Epithelia

Epithelia

1. covers "free" body surfaces

   • internal
     • epithelium
       • lining internal passageways
         • digestive, genitourinary, respiratory tracts for example
       • endothelium; lining blood vessels
         • arteries, veins, heart chambers, lymphatic vessels for example
       • mesothelium; lining internal body cavities
         • do not open to the exterior
         • peritoneal cavity
           • contains abdominopelvic viscera
         • pericardial cavity
           • contains the heart
         • pleural cavity
           • contain the lungs
   • external
     • epidermis; skin

2. classification by # of cell layers

   • simple; 1 layer
     • found at sites of molecular transfer
     • Simple Epithelia
       • simple cuboidal epithelia
         • line surfaces involved in secretion & absorption e.g. some glands, ducts
       • simple squamous epithelia
         • lines surfaces involved in passive transport e.g. alveoli, endothelium, mesothelium
       • simple columnar epithelia
         • lines surfaces involved in high rates of secretion & absorption, provides some protection e.g. stomach & small intestine where it has microvilli and fallopian tubes where it is ciliated
       • pseudostratified epithelia
         • usually ciliated, with goblet cells esp. in respiratory tract
   • stratified; >1 layer
     • protective
     • Stratified Epithelia
       • stratified squamous nonkertainized epithelia
         • lines surfaces requiring protection from abrasion, pathogens, chemical attack, must be kept moist by glandular secretion e.g. lining of mouth, throat, esophagus, rectum, anus, vagina
       • stratified squamous keratinized epithelia
         • lines surfaces requiring all under stratified squamous nonkeratinized epithelia... protects against dehydration, epidermal layer of the skin
       • transitional epithelia
         • lines surfaces which must allow stretch & recoil, urinary bladder
       • stratified cuboidal or columnar epithelia
         • rare

3. characteristics of covering epithelia

   • cellularity
   • polarity: apical, lateral & basal surface specializations
     • specialization of the apical surface
       • Microvilli
         • finger-like extensions
         • core: actin microfilaments cross-linked to each other, to the membrane, and to the terminal web
         • fxn is to ↑ SA to facilitate absorption, secretion
         • found in small intestine, kidney
       • Cilia
         • axoneme
           • 9(2) + 2 microtubules with associated proteins
         • motile finger-like extensions of apical surface
         • beat synchronously, fxn is moving fluid along surface of epithelial sheet
         • found in respiratory epithelia, fallopian (uterine) tubes
   • specialized intercellular contacts
     • Anchoring Junctions
       • Provide mechanical strength & stability; ability to withstand shear forces, anchors cytoskeletons of adjacent cells.
       • anchoring cells to each other
         • provides mechanical strength & stability; ability to withstand shear forces anchors cytoskeletons of adjacent cells
         • macula adherens or desmosomes
           • macula = "spot"; links the intermediate filament network esp. in skin
           • cadherens
             • integral transmembrane linker glycoprotein
             • proteinaceous plaque on cytoplasmic surface of membrane connect the intracellular moiety of cadherens to intermediate filament network of the cytoplasm
         • zonula adherens
           • zonule = "belt"  links the actin microfilament network of adjacent cells, esp. in small intestine
           • structure
             • integral transmembrane linker glycoproteins
             • intracellular attachment proteins
             • cytoskeletal filaments
           • cadherens
             • integral transmembrane linker glycoprotein
             • actin binding proteins connect cadherens' intracellular moiety to actin myofilaments of cytoskeleton
       • anchoring cells to the ECM
         • links the intermediate filament network of the cytoskeleton across the basal cell membrane to the underlying BM which contains collagen type IV, laminin, fibronectin etc.
         • hemidesmosomes
           • link the intermediate filament network of the cytoskeleton across the basal cell membrane to the underlying BM (contains collagen type IV, laminin, fibronectin etc.)
           • integrins
             • integral transmembrane linker glycoproteins
             • protenacious plaque on cytoplasmic surface of membrane connect the intracellular moiety of integrins to intermediate filament network of the cytoskeleton, extracellular moiety of integrin binds to laminin and fibronectin in the BM
         • focal contacts
           • used by motile, nonepithelial cells eg. WBCs adhesion molecules = integrins intracellular attachment proteins cytoskeletal elements = actin microfilaments integrins bind with fibronectin BM, subadjacent matrix transient binding permits locomotion, diapedesis
     • Occluding junctions; permeability barrier
       • zonula occludens or tight junctions
         • zonula: a belt like arrangement; seals the paracellular space around the circumference of the cell fascia occludens: discontinuous strips of TJs
         • functions
           • fxns to limit movement of membrane proteins
           • prevents back-diffusion of actively-transported molecules
           • anatomical basis of functional tissue compartments, e.g. BBB
           • prevent/control paracellular molecular movement
         • claudins
           • integral membrane glycoproteins which seal paracellular space
     • Communicating junctions; permits ionic/molecular mvmnt from cell to cell
       • gap junctions
         • also found in bone cells, cardiac myocytes, smooth muscle cells
         • connexon
           • molecular complex consisting of 6 subunits forms an intercellular pore
           • functions
             • open/close w/ conformational change
             • provide cytoplasmic continuity b/w adjacent cells
             • permit passage of ions & small molecules from cell to cell
             • co-ordinates cellular function
     • Junctional Complex
       • In some tissues, cell junctions occur together forming a junctional complex which includes tight junctions, zonula adherens and desmosomes...it is a prominent feature of the small intestine.
   • attached to BM and underlying CT
   • avascular but innervated
   • regeneration

