4.2 Epithelial Tissue

Epithelial tissue is the body's first line of defense, covering surfaces and lining organs. It's tightly packed, regenerates quickly, and performs crucial functions like protection, absorption, and secretion. Understanding its features is key to grasping how our bodies maintain barriers and regulate exchanges.

Simple and stratified epithelia have distinct structures and roles. Simple epithelia, with a single cell layer, handle diffusion and filtration. Stratified epithelia, with multiple layers, provide protection. Specialized junctions between cells ensure tissue integrity and allow communication.

Epithelial Tissue

Features of epithelial tissue

Top images from around the web for Features of epithelial tissue

• 

  Epithelial Tissues | Biology for Majors II View original

  Is this image relevant?

• 

  Epithelial Tissue | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Epithelial Tissues | Biology for Majors II View original

  Is this image relevant?

• 

  Epithelial Tissue | Anatomy and Physiology I View original

  Is this image relevant?

1 of 2

Top images from around the web for Features of epithelial tissue

• 

  Epithelial Tissues | Biology for Majors II View original

  Is this image relevant?

• 

  Epithelial Tissue | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Epithelial Tissues | Biology for Majors II View original

  Is this image relevant?

• 

  Epithelial Tissue | Anatomy and Physiology I View original

  Is this image relevant?

1 of 2

• Covers body surfaces, lines hollow organs and body cavities, and forms glands
• Always has one free (apical) surface exposed to the outside or lumen of an organ
• Lacks blood vessels but innervated in some regions
• Tightly packed with little intercellular material
• Regenerates quickly when damaged due to high mitotic activity
• Functions:
  • Protects from abrasion, dehydration, and pathogen invasion
  • Absorbs substances (nutrients, gases)
  • Secretes substances (mucus, enzymes, hormones)
  • Filters to regulate exchange between compartments (blood and interstitial fluid)
  • Senses stimuli in some locations (taste buds, olfactory epithelium)

Simple vs stratified epithelia

• Simple epithelia consist of a single layer of cells
  • Simple squamous epithelium has thin, flat cells involved in diffusion and filtration (capillary walls, pericardium, pleura)
  • Simple cuboidal epithelium has cube-shaped cells involved in secretion and absorption (small ducts, kidney tubules)
  • Simple columnar epithelium has tall, rectangular cells involved in secretion and absorption (stomach and intestinal lining)
    • Some have microvilli (brush border) to increase surface area for absorption
    • Some are specialized for mucus secretion (goblet cells in respiratory and digestive tracts)
  • Pseudostratified epithelium appears layered but all cells contact the basement membrane (respiratory tract)
• Stratified epithelia consist of multiple layers of cells
  • Stratified squamous epithelium has flattened top layers that protect against abrasion and water loss
    • Keratinized type has dead surface cells filled with keratin for waterproofing (epidermis of skin)
    • Non-keratinized type has living, moist surface cells (esophageal and vaginal lining)
  • Stratified cuboidal and columnar epithelia are rare, found in some glands and ducts (large ducts of salivary glands, male urethra)
  • Transitional epithelium can stretch and change shape (urinary bladder)

Junctions in epithelial tissue

• Tight junctions (zonula occludens) are sealing junctions that prevent leakage between cells
  • Composed of transmembrane proteins that fuse membranes of adjacent cells
  • Regulate paracellular transport and maintain cell polarity
• Anchoring junctions mechanically attach cells to each other or the basement membrane
  • Desmosomes (macula adherens) are spot-like junctions that link intermediate filaments and provide tensile strength
  • Adherens junctions (zonula adherens) are belt-like junctions that link actin filaments and allow coordinated cell movement
  • Hemidesmosomes attach basal cells to the basement membrane
• Gap junctions (nexus) are channels that allow direct communication between cells
  • Composed of connexin proteins that form tunnels between cells
  • Permit passage of ions, nutrients, and signaling molecules to coordinate cellular activities and maintain homeostasis

Epithelial tissue organization and interactions

• Basement membrane separates epithelium from underlying connective tissue
  • Provides structural support and attachment for epithelial cells
  • Regulates molecular exchange between epithelium and connective tissue
• Cell adhesion molecules maintain epithelial integrity and organization
  • Cadherins mediate cell-cell adhesion in adherens junctions and desmosomes
  • Integrins connect cells to extracellular matrix components in the basement membrane
• Epithelial-mesenchymal transition allows epithelial cells to acquire mesenchymal properties
  • Important in embryonic development, wound healing, and cancer metastasis

Endocrine vs exocrine glands

• Endocrine glands are ductless glands that secrete hormones directly into the bloodstream
  • Hormones are chemical messengers that regulate distant target cells (insulin, thyroid hormones)
  • Secretory cells are usually arranged in clusters or cords (pancreatic islets, adrenal medulla)
  • Secretion occurs by exocytosis of secretory vesicles into interstitial fluid, then diffusion into blood capillaries
• Exocrine glands secrete substances through ducts onto epithelial surfaces
  • Secretions include sweat, saliva, mucus, enzymes, and other products
  • Classified by structure:
    1. Simple glands have unbranched ducts (tubular or acinar)
    2. Compound glands have branched ducts (tubular, acinar, or tubuloalveolar)
  • Secretion methods:
    1. Merocrine (eccrine) secretion involves exocytosis without loss of cytoplasm (sweat glands)
    2. Apocrine secretion involves release of secretory product with apical portion of the cell (mammary glands)
    3. Holocrine secretion involves disintegration of entire cell to release its product (sebaceous glands)