Epithelium I - Membrane (1) (1) PDF

EPITHELUIM 1: MEMBRANE  Learning Objectives 1. Identify and describe the four major types of tissues. 2. Identify and describe the various types of Epithelial tissue. 3. Identify the location and describe the various functions of Epithelial tissue.  Introduction...

EPITHELUIM 1: MEMBRANE  Learning Objectives 1. Identify and describe the four major types of tissues. 2. Identify and describe the various types of Epithelial tissue. 3. Identify the location and describe the various functions of Epithelial tissue.  Introduction Tissues; groups of similar cells and extracellular products that carry out a common function 4 principal types of tissues in the body: – Epithelial tissue – Connective tissue – Muscle tissue – Nervous tissue.  Epithelial Tissue: Introduction Little extracellular matrix (ECM) Derived from all 3 germ layers All epithelium lie on the basement membrane BASAL MEMBRANE Compose d BASAL Held together RETICULAR by LAMINA attached LAMINA Protein LAMINA LAMINA LUCIDA DENSA  Functions of Epithelial Tissue ❖Protection of underlying tissues ❖Absorption (taking molecules into cells) ❖Diffusion (movement of molecules down their concentration gradients) ❖Secretion (releasing molecules from cells)  Characteristics of epithelial Tissue ❖ Polarity (Apical surface differs from basal surface) ❖ Avascular (has no blood vessels) but innervated (has nerves). It receives nutrients from underlying connective tissue where capillaries lie. ❖ Regeneration (lost cells are quickly replaced by mitosis) ❖ Specialized contacts (cells joined by special junctions) Types of epithelial tissue 2 main groups according to their structure and function: – covering epithelia – glandular epithelia.  Covering Epithelia Cells are organized in layers that cover the external surface or line the cavities of the body Classification 1. According to the number of cell layers and the morphological features of the cells in the surface layer. – Simple epithelium; one layer of cells – Stratified epithelium; more than one layer  Classification 2. Classified according to cell shape of its superficial layer: squamous, cuboidal, columnar 3. The presence of surface specializations i.e. cilia and keratin. E.g. Stratified squamous keratinized epithelium (skin) and ciliated pseudostratified columnar epithelium in the respiratory passages. According to the number of cell layers According to shape of cells Type Cell shape Example Squamous Squashed Endothelium (lines blood vessels), mesothelium (serous lining of ce lom) C uboidal C ubed Wa lls of gland s C olumnar C olumns Lining of gut tube; sometimes with cilia like lining of uterine tube Pseudo-stratified Flat ce lls give rise With c ilia in respiratory to c olumns tubes to move muc ous/partic les out of lungs According to apical specialisation  Simple squamous epithelium Simple squamous epithelia is composed of flattened, irregular shaped cells forming a continuous surface also called pavement epithelia. Squamous –compared to fish scales. Is supported by basement membrane. Lines surfaces involved in passive transport (diffusion ) of gases or fluids (capillaries). Lines pleural, pericardial and peritoneal cavities and blood vessels (endothelium).  Simple squamous epithelium Endothelium that lines blood and lymph vessels and the mesothelium that lines certain body cavities, such as the pleural and peritoneal cavities, and covers the viscera are examples of  Stratified squamous epithelium Has many cell layers. Basal cells are cuboidal and surface cells are flat. Epithelium well adapted to withstand abrasion  Stratified squamous epithelium Location: Oesophagus Vagina Uterine cervix Mouth Anal canal pharynx *Sites subjected to mechanical abrasion  Keratinzed Stratified squamous epithelium Has keratin Location: Epidermis of the skin  Simple Cuboidal Epithelium Cells appear square; polygonal on surface view. Nucleus is round and located in the center of the cell.  Simple Cuboidal Epithelium Location lines small ducts and tubules that may have excretory, secretory or absorptive functions; e.g. Collecting tubules of the kidneys, small excretory ducts of the  Stratified Cuboidal Epithelium Multiple layers of cuboidal cells Location: Excretory ducts of salivary and sweat glands  Simple columnar epithelium Cells are taller, appear columnar in sections at right angles to the basement membrane. Nuclei are elongated and usually near the base. Can sometimes be central and occasionally apical.  