Simple Epithelial Tissue - Study Notes

Tissues

  • Tissues are groups of cells that work together in functionally related groups called tissues.
  • How is this done?
    • Attachments
    • Communication
  • Types of tissues:
    • Epithelial – lining and covering
    • Connective – support
    • Muscle – movement
    • Nervous – control

Epithelial Tissue – General Characteristics & Functions

  • Covers a body surface or lines a body cavity
  • Forms most glands
  • Functions of epithelium:
    • Protection
    • Absorption, secretion, and diffusion
    • Filtration
    • Forms slippery surfaces (mucus secretion)

Special Characteristics of Epithelia

  • Cellularity: cells are in close contact with each other with little or no intercellular space between them
  • Specialized contacts: may have junctions for both attachment and communication
  • Polarity: epithelial tissues always have an apical and basal surface
  • Support by connective tissue: at the basal surface, both the epithelial tissue and the connective tissue contribute to the basement membrane
  • Avascular: nutrients must diffuse from basal layer
  • Innervated
  • Regenerative: epithelial tissues are highly mitotic

Epithelium–Connective Tissue Interface

  • Epithelium sits on a basement membrane that comprises basal lamina (secreted by epithelial cells) and reticular fibers (from connective tissue)
  • Apical modifications include:
    • Cilia
    • Microvilli
  • Cell junctions:
    • Tight junction
    • Adherens junction
    • Desmosome
    • Gap junction
  • Basal region:
    • Basal lamina
    • Reticular fibers
  • Underlying connective tissue contains:
    • Nerve endings
    • Capillaries
  • Basement membrane separates epithelium from connective tissue

First name of tissue indicates number of layers

  • Simple – one layer of cells
  • Stratified – more than one layer of cells

Classifications of Epithelia

  • Last name describes the shape of cells:
    • Squamous – cells wider than tall (plate or “scale” like)
    • Cuboidal – cells are as wide as tall, as in cubes
    • Columnar – cells are taller than they are wide, like columns

Naming Epithelia

  • Naming includes both the layers (first) and the shape of the cells (second), e.g. stratified cuboidal epithelium
  • The name may also include accessory structures:
    • Goblet cells
    • Cilia
    • Keratin
  • Special epithelial tissues (don’t follow naming convention):
    • Pseudostratified
    • Transitional

Simple Squamous Epithelium

  • Description: single layer of flat cells with disc-shaped nuclei
  • Special types:
    • Endothelium (inner lining) – slick lining of hollow organs
    • Mesothelium (middle lining) – lines peritoneal, pleural, and pericardial cavities
    • Covers visceral organs of those cavities
  • Function:
    • Passage of materials by passive diffusion and filtration
    • Secretes lubricating substances in serous membranes
  • Location:
    • Renal corpuscles (kidneys)
    • Alveoli of lungs
    • Lining of heart, blood and lymphatic vessels
    • Lining of ventral body cavity (serosae/serous membranes)
  • Photomicrograph (illustrative): Simple squamous epithelium forming part of the alveolar walls (magnification 400×400\times). Simple squamous lining the walls of the capillary; Air sacs of lung tissue; Nuclei of squamous epithelial cells

Simple Cuboidal Epithelium

  • Description: single layer of cube-like cells with large, spherical central nuclei
  • Function: secretion and absorption
  • Location: kidney tubules, secretory portions of small glands, ovary surface
  • Photomicrograph: Simple cuboidal epithelium in kidney tubules (magnification 400×400\times).

Simple Columnar Epithelium

  • Description: single layer of column-shaped (rectangular) cells with oval nuclei
  • Some bear cilia at their apical surface
  • May contain goblet cells
  • Function: absorption; secretion of mucus, enzymes, and other substances
  • Ciliated type: propels mucus or reproductive cells by ciliary action
  • Location:
    • Non-ciliated form: lines digestive tract, gallbladder, ducts of some glands
    • Ciliated form: lines small bronchi, uterine tubes, and uterus

Pseudostratified Columnar Epithelium

  • Description: All cells originate at basement membrane; Only tall cells reach the apical surface; May contain goblet cells and bear cilia; Nuclei lie at varying heights within cells; Gives false impression of stratification
  • Function: secretion of mucus; propulsion of mucus by cilia
  • Locations:
    • Non-ciliated type: ducts of male reproductive tubes; ducts of large glands
    • Ciliated variety: lines trachea and most of upper respiratory tract