Epithelia

1. epithelium
   1. lining internal passageways

      Annotations:

      • digestive, genitourinary, respiratory tracts for example
   2. endothelium; lining blood vessels

      Annotations:

      • arteries, veins, heart chambers, lymphatic vessels for example
   3. mesothelium; lining internal body cavities

      Annotations:

      • do not open to the exterior
      1. peritoneal cavity

         Annotations:

         • contains abdominopelvic viscera
      2. pericardial cavity

         Annotations:

         • contains the heart
      3. pleural cavity

         Annotations:

         • contain the lungs

1. epidermis; skin

Annotations:

• found at sites of molecular transfer

1. Simple Epithelia
   1. simple cuboidal epithelia

      Annotations:

      • line surfaces involved in secretion & absorption e.g. some glands, ducts
   2. simple squamous epithelia

      Annotations:

      • lines surfaces involved in passive transport e.g. alveoli, endothelium, mesothelium
   3. simple columnar epithelia

      Annotations:

      • 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
   4. pseudostratified epithelia

      Annotations:

      • usually ciliated, with goblet cells esp. in respiratory tract

Annotations:

• protective

1. Stratified Epithelia
   1. stratified squamous nonkertainized epithelia

      Annotations:

      • 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
   2. stratified squamous keratinized epithelia

      Annotations:

      • lines surfaces requiring all under stratified squamous nonkeratinized epithelia... protects against dehydration, epidermal layer of the skin
   3. transitional epithelia

      Annotations:

      • lines surfaces which must allow stretch & recoil, urinary bladder
   4. stratified cuboidal or columnar epithelia

      Annotations:

      • rare

1. specialization of the apical surface
   1. Microvilli
      1. finger-like extensions
      2. core: actin microfilaments cross-linked to each other, to the membrane, and to the terminal web
      3. fxn is to ↑ SA to facilitate absorption, secretion
      4. found in small intestine, kidney
   2. Cilia
      1. axoneme
         1. 9(2) + 2 microtubules with associated proteins
      2. motile finger-like extensions of apical surface
      3. beat synchronously, fxn is moving fluid along surface of epithelial sheet
      4. found in respiratory epithelia, fallopian (uterine) tubes

1. Anchoring Junctions

   Annotations:

   • Provide mechanical strength & stability; ability to withstand shear forces, anchors cytoskeletons of adjacent cells.
   1. anchoring cells to each other

      Annotations:

      • provides mechanical strength & stability; ability to withstand shear forces anchors cytoskeletons of adjacent cells
      1. macula adherens or desmosomes

         Annotations:

         • macula = "spot"; links the intermediate filament network esp. in skin
         1. cadherens

            Annotations:

            • integral transmembrane linker glycoprotein
            1. proteinaceous plaque on cytoplasmic surface of membrane connect the intracellular moiety of cadherens to intermediate filament network of the cytoplasm
      2. zonula adherens

         Annotations:

         • zonule = "belt"  links the actin microfilament network of adjacent cells, esp. in small intestine
         1. structure
            1. integral transmembrane linker glycoproteins
            2. intracellular attachment proteins
            3. cytoskeletal filaments
         2. cadherens

            Annotations:

            • integral transmembrane linker glycoprotein
            1. actin binding proteins connect cadherens' intracellular moiety to actin myofilaments of cytoskeleton
   2. anchoring cells to the ECM

      Annotations:

      • 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.
      1. hemidesmosomes

         Annotations:

         • link the intermediate filament network of the cytoskeleton across the basal cell membrane to the underlying BM (contains collagen type IV, laminin, fibronectin etc.)
         1. integrins

            Annotations:

            • integral transmembrane linker glycoproteins
            1. 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
      2. focal contacts

         Annotations:

         • 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
2. Occluding junctions; permeability barrier
   1. zonula occludens or tight junctions

      Annotations:

      • zonula: a belt like arrangement; seals the paracellular space around the circumference of the cell fascia occludens: discontinuous strips of TJs
      1. functions
         1. fxns to limit movement of membrane proteins
         2. prevents back-diffusion of actively-transported molecules
         3. anatomical basis of functional tissue compartments, e.g. BBB
         4. prevent/control paracellular molecular movement
      2. claudins

         Annotations:

         • integral membrane glycoproteins which seal paracellular space
3. Communicating junctions; permits ionic/molecular mvmnt from cell to cell
   1. gap junctions

      Annotations:

      • also found in bone cells, cardiac myocytes, smooth muscle cells
      1. connexon

         Annotations:

         • molecular complex consisting of 6 subunits forms an intercellular pore
         1. functions
            1. open/close w/ conformational change
            2. provide cytoplasmic continuity b/w adjacent cells
            3. permit passage of ions & small molecules from cell to cell
            4. co-ordinates cellular function
4. Junctional Complex

   Annotations:

   • 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.

1. may remain a simple epithelium
2. may develop into stratified epithelium
3. may develop into a gland
   1. Glandular Epithelia
      1. Exocrine glands; secrete onto an epithelial surface via a duct

         Annotations:

         • We will be focusing on exocrine glands
         1. Multicellular
            1. acinus (secretory portion)
            2. duct which conducts the secretory product to the epithelial surface and may modify the composition of the secretory product
         2. classification by secretion type
            1. mucous glands

               Annotations:

               • secretes mucinogens
            2. serous glands

               Annotations:

               • secretes a protein-rich watery solution
            3. seromucous glands

               Annotations:

               • some serous and some mucous secretory cells
         3. classification by mode of secretion
            1. merocrine

               Annotations:

               • secretory product released by exocytosis
            2. apocrine

               Annotations:

               • some cytoplasm released with product
            3. holocrine

               Annotations:

               • entire cells shed into lumen of exocrine gland
         4. classification by structure
            1. structure of duct portion
               1. simple glands

                  Annotations:

                  • unbranched ducts
               2. compound glands

                  Annotations:

                  • branched ducts
            2. shape of secretory unit
               1. tubular or branched tubular

                  Annotations:

                  • elongated
               2. acinar (alveolar) or branched acinar

                  Annotations:

                  • spherical shaped
         5. Unicellular: Goblet Cells
            1. mucous-secreting glands
               1. secrete mucous onto exposed surface of epithelial sheet to protect surface-lining cells eg. respiratory epithelium
      2. Endocrine glands; secretes internally into surrounding tissues
         1. Unicellular
            1. may exist within some covering epithelia as part of the diffuse neuroendocrine system
               1. secrete across their basal aspect into underlying CT
                  1. cell product taken up by underlying BVs for systemic distribution
         2. Multicellular
            1. invaginating epithelial tissue loses its attachment to epithelium ∴ ductless multicellular glands, secretory cells surrounded by BM & CT
               1. secretes cell product into interstitial fluid surrounding glandular tissue
                  1. cell products taken up by surrounding BVs for systemic districution