Simple columnar epithelium Location: lines the intestine Stomach Gall bladder Uterus *absorbs nutrients from the lumen of the intestine  Simple columnar ciliated epithelium Some simple columnar epithelia bear surface cilia on majority of the cells with a few among them being non-ciliated that usually have a secretory function. Cilia are motile and propel fluid Location: oviduct  Pseudostratified columnar ciliated epithelium Majority of cells are ciliated Cells are attached to the basement membrane and appear in section to be in more than on layer. Nuclei are at different levels and not all cells extend to the free surface, creating the illusion of cellular  Pseudostratified columnar ciliated epithelium Location: Lines the upper respiratory tract (respiratory epithelium) Cilia propel mucus which is produced by nonciliated goblet cells  Stratified columnar epithelium Multiple layers of columnar cells Location: Lines the mucous membrane of the conjuctiva Lobar ducts of salivary glands Certain parts of a male urethra  Transitional epithelia or Urothelia Found only in the urinary tract Allows a great degree of stretch Has some features intermediate (transitional) between stratified cuboidal and stratified squamous epithelia In non-distended state, it appears to have 4 to 5 cell layers thick  Transitional epithelia or Urothelia Basal cells are cuboidal, intermediate cells are polygonal and surface cells (umbrella or dome cells) are large and round In distended state, appears to have 2 or 3 cell layers (actual number of layers remain constant) and the intermediate and surface layers are extremely flattened.  Other types of epithelium Neuroepithelial cells are cells of epithelial origin with specialized sensory functions (eg, cells of taste buds and of the olfactory mucosa).  Other types of epithelium Myoepithelial cells are branched cells that contain myosin and a large number of actin filaments. They are specialized for contraction, mainly of the secretory units of the mammary, sweat, and salivary glands  Intercellular adhesion Epithelia cells adhere strongly to neighbouring cells 3 classes of intercellular junctions – Tight junctions/ occluding junctions/zonula occludens – Adherent junctions (intermediate) – Gap junctions communication  Characteristic features of epithelium Tight junctions/Occluding junctions Forms a band completely encircling each cell Creates diffusion barriers Prevents passage of materials across the cellular layer thru intercellular space (eg urinary bladder)  Intercellular adhesion Medical significance Defects in occludins may compromise foetalblood brain-barrier leading to severe neurological disorder  Intercellular adhesion Adherent junction/ zonula adherens Encircles epithelial cells immediately below zonula occludens Mediated by cadherins (ca2+ dependent) Desmosomes/ macula adherens “adhering spot”, button or snap between adjacent epithelial cells a small region that holds cells together and provides resistance to mechanical stress at a single point  Hemidesmosomes Anchors cytoskeleton to basal lamina Skin disorder blistering Autoimmune disease against desmosomal proteins, (skin), results in a skin disease called pemphigus vulgaris. Results in widespread blistering Intercellular adhesion Gap junction Consists of aggregated transmemebrane protein complexes that form circular patches Permit interceullular exchange of molecules  Specialisation of the apical surface Apical end specialised projections Function to increase surface area for absorption, and move substances along epithelial surface Microvili, stereocilia, cilia  Microvilli Visible EM as brush or striated borders 1µm long and 0.5µm Contain actin filaments capped and cross linked to each other Function: absorption  Stereocilia Restricted to absorptive epithelial cells linning epididymis, proximal part of the ductus deferens Actin filaments Longer than microvilli  Cilia Contain arrays of microtubules It’s Motile 5-10µm long, 0.2µm wide Each has a core structure consisting of 9 peripheral microtubular doublets 9 doublets form an array around 2 central microtubules called axoneme  Clinical application Tumors that arise from epithelial cells may be benign (nonmalignant) or malignant Malignant tumors arising from epithelia are called carcinomas those arising from glandular epithelial cells are called adenocarcinomas  References Mescher AL Junqeira’s Basic histology 13th Edition Young B. et al Wheaters functional histology 5th edition Standring S. Grays Anatomy 39th Edition  The End Thank you