Epithelial tissue as barriers

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Diffusion barriers

● Epithelial tissue allows to divide extracellular space into compartments

● Examples: intestine, kidney, exocrine glands, capillaries in brain (hemato-encephalic barriers), plexus choroideus (barriers blood-liquor), thymus, testes

Junction complex

● Zonula occludens – tight junction – intercellular space disappears

● Zonula adherens – adhesive junction – actin attachment to terminal web

● Macula adherens – desmosome – attachment of intermediate filaments - cytokeratins

Zonula occludens- tight junction

● Claudins and occludins – intercellular space disappears – it prevents paracellular transport of substances = diffusion barrier

● Attachment of actin filaments using adaptor proteins

Zonula occludens

● Tight junction – belt-like junction surrounding all surface of cell

● Permeability varries in different epithelial cells:

● Impermeable in collecting tubules and capillaries in brain

● Almost permeable in small intestine and proximal tubules in kidney

Basement membrane

● Interaction between cell and matrix – cellular anchorage

● Structure – proteins and proteoglycans

● Lamina basalis● Lamina rara (lucida)● Lamina densa

● Lamina fibroreticularis

Basement membrane● Lamina basalis – cellular attachment to

background● Lamina rara - integrins (integral proteins of

membrane)● Lamina densa – collagen IV, laminin, perlecan

● Lamina fibroreticularis – microfibrils and anchoring fibers – fibrillin and collagen VI – attachment to collagen fibers (collagen III – reticular fibers)

● Extraordinally – two fused basal laminas (kidney, lung) = l.rara +l.densa (thicker) +l.rara

Barriers

Barriers

Paracellular transport – through zonula occludens – it is dependent on the juction tightness – channel for water are formed by claudins

Occludins prevent the water leakage

● Transcellular transport – ion channels and carriers – selective transportation

How substances are transported across the epithelial cell?

● Diffusion (O2, CO2, NO)● Facilitated diffusion (carriers, ion channels,

aquaporins)● Active transport (pumps with ATPase)● Endocytosis (pinocytosis, phagocytosis)● Transcytosis● Exocytosis

Aquaporins

● Transmembranous proteins with hydrophilic channel Specific type of aquaporin – presence in collecting ducts of kidney or in large intestine

Difuse barrier in transporting epithelial cells

● Impermeable junctions - ileum snd colon – water and ions have to pass through cell (transcellular transport) – controlled (Na/K ATPase)

● Permeable junction – duodenum and jejunum – resorption of ions and nutrients – water passes through paracellular spaces – according to concentration gradient

Pinocytosis

Invagination of plasmalemma

● Clathrin mediated endocytosis

● Clathrin independent endocytosis

● Caveolae

● Clathrin and caveolin independent endocytosis

● Vesicle – closed

by dynamin

Clathrin mediated endocytosis

● Clathrin coat – adaptor protein - coated vesicles (diameter 100 nm)

● Transported molecule is bound to the receptor – receptor mediated endocytosis - LDL, transferrin

Clathrin independent endocytosis

● Caveolae (70nm) smooth invagination – very frequent in endothelial cells

● Special structure of membrane – high content of cholesterol and sfingolipids, caveolin on the cytosolic side of membrane

● Function:● Endocytosis of viral particles● Transcytosis● Start of signal cascades (insulin – glut 4)

Transcytosis

● Transport of macromolecules across barriers – endothelial cells

● Transport across placenta● Transport of IgA across mucosa

Endothelium

● Lining of vessels

● Simple squamous epithelium

● Different permeability according to the tissue demand – from very permeable (bone marrow) to impermeable in the brain (hemato-encephalic barrier)

Function of endothelium

Exchange of gases - diffusion

Transport of substances - pinocytosis – caveolae

Transcellular transport of ions, water or organic substances

Control of the vessel growth

Synthesis of vasoactive substances - NO, prostacyclin, endothelin1

Regulation of immune reactions – it allows leucocytes to leave vessels – E-selectin

Haemostasis (blood clotting) – von Willebrant factor – Weibel-Palade granules within endothelial cells (arterioles)

