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Epithelium
Organisation of the Body
| Question | Answer |
|---|---|
| Main tissues in the body | Epithelium - aggregated polyhedral cells Muscle - elongated contractile cells Nervous - intertwining elongated processes Connective - several types; fixed and wandering |
| Functions of epithelium | Barrier between outside and inside organism Protection both mechanical and chemical from radiation, desiccations, toxins, pathogens, trauma Absorption in kidney and intestine Secretion in respiratory and alimentary tract |
| Forms of epithelia | Covering and lining epithelia - sheets that cover the body on external and internal surfaces Glandular/secretory epithelium - originated from invaginated epithelial cells as 3D secretory units |
| Classification by shape | Squamous - permeability - transport across Cuboidal - lining - absorption/secretion Columnar - protection, lubrication, absorption/secretion |
| Classification by stratification | Simple - single layer Stratified - several layers (can be keratinised) Pseudostratified - appearance of layers but none present Transitional - several layers may change shape |
| Classification by function | Protection - covering and lining epithelia Absorption - absorptive epithelia Transport - of molecules Secretion - glandular epithelia |
| Classification by specialisation | Cilia - movement of particles Microvilli - increase absorption area |
| Simple squamous epithelium | Lines blood vessels, lymphatic vessels, loop on Henle and alveoli A single layer of flat cells, nuclei appear as bumps For diffusion, secretion, some protection and secretion/absorption |
| Simple cuboidal epithelium | Kidney tubules, glands and ducts, terminal bronchioles and choroid plexus Single layer of cube cells with microvilli or cilia Active transport and facilitated diffusion result in secretion/absorption and movement of mucus |
| Simple columnar epithelium | Glands and ducts, bronchioles, uterus, stomach, intestines Single layer of tall, narrow cells. Some have cilia or microvilli - goblet cells are present Movement of particles out of bronchioles, secretion by cells of glands, stomach and intestine |
| Stratified squamous epithelium | Non-keratinised - mouth, larynx, oesophagus Keratinised - skin Multiple layers of cells cuboidal at the base but stratified at the top. Keratinised - cytoplasm replaced by keratin For protection against abrasion and infection |
| Stratified cuboidal epithelium | Sweat glands, ovarian follicular cells, salivary gland duct Multiple layers of somewhat cube shaped cells Secretion, absorption and protection against infection |
| Pseudostratified columnar epithelium | Lining of nasal cavity, sinuses, auditory tubes, trachea and bronchi Single layer of cell, with some reaching the free surface and others not. Ciliated and associated with goblet cells Synthesise and secrete mucus onto free surface and move mucus |
| Transitional epithelium | Lining of bladder, ureters and superior urethra Stratified cells that appear cuboidal when not stretched and squamous when stretched by fluid Accommodated fluctuations in the volume of fluid in an organ or tube |
| Specialisations related to function | Cilia - microtubule core in respiratory tract Microvilli - microfilament core in the small intestine |
| Microvilli | On columnar epithelium of gut To increase surface area for absorption Microfilament core - actin Allows cell to change shape Give a brush border appearance |
| Cilia | On pseudostratified columnar of respiratory tract To propel mucus up and out - mucociliary escalator Microtubule core Motor protein dynein moves along tubule enabling cilia to move |
| How do cilia move | Axoneme core = 9+2 arrangement of microtubules Motor protein to generate force - dynein Microtubule gliding powered by ATP hydrolysis |
| Exocrine glands | Retain contact with surface through ducts Secrete products on to external or internal epithelial surface Can take a simple tubular/acinar shape, or be more complicated in compound tubuloacinar shapes |
| Endocrine glands | Ductless - lost connection to originating epithelium Secrete products into blood or lymphatic vessels |
| Goblet cells | Unicellular glandular epithelial cell Basal region sits on basal lamina Apical portion - the theca - faces the lumen Theca filled with membrane-bound secretory droplets Secrete mucus to lubricate cell surface in respiratory tract |
| Structure of pseudostratified columnar cells | Cilia of cells beat up and out to shift mucus produced by goblet cells Beneath epithelium - connective supporting tissue (Lamina propria) Epithelium and lamina propria = mucosa Beneath mucosa - submucosa and adventitia (connective tissue Larger glands) |
| Embryological origin of epithelia | Can be derived from any of the three germ layers Location dictated which layer it was formed from |
| Ectodermal epithelia | Stratified or pseudostratified Oral and nasal mucosa Cornea Skin Glands of skin Mammary glands |
| Ectodermal epithelial | Simple Lining of respiratory tract Small intestine Pancreas Gall bladder Liver |
| Mesodermal epithelium | Simple Kidney Pleurae Peritoneum Pericardium Mesothelium Reproductive systems Endothelial cells of blood vessels |
| Basement membrane | Boundary between epithelium and connective tissue which surrounds and supports epithelium Acts as a mechanical barrier (protects anything moving into connective tissue) Comprises basal lamina and reticular lamina |
