EPITHELIUM

• Fundamental tissue types

  • Connective tissue
  • Epithelium
  • Muscular tissue
  • Nervous tissue
• Epithelium


  A cohesive sheet of epithelial cells (or epithelial layer)
• Epithelia are derived from ectoderm, mesoderm, and endoderm
• Epithelia also forms most glands
• Epithelial tissue


  Covers internal and external surfaces throughout the body
• Epithelium


  External facing surfaces
• Endothelium


  Internal facing surfaces (e.g. blood vessels)
• Unlike connective tissues, there is little extracellular material in epithelia
• Epithelia

  • All (with a few exceptions) have one free surface and one basal surface in contact with a basement membrane
  • The basement membrane separates epithelia from all other tissues, including its blood supply
• Epithelial cells


  Are cohesive, joined to neighbours by continuous bands of junction complexes
• Tight junctions


  Together with the basement membrane can form a selective filter
• Functions of epithelia


  • Protection
  • Barrier
  • Permitting the passage of substances
  • Secretion
  • Absorption
• Classification of epithelia


  • According to the number of layers
  • According to the shape of the cells
• Epithelia: classification by layers

  • Simple
  • Stratified
  • Pseudostratified
• Epithelia: classification by shape

  • Squamous
  • Cuboidal
  • Columnar
• Simple squamous epithelium is a single layer of very flat cells with minimal cytoplasm
• Simple squamous epithelium

  Functions: Diffusion, Filtration, Lining
• Simple cuboidal epithelium

  Cell height is similar to cell width, with rounded central nuclei, functions in active transport and facilitated diffusion (secretion and absorption)
• Some simple cuboidal epithelial cells have cilia to move mucus or microvilli to increase surface area
• Simple cuboidal epithelium is found in the lining of ducts of many glands, kidney tubules, choroid plexus, and respiratory tract
• Simple columnar epithelium

  Cell height is greater than width, with elongated nuclei towards the base, some have cilia or microvilli, contain numerous organelles to support complex functions like secretion and absorption
• Simple columnar epithelium is found in some glands, bronchioles of lungs, fallopian tubes, small and large intestine
• Pseudostratified columnar epithelium


  All cells are in contact with the basement membrane, not all cells have a free surface, appear stratified because nuclei are at different levels
• Pseudostratified columnar epithelium is mostly ciliated and associated with mucus producing cells (goblet cells), found in the nasal cavity and bronchi
• Stratified squamous epithelium


  Multiple layers of cells, bottom layer is germinal, cuboidal in the base layer and progressively flatter towards the surface, protects against abrasion, caustic substances, water loss and infection
• Types of stratified squamous epithelium


  • Unkeratinised: moist, oxygen availability, all cells are alive (e.g. pharynx)
  • Keratinised: surface layer dead, dried, scale like flakes (e.g. skin epidermis)
• Transitional epithelium


  Specialised stratified epithelium that can be stretched without loss of function, lines cavities which expand (e.g. urinary bladder), protects underlying tissue from corrosive substances
• Transitional epithelium - unstretched state

  • Five or more layers of cuboidal or columnar cells, often cuboidal with a 'dome' shaped free surface, middle layers composed of round/pear shaped cells, bottom layer cubical - cone shaped cells resting on an incomplete basement membrane
• Transitional epithelium - stretched state

  • Cell layers decrease, cells become more flattened - cuboidal to squamous
• Glandular epithelium


  One or more cells that synthesise and secrete an aqueous fluid = gland
• Types of glands


  • Endocrine: ductless, secrete directly into blood
  • Exocrine: secrete to exterior via ducts
• Unicellular gland

  Single gland cell in epithelium (e.g. goblet cells in large and small intestine and respiratory passages)
• Exocrine glands

  Connected to the surface epithelia by ducts, can be simple (each duct connected to a single secretory unit) or compound (numbers of ducts from secretory unit unite to form a large duct)
• Exocrine gland duct shapes

  • Tubular: test tube shaped
  • Globular shaped
• Modified epithelia


  • Germinal epithelium
  • Sensory epithelium
  • Myoepithelium
  • Pigmented epithelium
  • Ependymal epithelium
  • Cuticular epithelium