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 complexes- Tight 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 Also zonula adherens Belt just below tight junctions Cell-cell contact by CAMs Cytoskeleton-actin

Epithelial junctions:Gap 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 nm Provide structural support to physical stress Prevent shearing forces e.g epidermis

Functions of epithelia Body barrier e.g microorganisms Prevents water loss/dessication Maintenance of internal environment Regulated vectorial transport

Types of capillaries Type 1: continuous 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 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+/Cl- contransporter, 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 Controlled by Neural/hormonal signals Neural:enteric&sympathetic Hormonal: Aldosterone Paracrine:gastric histamine for HCl Mechanisms: 1.Gene expression:Synthesis/degradation of specific transporters 2.Change in activity of membrane transporters/post-translational modification of pre-existing transport proteins(channel gating) 3.Recruitment/Retrieval of transporters from membrane by endocytosis or insertion into membrane from intracelluler vesicular pool 4.Change in paracellular(leak) pathway 5.Luminal supply of transported species