Lecture 6 - Epithelial Tissue PDF

Lecture 6 Epithelial Tissue Presented by : Histology Department  LEARNING OUTCOMES (LOs) 1- Describe the general characteristics of the epithelial tissue. 2- Classify types of epithelium. 3- Describe the microscopic structure of each type of epithelial tissue. 4- Correlate the microscop...

Lecture 6 Epithelial Tissue Presented by : Histology Department  LEARNING OUTCOMES (LOs) 1- Describe the general characteristics of the epithelial tissue. 2- Classify types of epithelium. 3- Describe the microscopic structure of each type of epithelial tissue. 4- Correlate the microscopic structure to the function of each type of epithelial tissue. 5-Classify glandular epithelium 6-Describe the structure of Neuroepithelium. 7-Describe the structure of Myoepithelium.  LEARNING OUTCOMES (LOs) 8. Define epithelial polarity 9. List the specializations of the different epithelial cell surfaces. 10. Describe the structure and correlated functions of the different types of the cell junctions. 11. Mention examples of sites of the different types of the cell junctions. 12. Define the junctional complex. 13. Describe basal surface specializations. 14. Describe apical surface specializations. 15. Correlate changes in the structure of different epithelial cell 3 surface specializations with related clinical conditions. Tissue: group of Epithelial Introduction tissue cells with similar tructure & function 4 Nervous Connective tissue Basic tissue Tissues Muscular tissue  General characteristics of Epithelium 1. COVERS SURFACES & LINES CAVITIES. 2. Composed mainly of tightly packed cells with Little intercellular substances. 3. The cells are separated from the underlying connective tissue by the basal lamina. 4. The cells are bound together by junctional complexes & form sheets. 5. Avascular: No blood vessels enter inbetween the cells but nerves can. Nourishment?? Occurs by diffusion. 6. The cells are continuously renewed and replaced.  TypesTypes of Epithelium of Epithelium I- surface II- Glandular III- Neuro- IV- Myo- epithelium epithelium epithelium epithelium Covering Lining Secretory Sensation Contraction surface cavities Mescher AL: Junqueira’s Basic Histology: Text and Atlas, 12th Edition. http://www.accessmedicine.com Nice to know  Surface epithelium (Epithelial membranes) Covers the body & also lines the cavities. Simple Stratified Two or more layers (named for the shape One layer (same shape) of cells in the top layer) ❑ Simple squamous ❑ Stratified squamous ❑ Simple cubical ❑ Stratified cubical ❑ Simple columnar ❑ Stratified columnar ❑ Pseudostratified columnar ❑ Stratified transitional Nice to know  Types of Simple Epithelium Simple Simple Simple Pseudostratified Squamous Cuboidal Columnar columnar  1-Simple squamous epithelium ✓ One layer of flat cells (scale like). ✓ Each cell is formed of a thin cytoplasm and flat nucleus. ✓ Sites: - Lining the lung alveoli. - Lining the blood vessels (endothelium) - Lining serous membranes (mesothelium) : pleura, pericardium, peritoneum, Nice to know  Function of simple squamous epithelium Transport: as it is thin, & substances can pass through it. For example: oxygen passes to the blood, through the simple squamous epithelium that lines the lung alveoli  2-Simple cubical epithelium One layer of cubical (length= width) cells resting on the basement membrane. Each cell has a single rounded central nucleus. Site: -Thyroid follicle - Ducts  Determine the epithelium in the photo. Simple cubical Simple squamous  3-Simple columnar epithelium One layer of columnar cells (cells are more tall than wide). Each cell has a single, basal oval nucleus. Sites: lining the stomach, intestines, and some areas of the respiratory and reproductive tracts. Sometimes in some areas (e.g: intestines) you can see “open spaces” among the cells of columnar epithelium. These are specialized goblet cells that produce mucus. ❑ Goblet cells ❑ Columnar epithelial cells The function of simple columnar epithelium Mainly absorption. Sometimes secretion.  