Organisation of the Body
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| Main tissues in the body | Epithelium - aggregated polyhedral cells
Muscle - elongated contractile cells
Nervous - intertwining elongated processes
Connective - several types; fixed and wandering
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| 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
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| 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
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| Classification by shape | Squamous - permeability - transport across
Cuboidal - lining - absorption/secretion
Columnar - protection, lubrication, absorption/secretion
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| 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
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| Classification by function | Protection - covering and lining epithelia
Absorption - absorptive epithelia
Transport - of molecules
Secretion - glandular epithelia
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| Classification by specialisation | Cilia - movement of particles
Microvilli - increase absorption area
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| 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
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| 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
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| 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
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| 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
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| Stratified cuboidal epithelium | Sweat glands, ovarian follicular cells, salivary gland duct
Multiple layers of somewhat cube shaped cells
Secretion, absorption and protection against infection
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| 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
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| 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
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| Specialisations related to function | Cilia - microtubule core in respiratory tract
Microvilli - microfilament core in the small intestine
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| 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
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| 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
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| How do cilia move | Axoneme core = 9+2 arrangement of microtubules
Motor protein to generate force - dynein
Microtubule gliding powered by ATP hydrolysis
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| 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
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| Endocrine glands | Ductless - lost connection to originating epithelium
Secrete products into blood or lymphatic vessels
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| 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
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| 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)
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| Embryological origin of epithelia | Can be derived from any of the three germ layers
Location dictated which layer it was formed from
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| Ectodermal epithelia | Stratified or pseudostratified
Oral and nasal mucosa
Cornea
Skin
Glands of skin
Mammary glands
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| Ectodermal epithelial | Simple
Lining of respiratory tract
Small intestine
Pancreas
Gall bladder
Liver
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| Mesodermal epithelium | Simple
Kidney
Pleurae
Peritoneum
Pericardium
Mesothelium
Reproductive systems
Endothelial cells of blood vessels
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| 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
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| Basal lamina | Synthesised by epithelial cells and contains anchoring fibrils of type 4 collagen which extends into reticular lamina
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| Reticular lamina | Produced by connective tissue and contains fibres and collagen bundles around which fibrils loop
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| 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
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| Apical domain | Rich in ion channels and carrier proteins
Protection
Secretion
Absorption
Regulation of nutrient and water uptake
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| Basolateral domain | Junctional specialisation (connects cells)
Cell contact
Adhesion
Communication
Ion channels
Receptors
Basal infoldings
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| Epithelial junction on basolateral domain | Tight junctions
Anchoring junctions
Gap junction
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| Tight junctions | Occluding junctions - block intercellular transport
Zona occludens
Connects in a band around cells
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| Anchoring/adhesive junctions | Link adjacent cytoskeletons
Adhering junctions - zonula adherens - connects cells in a band
Desmosomes - macula adherens
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| Gap junctions | Allow communication
Channels allow passage of small molecules and ions between cells
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| Junctional complex | Comprised of zonulae occludens, zonulae adherens and desmosomes
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| 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)
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| 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
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| Cadherins at desmosomes | Attached to a cytoplasmic plaque
Adhesion molecules linked to intermediate filament cytoskeleton
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| Pemphigus | Blistering disease affecting keratinocytes and mucous membranes
Autoantibodies against desmosome protein desmoglein
Mainly affects women between 50-60
Treated by corticosteroids and plasmapheresis
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| 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
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| 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
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| Causes of pemphigus | Genetic - several HLA genes
Environment - drugs or viruses or diet
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| 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
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| 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
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| 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
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| 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
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| Connexins in gap junctions | Proteins of gap junctions
Each gap junction has 6 pairs of connexins (one in each cell)
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| 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)
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| 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
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| Co-transporters in gut epithelium | Na/glucose cotransporter
Na/phosphate cotransporter
Na/iodide cotransporter
Na/K/Cl cotransporter
Na/Cl cotransporter
K/Cl cotransporter
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| 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
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| Glycocalyx | Visible at the surface of gut epithelium under electron microscopes
Where carbohydrate residues are attached to transmembrane proteins of the plasmalemma
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| How do occluding junctions appear in freeze fracture | Branching and interconnected sealing ridges
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| 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
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| Dysplasia of epithelium - abnormal differentiation |
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