Epithelial Connective Muscular Nervous Tissues Aside from the cells, each tissue contains extracellular matrix which varies from each other ECM functions for structural support network, cell migration, wound healing, and differentiation. Components: proteins, sugars, and water Epithelial Tissue Polyhedral cells that are bound tightly together structurally and functionally to form sheet like or tubular structure. ECM is present but found in small amount only. Greek root words: “epi” – on or upon “thele – nipple Epithelial Tissue Location Epithelial tissues are found all over the body Cavity linings of organs Body surfaces (external and internal) Glands Epithelial Tissue Functions Protection Secretion Absorption Excretion Contractile Motion Sensory Characteristic Features of Epithelial Cells Epithelial tissues vary in shape (columnar, cuboidal, and squamous) Adjacent to connective tissue Lamina Propia – connective tissue that underlies the epithelia Papilae – small evaginations Occurs on epithelial tissues subject to friction Ex. Skin and tongue Characteristic Features of Epithelial Cells Epithelial cells generally shows polarity Organelles and membrane proteins are distributed evenly within the cell Parts Basal Surface – contacting ECM and connective tissue Lateral Surface – adjoin neighboring cells; cell membranes have numerous folds Apical Surface – facing lumen and extracellular space Basement Surface Basal surface of all epithelia that rests on a thin extracellular matrix Glycoproteins and other components Functions Semipermeable properties Structural support Tissue regeneration Compartmentalization Basement Surface Basal Lamina Fine extracellular layer Thin meshwork of: Type IV collagen Laminin Nidogen and Perlecan Basement Surface Reticular Lamina Diffuse and fibrous layer Adjacent to the connective tissue Components Type III collagen – bound to basal lamina Type VII collagen – anchoring fibrils Both are produced by the cells of connective tissue. Basement Surface Lateral Surface Adjoin neighboring cells Cell membranes have numerous folds = increase area and functional capacity of the surface. Intracellular adhesion and other junctions Tight/Occluding Junctions Adherent/Anchoring Junctions Desmosomes Gap Junctions Hemidesmosomes Lateral Surface Lateral Surface Tight/Occluding Junctions Most apical of all the junctions “zonula” = junction forms a band; encircling the cell Formed by interacting transmembrane proteins such as claudin and occludin. Seal between the cell and tight Lateral Surface Tight/Occluding Junctions Serve as a fence restricting movements of membrane lipids and proteins at the apical surface into the lateral and basal surfaces Separate tissue spaces Apical Basolateral Lateral Surface Tight/Occluding Junctions Lateral Surface Adherent/Anchoring Junctions Zonula adherens Encircles the epithelial cell and it is below the tight junction Cadherins protein component creating strong attachment holding the epithelium together Binded by the presence of your calcium ions Lateral Surface Adherent/Anchoring Junctions Catenins Cytoplasmic end, cadherins now are linked to actin filaments and actin- binding proteins (vinculin). “Terminal Web” Actin filaments + adherent junction AJ + Apical pole of the epithelial cell function like a plastic band that hold cells together. Lateral Surface Adherent/Anchoring Junctions Lateral Surface Desmosomes/Macula Adherens Resembles a single “spot-weld” and does not form a belt around the cell Disk-shaped structures at the surface of one cell that are matched with identical structures at an adjacent cell surface. Lateral Surface Desmosomes/Macula Adherens Contains more cadherin family protein called desmogleins and desmocollins. Cytoplasmic ends bind plakoglobins and desmoplakins (attachment plaque; electron dense) Attachment plaque bind intermediate filaments proteins (tonofilaments/keratin) Lateral Surface Desmosomes/Macula Adherens Provides strong adhesion between cells Gives mechanical strength to tissues Lateral Surface Hemidesmosomes Type of anchoring junction found in the basal epithelial surface. They resemble a half desmosome Composed of transmembrane integrins Integrins bind to basal lamina specifically to the laminin Lateral Surface Hemidesmosomes Lateral Surface Gap Junctions Points of cell contact composed of transmembrane gap junction protein (connexins) to form hexameric complexes (connexons) that allow passage of small molecules from one cell to the other. Lateral Surface Gap Junctions Capable of controlling its passageway Calcium ions main factor on the opening and closing of the gap junctions Another factor is pH and voltage Lateral Surfaces Gap Junctions Apical Surface Apical surface features/structures found in cuboidal and columnar cells to increase the surface area for better absorption or to move substances along the surface Microvilli Stereocilia Cilia Apical Surface Microvilli Specialized for absorption in the apical cell surface Uniform in length Brush/Striated Border – microvilli- covered surface of epithelial cells Apical Surface Microvilli Microvilli interacts with the glycocalyx which include membrane-bound proteins and enzymes for digestion. Apical Surface Microvilli Each microvilli contains bundled actin filaments capped and bound to the surrounding plasma membrane. Actin filaments insert into the terminal web of cortical microfilaments at the base of the microvilli. Apical Surface Stereocilia Less common type of apical structure Seen on the absorptive epithelial cells lining the male reproductive system Increase cells’ surface area = facilitating absorption Apical Surface Stereocilia Ears – stereocilia have motion detecting functions found in the inner ear sensory cells Stereocilia are much longer, less motile, and show branching compared to microvilli. Apical Surface Cilia Long, highly motile apical structures, and larger than microvilli. Contains internal arrays of microtubules Primary cilium – not motile, rich with