4-Pseudostratified columnar epithelium The cells are overcrowded, some cells are tall and others are short and not reaching the surface. So, the epithelium appears to be two cell thick, but in fact it is only ONE cell thick as all the cells rest on the basal lamina (basement membrane). This is the reason it is called pseudo (or false) stratified epithelium. In some places the epithelium has cilia that extend from the cells. In other places it has goblet cells. Sites: - Trachea - Bronchi.  II-Stratified epithelium Stratified squamous non- keratinized: consists of several layers of cells of different shapes. ✓ basal cells are cubical or columnar FUNCTION: regeneration ✓ intermediate cells are polygonal Mescher th AL: Junqueira’s Basic Histology: Text and Atlas, 12 Edition. http://www.accessmedicine.com which gradually flatten ✓ surface cells: are squamous cells. - Sites: in wet surfaces of all openings leading to the skin: Oral cavity Tongue Cornea Anal canal  Stratified Squamous keratinized epithelium -The flat cells of the top layer are changed into non-living layer of keratin. -This type is tough and resists friction and is impermeable to water. Sites: epidermis of the skin Ross MH, Pawlina W: Histology A Text & Atlas with correlated Cell & Molecular Biology, 6th Edition.  Function of stratified epithelium Protection. Stratified squamous epithelium protects the body against invasion by microbes & microorganisms. Stratified squamous keratinized ep is found in the skin. One way of preventing infections, therefore, is to take good care of your skin. Don’t let it become cracked, and guard against cuts and scratches.  Stratified transitional epithelium ❑ Stratified transitional epithelium found in body areas subjected to stress and is able to stretch; e.g. urinary bladder. ❑ In unstretched case: 10 layers of cells of different shape & sizes are present. The top layer is large cubical with rounded one or 2 nuclei. ❑ In stretched condition: the number of cell layers decreases to expand the area, and cell shape changes from roughly cuboidal to nearly squamous (flat) in appearance. ❑ Function: keeps the bladder wall from tearing under the pressures of stretching. Stratified transitional epithelium  II-Glandular Epithelium ❑ Simple cuboidal epithelium adapted for secretory function. ❑ Glands are classified according to number of cells into : 1- Unicellular gland: are goblet cells. The gland is formed of one cell scattered among other epithelial cells. Sites: in trachea and intestine 2- Multicellular: The gland is formed of many cells. Cuboidal cells forming wall of gland Nice to know ❑ Goblet cells  Classification of Glands Multicellular glands are : divided according to presence or - absence of ducts into: ❑ Endocrine glands: The cells loose their connection to the surface and the secretion passes to capillaries (to blood). e.g thyroid gland ❑Exocrine gland: The cells are connected to the surface by a duct. e.g. salivary glands. Nice to know  Multicellular Glands are also classified according to type of secretion into: 1-Mucous glands (acini) Secrete a viscous thick secretion containing mucin. 2-Serous glands (acini) Secrete a watery secretion rich with enzymes 3-Mixed glands (Seromucous acinus) Secrete both mucous and serous secretions.  Multicellular Glands (According to type of secretion) Mucous Serous acini acini Foundation of Basic Medical Sciences I Module  Difference between Mucous and Serous acini Mucous acini Serous acini Size Large Smaller (Diameter) Lumen Wide Narrow Cross few number of cuboidal cells larger number of pyramidal cells section well-defined boundaries ill-defined cell boundaries Cytoplasm Pale basophilic and Basophilic with Apical acidophilic vacuolated (due to mucous) secretory granules. Nucleus single flattened nucleus single rounded nucleus Secretion Mucous ( a viscous thick Watery containing enzymes secretion)  III- Neuroepithelium A special type of epithelium which is modified to receive stimuli (sensation) and convert them into electrical impulses. Sites: - Taste buds in the tongue - Olfactory mucosa - Inner ear receptors