Epithelial Tissue

• Composed of only four basic tissue types: epithelial, connective, muscular, and nervous tissues.
• Organs typically consist of two main components: the parenchyma, which carries out the organ's specialized functions, and the stroma, which provides support. Unlike in the brain and spinal cord, the stroma is connective tissue
• Principal function of epithelial tissue includes:

  1. Covering, lining, and protecting surfaces (eg, epidermis)
  2. Absorption (eg, the intestinal lining)
  3. Secretion (eg, parenchymal cells of glands)
• Main Characteristics of four basic types of tissues

  A) Epithelial

  B) Aggregated polyhedral cells

  C) small amount

  D) Lining of surface or body cavities; glandular secretion

  E) Connective

  F) Several types of fixed and wandering cells

  G) Abundant amount

  H) Support and protection of tissues/organs

  I) Muscle

  J) Elongated contractile cells

  K) Moderate amount

  L) Strong contraction; body movements

  M) Nervous

  N) Elongated cells with extremely fine processes

  O) Very small amount

  P) Transmission of nerve impulses

  
• Characteristics Features of Epithelial Cells

  1. Lamina Propia - connective tissue underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems
  2. Papillae - can increase the area of contact between the two tissues by small evaginations.
  3. Basal pole - region of the cell contacting the ECM and connective tissue
  4. Apical pole - opposite end, usually facing a space.
  5. Lateral surfaces - cuboidal or columnar cells are the regions that adjoin neighboring cells.
• Basement Membrane - thin extracellular, felt-like sheet of macromolecules (semi-permeable)

  2 parts

  1. Basal Lamina - a thin, electron-dense, sheet like layer of fine fibrils.
  2. Fibrous Reticular Lamina - more diffuse and fibrous
• Characteristics of ff:

  1. Reticular Lamina- contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen.
  2. Basal lamina

  • Type IV collagen - monomers form a two-dimensional network that resembles the mesh of a window screen.
  • Lamina - large glycoproteins attach to transmembrane integrin proteins in the basal cell membrane and project through the mesh formed by type IV collagen.
  • Nidogen and perlecan - short, rod-like protein and a proteoglycan cross-link laminins to the type IV collagen network
• Function of Intercellular Adhesion & Other Junctions:

  Tight or occluding junctions - form a seal between adjacent cells.

  Adherent or anchoring junctions - sites of strong cell adhesion.

  Gap junctions - channels for communication between adjacent cells.
• Epithelial cell junctions, their major structural features and functions, andmedical significance.

  A) Occludins, claudins, ZO proteins

  B) Actin Filaments

  C) E-cadherin, catenin complexes

  D) Actin Filaments

  E) Cadherin family proteins (desmogleins, desmocollin)

  F) Intermediate filaments (keratins)

  G) Integrins

  H) Intermediate filaments

  I) Connexin

  J) None

  
• Additional Information about Epithelial Cell Junctions

  • Zonula - junction forms a bandcompletely encircling each cell.
  • Tight junctions (zonulae occludens) - seal between the two cell membranes is due to tight interactions between the transmembrane proteins: claudin and occludin.
  • Adherens junction (zonula adherens) - anchoring a cell to its neighbors by cadherins, binds catenins that link to actin filaments with actin-binding proteins
• Additional Information about Epithelial Cell Junction?

  • Desmosome (Macula adherens) - single “spot-weld” and does not form a belt around the cell.
  • Gap junction (Nexus) - transmembrane gap junction proteins, connexins, form hexameric complexes called connexons, each of which has a central hydrophilic pore about 1.5 nm in diameter.
  • Anchoring junction (hemidesmosomes) - integrins of hemidesmosomes bind primarily to laminin molecules in the basal lamina.
• Specialization of the Apical Cell Surface (Absorption)

  • Microvilli - small membrane projections with cores of actin filaments that generally function to increase epithelial cells’ apical surface area for absorption. (lining small intestine)
  • Stereocilia - long microvilli with specialized mechanosensory function in cells of the inner ear and for absorption in tissues of the male reproductive tract. (lining male reproductive)
• Specialization of the Apical Cell Surface (Absorption)

  • Cilia - Larger structures called axonemes have a well-organized core of microtubules in a 9 + 2 arrangement. Dynein-based sliding of microtubules in the axoneme causes ciliary movement, propelling material along an epithelial surface. (respiratory tract)
• Common types of Covering Epithelia

  
• Secretory Epithelia & Glands

  • Glands - to produce and secrete various macromolecules
  • Goblet Cell (mucous glands) - secretes lubricating mucus that aids the function of these organs. (lining of the small intestine and respiratory tract)
  • Exocrine glands - have epithelial ducts carrying secretions to specific sites; the ducts of simple glands are unbranched and those of compound glands are branched.
  • Endocrine glands - lack ducts; secreted substances are hormones carried throughout the body by the interstitial fluid and blood, with specificity produced by the hormone receptors of target cells
• Secretory Epithelia & Glands

  • Glands have three basic secretory mechanisms: merocrine, which uses exocytosis; holocrine, in which terminally differentiated cells filled with lipid products are released; and apocrine, in which apical, product-filled areas of cells are extruded.
  • Serous glands - producing largely enzymes (proteins) via exocrine glands.
• Additional Information about Secretory Epithelia & Glands

  • Glands can be simple (ducts not branched) or compound (ducts with two or more branches).
  • Secretory portions can be tubular (either short or long and coiled) or acinar (rounded and saclike); either type of secretory unit may be branched, even if the duct is not branched.
  • Compound glands can have branching ducts and can have multiple tubular, acinar, or tubuloacinar secretory portions.
• 1. Merocrine secretion - most common method of protein or glycoprotein secretion and involves typical exocytosis from membrane-bound vesicles or secretory granules.
  2. Holocrine secretion - Cells accumulate product as they enlarge and undergo terminal differentiation. This process culminates in complete cell disruption, releasing the product and cell debris into the gland's lumen. (sebaceous glands producing lipid rich material)
  3. Apocrine secretion - Product accumulates at cells' apical ends, where portions are pinched off to release the product surrounded by cytoplasm and membrane.
• Exocrine glands with merocrine secretion - either serous or mucous according to the nature of their secretory products.

  • Mucous cells (goblet cells) - contain heavily glycosylated proteins called mucins. When mucins are released formed a mucus layer.
  • Salivary glands with seromucous glands - having both serous acini and mucous tubules with clustered serous cells
  • Endocrine glands lack myoepithelial cells - either for protein or steroid hormone synthesis, with cytoplasmic staining characteristic of RER or SER.
• Transcytosis - cross the thin cells in both directions and release their contents on the opposite side by exocytosis. (between the

  apical and basolateral membranes, simple cuboidal/columnar)
• Transcellular transport - transfer of ions (byion pumps) and water (via the membrane channels called aquaporins) in either direction across the epithelium.
• Renewal of epithelial cells - cells are renewed continuously by mitotic activity and stem cell populations.

  • Rate of renewal varies widely; it can be fast in tissues such as the intestinal epithelium, which is replaced every week, or slow, as in the large glands
• Structural classes of exocrine glands, features of each class, and examples.