receptors and signal transduction complexes Apical Surface Cilia Motile cilia are abundant in cuboidal and columnar cells Core Structure: nine microtubule doublet (9+2 assembly /axoneme) Kinesin and Cytoplasmic Dynein Motors Apical Surface Cilia Axonemes are continues with those in basal bodies just below the cell membrane. Basal bodies have similar structure with centrioles (9+3). Forms rootlets anchoring the entire structure to the cytoskeleton Apical Surface Cilia Exhibit rapid beating patterns that move on one direction of fluid along the epithelium. Movement due to conformation of the axoneme making them stiff, but elastic. Apical Surface Cilia Movement utilizes ATP Dynein arms slide to adjacent doublet, creates bend produce sliding movements Beating motion of the cilium. Apical Surface Types of Epithelia Covering or Lining Epithelia Secretory Epithelia and Glands Arbitrary since there are lining epithelia that can also secrete and glands that are distributed among the lining of the cells. Covering or Lining Epithelia This type of epithelia covers the surface or line the cavities of an organ. Can be classified in terms of layers Can be classified in terms of cell shape Simple Squamous Epithelium Composed of thin polyhedral cells Function regulates the passage of substances into the underlying tissue. Organs Blood vessels Mesothelium Eye Organ Linings Simple Cuboidal Epithelium Composed of one layer of cuboidal cells Function For covering and secretion Organs Renal collecting tubules Thyroid follicles Ovaries Ducts Simple Columnar Epithelium Composed of one layer of tall but not wide cells Function For protection, lubrication, absorption, and secretion Organs GI Tract Gall Bladder Most cells with cilia Stratified Squamous Epithelium Two or more layers of squamous epithelia. Types Stratified Squamous Keratinized Epithelium Stratified Squamous Nonkeratinized Epithelium Stratified Squamous Keratinized Epithelium Undergoes the process of keratinization Upward movement to the skin surface keratin filaments Increased number of desmosomes = irregular in shape Helps prevent dehydration. Organ Epidermis of the skin Stratified Squamous Nonkeratinized Epithelium Layers of squamous epithelium that retain their nuclei and metabolic functions. Function Protection Secretion Prevents Water Loss Organs Vagina Esophagus Mouth Anal Canal Larynx Stratified Cuboidal Epithelium Layers of cuboidal epithelium Function Protection and Secretion Organs Sweat Glands Ureters Renal Calyces Stratified Columnar Epithelium Layers of columnar epithelium Rare type of epithelium Function Protection and mucous secretion Organ Conjunctiva Anus Urethra Pseudostratified Epithelium Layers of cells with nuclei at different levels Not all cells reach the surface but all adhere to the basal lamina Function Protection Secretion Cilia mediated transport Organ Trachea Bronchi Transitional Epithelium Also called the urothelium that posses umbrella cells. Function Protection and distensibility Organ Bladder Ureters Secretory Epithelia and Glands Glands - Epithelial cells that function mainly to produce and secrete various macromolecules Store, release and release Proteins (pancreatic secretions) Lipids (sebaceous glands) Carbohydrates + Protein ( salivary glands) Milk (mammary glands) Water + ions (sweat glands) Secretory Epithelia and Glands Scattered secretory cells = unicellular glands Common in simple cuboidal, simple columnar, and pseudostratified Goblet Cells – small intestine and respiratory tract Secretory Epithelia and Glands Exocrine Glands Glands that remained connected with the surface epithelium Endocrine Glands Lose the connection to their original epithelium; lacks ducts Adjacent to blood vessels (capillaries) Exocrine Glands Acini/Alveoli – saclike; secretory portion of the exocrine glands Duct – conducting portion Types Simple Compound Exocrine Glands Exocrine Glands Exocrine Glands: Secretory Mechanisms Merocrine Glands Most common methods of secretion Involves exocytosis from membrane- bound vesicle or secretory granules Ex. Salivary Glands Secretions can be serous or mucous Serous - digestive enzymes Mucous - respiratory secretions mucins + water = mucus Serromucous – salivary glands Holocrine Glands Secretions is produced by the disintegration of the secretory cells. Cells become filled with product as they divide Ex. Sebaceous glands and Meibomian Glands Apocrine Glands Secretion involves loss of membrane- enclosed apical cytoplasm. Releases lipid droplets Ex. Mammary Glands and Sweat Glands* Exocrine Glands Sebaceous glands vs. Sweat Glands Sebaceous Glands – glands found in the hair follicles that secrete sebum oily substance that protects your skin from drying out Sweat Glands - coiled tubular structures that secrete sweat Merocrine Apocrine Transport Across Epithelia Transcellular transport Transfer of ions via ion pumps and water via membrane channels Tight junction prevent paracellular diffusion Transcytosis Specialized transport through which an extracellular cargo is endocytosed, shuttled across the cytoplasm in membrane‐bound vesicles, and secreted at a different plasma membrane surface. Transport Across Epithelia Medical Applications Kartagener Syndrome Immotile cilia syndrome caused by mutation (autosomal recessive) Causes non-functional cilia leading to respiratory tract infections Also causes male and female infertility. Medical Applications Chronic Bronchitis Common to smokers Increase number of goblet cells in the lungs = increase mucus production; decrease ciliated cells Obstruction of airways Pseudostratified transforms to stratified squamous epithelium Medical Applications Acne/ Acne vulgaris Holocrine sebaceous glands Excessive sebum + keratin triggered by the surge of testosterone during puberty Propionibacterium acnes causes inflammation Medical Applications Tumors Benign and malignant Arise from most types of epithelial cells Carcinomas – epithelial origin Adenocarcinomas – glandular epithelial tissue