for hearing and equilibrium. Nice to know  IV-Myo-epithelium -Specialized epithelial cells for contraction. -Contain actin, myosin and intermediate filaments. -Present around mucus & serous acini, their contraction squeeze secretions into the ducts. Nice to know 27  Metaplasia ❑ In smokers: the pseudo-stratified columnar epithelium of the respiratory tract due to the irritation of smoke may be transferred to stratified squamous epithelium. Nice to know 28  Epithelial Polarity Definition: asymmetrical distribution of organelles and membrane proteins within the cell. - Basal pole (basal surface): region of the cell facing CT. - Apical pole (apical surface): region of the cell facing a space. - Lateral surface: the region of the cell facing the neighboring cells. -Each surface has specific specializations  Junctions Lateral surface Lateral inter-digitation Basal lamina Specialization of Epithelial Basal Basal infoldings cell surfaces surface (Modifications) Hemidesmosome Microvilli Apical surface Cilia Stereocilia  Apical Surface Specializations 1-Cilia 2-Flagella 3- Microvilli 4- stereocilia For cilia & flagella revise the cell. 1- Cilia (kinocilia) lM: Hair-like processes present on the apical surface of certain cells (e.g. in respiratory epithelium).( motile). EM: Each cilium is formed of 1- Basal body:-Formed of 9 peripheral triplet microtubules(9 X 3) 3- Shaft (axoneme): It is formed of 9 peripheral doublets and in the central part there are 2 central singlets (9x2 + 2) 31  Basal body of cilia Shaft of Cilia https://img.scoop.it/G8EVU-CuFLSWJDfyPCgNlzl72eJkfbmt4t8yenImKBVvK0kTmF0xjctABnaLJIm9 http://cytochemistry.net/cell-biology/cilia.htm 2. Flagella ( Sperms) They have the same structure as cilia (contain similar arrangement of microtubules). Nice to know It is single and longer than cilia.  Apical Surface Specializations 3-Microvilli -Small cytoplasmic Finger-like projections (1μm long, 0.08μm wide) extending from the apical cell surface. - Site : apical cell surface. Nice to know - In the intestine, microvilli on the cell surface forms a brush border, to increase the surface area to aid the absorption of nutrients. - The cores contains actin microfilaments. 33  4 –Stereo-cilia They are not cilia They are very long and irregular microvilli They are found in the male reproductive tract to increase the surface area for fluid absorption. Nice to know 34  Lateral Surface Specializations 1- Lateral interdigitations Are finger-like projections that interlock adjacent epithelial cells together. Nice to know 35  Lateral Surface Specializations 2- Cell Junctions Def.: Sites of contact between lateral surfaces of cells. - Trans membrane protein in adjacent cell membranes are the main structures of cell junctions. Types: 1- Occluding 2- Adhering 3- Communicating  Lateral Surface Specializations 2- Cell Junctions Types Occluding (tight Adhering Communicating ) junctions (anchoring) (gap) junctions junctions Intercellular Cell membrane Cell membranes Membrane space of both cells of both cells come proteins form fuse together in close contact channels between with a minimal adjacent cells space Transmembrane Occludins Cadherins Connexins protein Claudins 37  Cell Junctions Zonula: If the junction extends around the entire cell perimeter like a ring or belt Fascia: If the junction is strip-shaped or band- shaped Macula: If the junction is spot like Nice to know 38  1- Occluding Junction (Tight Junction) - At the apical part of the cell. At the junction, neighboring cell membranes fuse together then diverge, then fuse again several times like a zipper. - It is formed of the transmembrane proteins called: claudins & occludins. Their cytoplasmic part bind to actin microfilaments. - It forms the barrier function of the membrane. Nice to know 39  Sites of Occluding junction --Columnar cells of the intestine -Endothelium of blood capillaries in the brain Functions of occluding junctions: 1- Prevent passage of toxic & harmful material to brain 2- Prevent movement of molecules & ions to the inter- cellular space (prevent paracellular transport). (Histology Department) 40  Adhering Junctions - The cell membranes don’t fuse, but come in close contact with the neighbouring cell membrane leaving a minimal intercellular space. - Cadherin is the transmembrane protein that joins to hold neighbouring membranes. - It can be either - Zonula adherens or macula adherens. 