| Basal lamina | Synthesised by epithelial cells and contains anchoring fibrils of type 4 collagen which extends into reticular lamina |
| Reticular lamina | Produced by connective tissue and contains fibres and collagen bundles around which fibrils loop |
| Polarity of epithelial cells | Apical-basal polarity Two domains - apical and basolateral Domains are distinct - morphologically, biochemically and functionally Specialised functions are associated with different domains |
| Apical domain | Rich in ion channels and carrier proteins Protection Secretion Absorption Regulation of nutrient and water uptake |
| Basolateral domain | Junctional specialisation (connects cells) Cell contact Adhesion Communication Ion channels Receptors Basal infoldings |
| Epithelial junction on basolateral domain | Tight junctions Anchoring junctions Gap junction |
| Tight junctions | Occluding junctions - block intercellular transport Zona occludens Connects in a band around cells |
| Anchoring/adhesive junctions | Link adjacent cytoskeletons Adhering junctions - zonula adherens - connects cells in a band Desmosomes - macula adherens |
| Gap junctions | Allow communication Channels allow passage of small molecules and ions between cells |
| Junctional complex | Comprised of zonulae occludens, zonulae adherens and desmosomes |
| Tight junction transmembrane proteins | Particularly in the blood brain barrier Linked to cytoplasmic MAGUK proteins (membrane associated with guanylyl kinase-like domain) Zonula occludens (ZO-1,2,3) Principle proteins - Claudins and occludins (establish a fence function- high affinity) |
| Cadherins at anchoring junctions | Bind to each other intercellularly Adhesion molecules linked to actin filaments intracellularly Filaments run into terminal web - actin filaments stabilized by spectrin |
| Cadherins at desmosomes | Attached to a cytoplasmic plaque Adhesion molecules linked to intermediate filament cytoskeleton |
| Pemphigus | Blistering disease affecting keratinocytes and mucous membranes Autoantibodies against desmosome protein desmoglein Mainly affects women between 50-60 Treated by corticosteroids and plasmapheresis |
| Desmoglein | Adhesion glycoprotein Calcium binding transmembrane protein that binds desmosome to keratin Member of cadherin cell adhesion superfamily 4 isoforms - Desmoglein 1 and 3 most common in disease |
| Subtypes of pemphigus | Pemphigus vulgaris - blisters develop deep in epidermis Pemphigus foliaceus - blisters develop in superficial layers Paraneoplastic pemphigus - deep layers and neoplasm - 90% mortality |
| Causes of pemphigus | Genetic - several HLA genes Environment - drugs or viruses or diet |
| Examples of autoimmune blistering diseases | Pemphigoid - antibodies to proteins of dermis epidermis junction IgA-mediated dermatoses - aberrant deposits of IgA in BM Epidermolysis bullosa acquista - antibodies against type 7 collagen which attached dermis to epidermis |
| Epidermolysis bullosa | A group of genetic conditions leading to easy blistering of skin and mucous membranes Caused by mutations in keratin filaments -18 genes Keratin attached to cell membrane at site of desmosome and hemidesmosome junction Maintains integrity of epidermis |
| Types of epidermolysis bullosa | Epidermolysis bullosa simplex - KRT5 and KRT14 genes Dystrophic epidermolysis bullosa - COL7A1 gene Junctional epidermolysis bullosa - hemidesmosomes affects Lamin and collagen Kindler syndrome - blistering in young children |
| Treatment of epidermolysis bullosa | Wound care Pain control Controlling infections Nutritional support Prevention and treatment of complication In future, regeneration using transgenic stem cells may be possible |
| Connexins in gap junctions | Proteins of gap junctions Each gap junction has 6 pairs of connexins (one in each cell) |
| Epithelial cell transport | Can be between cells - paracellular Can be through a cell - transcellular Some cells transport ions and fluid from apex to base (e.g. kidney) and some from base to apex (e.g. sweat gland) |
| Transepithelial transport of ions | Selective exchange of ions Na+ enters cells by diffusion Actively extruded by basal NA/K ATPase to maintain low intracellular concentration Can drive transport of other molecules by co transport |
| Co-transporters in gut epithelium | Na/glucose cotransporter Na/phosphate cotransporter Na/iodide cotransporter Na/K/Cl cotransporter Na/Cl cotransporter K/Cl cotransporter |
| Modifications to intestinal lumen | Plicae circulares - transverse folds increase SA and decrease velocity of chyme movement Villi - epithelial covered protrusions containing capillary and lymphatic channels Microvilli - modifications of apical plasmalemma Crypts of Lieberkühn - glands |
| Glycocalyx | Visible at the surface of gut epithelium under electron microscopes Where carbohydrate residues are attached to transmembrane proteins of the plasmalemma |
| How do occluding junctions appear in freeze fracture | Branching and interconnected sealing ridges |
| Renewal of epithelial cells | High turnover - renewed by mitosis Epidermis - renewed at basal layer, migrate to surface with skin keratinised on route Small intestine replaced every 4-6 days Other epithelia - renewed until adulthood and only proliferate when damaged |
| Dysplasia of epithelium - abnormal differentiation |
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