Epithelial and membrane
transport
G Ogweno
Types of epithelial tissues
• Surface linning
• Glandular
• Cell shape: squamaous, cuboidal,
columnar
• Layers: simple, stratified, pseudostratified
• Examples: skin, intestine,
respiratory,uroepithelium
Epithelial polarity
• Apical surface: microvilli absorptive,cilia
motile
• Lateral junctional complexes
• Basal:basal lamina, basement membrane,
basal infoldings for ion transport,
hemidesmosomes for fixation
• Distribution of enzymes, transporters,
Antigen detection, signal transduction
Epithelial junctional complexesTight
• Also called zonula occludens
• Integral membrane proteins- occludins, claudins
• Determines permeability characteristics& cell
migration
• On apical side-continuous belt, divides into
apical and basal domains (fencing)
• Prevents Para cellular transport and binds cells
together; barrier; gating
• Common intestinal and skin
Epithelial junctions:adhering
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Also zonula adherens
Belt just below tight junctions
Cell-cell contact by CAMs
Cytoskeleton-actin
Epithelial junctions:Gap
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2 nm gap between adjacent cells
Hollow cylindrical structures-connexons
Form channels-Interecellular
Point of electrical coupling :low -resistance
communication,
• may be gated-responsive to physiological
stimuli
• Nexus in smooth muscles;intercalated disc
in cardiac muscles
Epithelial junctions:desmosomes
• Also called spot, macula adherens, button
like between adjacent cells
• Intermediate filaments-tonofilaments
cytoskeleton of adjacent cells
• Gap between of 25-30 nm
• Provide structural support to physical
stress
• Prevent shearing forces e.g epidermis
Functions of epithelia
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Body barrier e.g microorganisms
Prevents water loss/dessication
Maintenance of internal environment
Regulated vectorial transport
Types of capillaries
• Type 1: continuous-10-15 nm clefts, much
tighter in BBB.found pulmonary,brain,testis
• Type 2:fenestrated-thin fenestrated.small
intestine, exocrine glands,muscles
• Type 3: discontinuous-large gaps, can
pass cells, sinusoids e.g liver, bone
marrow
Epithelial vectorial transport
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Function of membrane transport protein polarity
Apical- absorption or reabsorption; basolateral-secretion
Create ionic gradients
Both Transcellular(tight) and paracellular (leaky) transport
depending on epithelium
Are specific for ions-cations vs anions
Transport proteins(channels,carriers&pumps) and kekyness
determine direction and type of solutes transport
Active transport is transcellular while paracelluler is passive
Na+/K+ ATPase exclusively basolateral except in choroid plexus
apical
K+ through channels recycled
Inwardly directed Na+ gradient driving force for apical Na+
channels, secondary active transport for other solutes e.g Na+glucose.Na-H+ exchanger
Epithelial membrane solute
transporters
• Placed either on apical or basolateral
membranes control absorption or secretion
• Na+: basolateral Na+/K+ ATPase pumps out,
creates inward gradient,Cl- paracellular
• K+: apical channel eflux;basal Na+/K+
• Glucose:apical secondary active
transport(SGLT), basal carrier exit(Glut)
• Cl-: secondary active uptake basal Na+/K+/Clcontransporter, passive cl-apical exit through
channels e.g CFTR;Na+ paracellularly into
lumen
Transepithelial water movement
• Passive, driven by osmotic pressure gradient
• Transepithelial(transcelluler): aquaporins
• Paracelluler(junctional):follows NaCl
gradient(secretion/reabsorption)& solvent drag
• Some eipthelia have high solute-transport and
low water permeability
• Hydraulic conductivity varies
• Isotonic movement occurs due to constitutive
expression of aquaporins and localized
interepithelial osmotic gradient
Regulation of epithelial transport
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Controlled by Neural/hormonal signals
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Neural:enteric&sympathetic
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Hormonal: Aldosterone
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Paracrine:gastric histamine for HCl
Mechanisms:
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Gene expression:Synthesis/degradation of specific transporters
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Change in activity of membrane transporters/post-translational
modification of pre-existing transport proteins(channel gating)
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Recruitment/Retrieval of transporters from membrane by
endocytosis or insertion into membrane from intracelluler
vesicular pool
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Change in paracellular(leak) pathway
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Luminal supply of transported species