Transport across the endothelial cells

● Transcellular (tight junction)● Paracellular – among cells● Continuous (somatic) capillaries – transcytosis

– caveolae and vesicles) tight junctions – small slits (4-8nm) – allow paracellular transport

● Continuous (impermeable) capillaries – brain Less permeable (100 times) + paracellular seal – hemato-encephalic barrier – participation of glial cells

Endothelium - glycocalyx

● Prevents blood clotting - unwetted surface

● It allows immune cells to pass endothelium to tissue (expression of E-selectin and integrins)

Examples

● Barrier blood-urine (kidney)● Barrier air-blood (lung)● Hemato-encephalic barrier

Blood primary urine barrier

● Fenestrated endothelium

● Basement membrane (laminin and heparan sulphate)

● Podocytes

● It stops protein transport including albumin due to the negative charge of heparan sulphate

Air – blood barrier

● Continuous capillaries (somatic!!!)

● Basement membrane● Epithelial cells in

alveoli – pneumonocyte I. (simple squamous epithelium)

● Surfactant

Air-blood barrier

Haemato-encephalic barrier● Endothelium - zonulae

occludentes (tight juctions) and few caveoles – it does not allow paracellular transport or transcytosis

● Continuous basal lamina

● Glial cells (astrocytes) - membrana gliae perivascularis

● It prevents transport of hydrophilic substances - they need carriers (glucose GLUT1)

● It prevent transport of substances that have function of neurotransmitters

Stratified epithelium as a barrier

●More layers– basal, intermediate, superficiale

●Gradual differentiation of cells

●Zonulae occludentes (tight junctions) are present among cells in upper third of epithelium

●Thick basement membrane

●Lamina propria or dermis (connective tissue)

Stratified squamous epithelium

● Keratinized – epidermis - skin

● Nonkeratinized - oral cavity, esophagus, anus,vagina, cornea and conjuctiva

● Cytokeratins, desmosomes and hemidesmosomes

Stratified squamous keratinized

● Stratum corneum● Stratum lucidum● Stratum granulosum● Stratum spinosum● Stratum basale

Keratinization

● Keratin is insoluble in water – prevents dehydratation

● Basal layer – low molecular cytokeratins (5,14)

● Stratum spinosum – high molecular cytokeratins (1,10)

● Keratinization of epithelium – stratum granulosum – keratohyaline granules – cytokeratins and proteins inducing aggregation – for example profillaggrin

● Aggregation in str. corneum – cytokeratins and fillagrin (caspase14) – cornified cell envelope

● Orthokeratinization - skin

Stratified squamous epithelium keratinized

● Intercellular spaces are infilled by polar lipids – ceramides (they are produced in vital cells of str. granulosum – multilamellar (Odland) corpuscles – visible only in EM + network of tight junctions (zonulae occludentes) in stratum granulosum

● It prevents water loss ● Only lipophilic substances may pass epithelium

Melanocytes

● Protection against UV irradiation● Melanosomes (organeles) – RER and GA –

fibrillar network + tyrosinase = start of melanine production

● Synthesis: tyrosine (AA) – DOPA (enzyme -tyrosinase) – polymerates to melanine

Melanocytes

● Melanosomes (organeles) – RER and GA – network and tyrosinase – start of melanine synthesis

● Synthesis: tyrosine – dopa (enzyme tyrosinase) – melanine

● Cytocrinia – microtubules and kinesin - granules are transmitted to keratinocytes of stratum spinosum

Stratified squamous non-keratinized epithelium

● Stratum basale● Stratum parabasale● Stratum intermedium● Stratum superficiale

● Content of glycogen

Transitional epithelium - urothelium

● Superficial cells (umbrella cells) – plasmalemma with invaginations –spindle-shaped vesicles – duplicatures of membrane

● Very effective barrier agains water leakage - tight junctiones - special protein – uroplakin

● Surface of umbrella cells – (superfacial cells) is more stainable due to presence of spindle-shaped (fusiform) vesicles and cytokeratin filaments – crusta urothelialis

Urothelium (ATPase)