41  Adhering Junctions 1- Zonula adherens - Forms a belt around the entire circumference of the cell immediately below tight junctions. Cadherins proteins span between the cell membranes. Intercellular space is 20 nm. The cytoplasmic part of cadherin are associated with actin filaments 42  Adhering Junctions 2- Macula adherens (Desmosomes) -Similar to Zonula adherens but form a spot like junction. - Intercellular space is 30nm. The link proteins are members of cadherin family called: desmoplakins & desmogleins. - Cadherins are anchored internally to intermediate filaments (tonofilaments). -Desmosomes :Provides structural and 43 mechanical link between cells.  Junctional Complex - Often, several types of epithelial junction are found in close proximity to each other. This is known as a junctional complex. Example: zonula occludens, zonula adherens & desmosome Found at the apex of intestinal epithelial cells. 44  Gap Junction (Communicating Junction or Nexus) - Present in epithelial and non epithelial tissues - The inter cellular space is 2-3 nm. - Six transmembrane proteins called (connexins) forms a tiny channel called connexon which join a similar channel from the adjacent cell forming a bridge. - Gap junction is not associated with any cytoplasmic filaments. - Communicating (gap) junctions: allow direct cell–cell communication by allowing diffusion of small molecules and ions between adjacent cells. 45  Gap Junction Sites: not only in epithelia but they can occur also in: Cardiac muscles, nerve tissue, bone cells and smooth muscles. Function: - It makes the cells act together in a coordinated manner. - They transmit nerve impulses. 46 Compare between the different types of junctions 47  Basal Surfaces Specialization Basal surface specialization include: 1- Hemidesmosomes. 2- Basal membrane infoldings. 3- Basal lamina. 48  1-Hemidesmosomes -Similar to half of a desmosome but: joins epithelial cells to the underlying basement membrane -The involved transmembrane protein is called : Integrin. Integrins bind to components of the basement membrane. -The intermediate filaments (tonofilaments )attached to the cytoplasmic plaques. 49  2-Basal membrane infoldings -These are multiple finger-like infoldings of the basal plasma membrane. - Mitochondria are present between these infoldings. -Found in ion transporting cell. -Function: ↑ Surface area of plasmalemma. In ion transporting cells (e.g. kidney tubules) 50  3-Basal lamina -It separates epithelial cells from the underlying CT. The basal lamina is a thin layer composed of type IV collagen + laminin, and fibronectin. -Formed of 2 layers by EM: 1- Lamina lucida 2- Lamina densa (Histology Department) 51  Pemphigus : pemphigus vulgaris & bullous pemphigoid Blistering disorders occurring due to antibodies against desmosomal proteins resulting in lack of cell to cell adhesion. eg. pemphigus 52  Food poisoning In Food poisoning, the toxins secreted by bacteria (Clostridium perfringens) will bind with claudin molecules of tight junction in intestinal cells. This will result in loss of tissue fluid into the intestinal lumen through the paracellular pathway. 53 Immotile cilia syndrome (genetic disease)  Celiac disease Celiac disease is caused by an immune reaction against the wheat protein gluten. This results in intestinal inflammation and decrease of microvilli of absorptive cells leading to malabsorption. 55  Match A B Cadherin Microvilli Occludin Adhering junctions Lamina lucida Ion transporting cells Core of Actin filaments Tight junction Basal membrane infoldings Basal lamina  References -Oxford hand book of medical sciences 2nd edition -The human body in health & disease 6 th edition - USMLE Step 1 Lecture notes 2018 - BRS 57 58