4. functions

   • physical protection
   • controlled permeability; transcellular vs. paracellular transport
   • provide sensation,including general & special senses
   • produce specialized secretions i.e. glandular epithelia

5. development of epithelia

   • originates as a simple sheet
     • may remain a simple epithelium
     • may develop into stratified epithelium
     • may develop into a gland
       • Glandular Epithelia
         • Exocrine glands; secrete onto an epithelial surface via a duct
           • We will be focusing on exocrine glands
           • Multicellular
             • acinus (secretory portion)
             • duct which conducts the secretory product to the epithelial surface and may modify the composition of the secretory product
           • classification by secretion type
             • mucous glands
               • secretes mucinogens
             • serous glands
               • secretes a protein-rich watery solution
             • seromucous glands
               • some serous and some mucous secretory cells
           • classification by mode of secretion
             • merocrine
               • secretory product released by exocytosis
             • apocrine
               • some cytoplasm released with product
             • holocrine
               • entire cells shed into lumen of exocrine gland
           • classification by structure
             • structure of duct portion
               • simple glands
                 • unbranched ducts
               • compound glands
                 • branched ducts
             • shape of secretory unit
               • tubular or branched tubular
                 • elongated
               • acinar (alveolar) or branched acinar
                 • spherical shaped
           • Unicellular: Goblet Cells
             • mucous-secreting glands
               • secrete mucous onto exposed surface of epithelial sheet to protect surface-lining cells eg. respiratory epithelium
         • Endocrine glands; secretes internally into surrounding tissues
           • Unicellular
             • may exist within some covering epithelia as part of the diffuse neuroendocrine system
               • secrete across their basal aspect into underlying CT
                 • cell product taken up by underlying BVs for systemic distribution
           • Multicellular
             • invaginating epithelial tissue loses its attachment to epithelium ∴ ductless multicellular glands, secretory cells surrounded by BM & CT
               • secretes cell product into interstitial fluid surrounding glandular tissue
                 • cell products taken up by surrounding BVs for systemic districution

6. classification by shape of cells @ exposed surface

   • cuboidal; height = width
   • columnar; height > width
   • squamous = flat

7. classification by surface specializations

   • microvilli?
   • cilia?
   • goblet cells?