Histo .2

• Glandular tissue (secretory epithelia) is epithelial cells that function mainly to produce and secrete various macromolecules

  
• Classification of glandular tissue based on:

  • Number of secretory cells
  • Nature of secretion
  • Mechanism of secretion
  • Shape of secretory unit
  • Arrangement and occurrence of duct system

  
• Secretory cells


  • They may synthesize, store, and release proteins, lipids, complexes of carbohydrates and proteins
  • Some glands (e.g. sweat glands) have little synthetic activity and secrete mostly water and electrolytes
• Exocrine glands


  Remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver the secreted material where it is used

  
• Endocrine glands


  Lose the connection to their original epithelium and therefore lack ducts. Thin-walled blood vessels (capillaries) adjacent to endocrine cells absorb their secreted hormone products for transport in blood to target cells throughout the body
• Classification of exocrine glands by number of secretory cells


  • Unicellular exocrine glands
  • Multicellular exocrine glands

  
• Examples of multicellular exocrine glands


  • Salivary glands
  • Sweat glands
  • Sebaceous glands
  • Pancreas
  • Mammary glands

  
• Structural organization of exocrine glands

  • Parenchyma (secretory cells)
  • Stroma (connective tissue)
  • Duct system

  
• Classification of exocrine glands by shape and branching pattern of duct


  • Simple (simple tubular, simple coiled tubular, simple branched tubular, simple branched alveolar)
  • Compound (compound tubular, compound alveolar, compound tubulo-alveolar)

  
• Classification of exocrine glands by shape of secretory unit


  • Tubular glands
  • Acinar glands
  • Alveolar glands
  • Saccular glands

  
• Classification of exocrine glands by mechanism of secretion

  • Merocrine secretion
  • Holocrine secretion
  • Apocrine secretion

  
• Merocrine secretion


  The most common method of protein or glycoprotein secretion, involving typical exocytosis from membrane-bound vesicles or secretory granules
• Holocrine secretion


  Cells accumulate product continuously as they enlarge and undergo terminal differentiation, culminating in complete cell disruption that releases the product and cell debris into the gland's lumen
• Apocrine secretion


  Product accumulates at the cells' apical ends, portions of which are then extruded to release the product together with small amounts of cytoplasm and cell membrane
• Nature of secretions in exocrine glands

  • Mucous glands
  • Serous glands
  • Mixed glands

  
• In addition to secretory cells, epithelia of many exocrine glands contain contractile myoepithelial cells at the basal ends of the secretory cells

  
• Endocrine glands lack myoepithelial cells and are specialized either for protein or steroid hormone synthesis
• Extracellular Matrix


  Substance between cells that provides support and structure
• Main Functions of Tissues


  • Epithelial
  • Connective
  • Muscle
  • Nervous
• Epithelial


  • Aggregated polyhedral cells
  • Small amount
  • Lining of surface or body cavities; glandular secretion
• Connective


  • Several types of fixed and wandering cells
  • Abundant amount
  • Support and protection of tissues/organs
• Muscle


  • Elongated contractile cells
  • Moderate amount
  • Strong contraction; body movements
• Nervous


  • Elongated cells with extremely fine processes
  • Very small amount
  • Transmission of nerve impulses
• Epithelial tissue


  Tissue in which cells are bound tightly together structurally and functionally to form a sheetlike or tubular structure with little extracellular material between the cells
• Epithelial tissue


  • Contains cells and basement membrane
  • Cells have an apical side facing the sheet's free surface and a basal side facing a basement membrane and underlying connective tissue
• Characteristic features of epithelial cells

  • Shapes and dimensions are quite variable, ranging from tall columnar to cuboidal to low squamous cells
  • Nuclei vary in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape
  • Columnar cells generally have elongated nuclei
  • Squamous cells have flattened nuclei
  • Cuboidal or pyramidal cells have more spherical nuclei
• Epithelial tissues are avascular (no blood vessels), so how they receive nourishment
• How epithelial tissues are specialized

  • Absorption
  • Transcytosis
  • Pinocytosis of material at the apical side
  • Exocytosis at the basolateral side (or vice versa)

  
• Basement membrane


  Thin extracellular layer of specialized proteins, usually having two parts: the basal lamina and the reticular lamina
• Function of basement membrane


  • Attach epithelia to connective tissue
  • Regulate (filter) substances passing from connective tissue into epithelia
  • Provide a guide or scaffold during tissue regeneration after injury
  • Compartmentalize epithelial cells from other tissues
• Intercellular junctions


  • Tight or occluding junctions
  • Gap or communicating junctions
  • Adherent or anchoring junctions
• Tight or occluding junctions


  Formed by interacting transmembrane proteins such as claudin and occludin; linear arrangements of these linked proteins surround the apical ends of the cells and prevent paracellular passage of substances (between the cells)
• Gap or communicating junctions


  Points of cell contact where both plasma membranes have numerous hexameric complexes of transmembrane connexons, each forming a channel allowing passage of small molecules from one cell to the other
• Adherent or anchoring junctions


  Formed by interacting proteins of the cadherin family, are points of strong attachment holding together cells of the epithelium
• Proteins of tight junctions provide the targets for certain common bacteria of medical importance
• Various blistering (bullous) diseases, such as pemphigus vulgaris, involving the epidermis or stratified squamous epithelia of the oral mucosa, are due to abnormal desmosome function caused by autoimmune reactions against specific desmogleins that reduce cell-to-cell adhesion
• Apical structures of epithelial cells


  • Microvilli
  • Stereocilia
  • Cilia
• Microvilli


  Small membrane projections with cores of actin filaments that generally function to increase epithelial cells' apical surface area for absorption
• Stereocilia


  Long microvilli with specialized mechanosensory function in cells of the inner ear and for absorption in tissues of the male reproductive tract
• Cilia


  Larger projecting structures with a well-organized core of microtubules (in a 9 + 2 arrangement called the axoneme) in which restricted, dynein-based sliding of microtubules causes ciliary movement that propel material along an epithelial surface