Epithelial Tissue

• What is epithelial tissue composed of?

  Closely aggregated polyhedral cells with very little extracellular substance
• What are the main functions of epithelial tissue?

  1. Covers surfaces
  2. Lines cavities
  3. Forms glands for secretion
  4. Acts as receptors for special senses
  5. Exhibits contractility through myoepithelial cells
  6. Serves as sources of germ cells in testes
• What are the characteristic features of epithelium?

  1. Highly cellular
  2. Arranged in sheets
  3. Small amount of extracellular matrix
  4. Closely apposed cells adhered by cell junctions
  5. Basal surface attached to basement membrane
  6. Exhibits polarity
  7. Avascular
  8. High capacity for renewal
  9. Derived from all 3 germ layers (ecto-, meso-, endoderm)
• What is the thickness of the basement lamina?

  20-100 nm
• What is the function of the basement lamina?

  It separates epithelium from underlying connective tissue
• What is the composition of the basement membrane?


  1. Basal lamina (product of epithelial tissue)
  2. Lamina fibroreticularis (product of connective tissue)
• What are the two general classifications of epithelial tissue?


  1. Covering / lining epithelium
  2. Glandular epithelium
• What is the function of glands in epithelial tissue?

  Production of secretions
• What are the two types of glands?

  1. Exocrine glands
  2. Endocrine glands
• What do exocrine glands secrete?


  Into ducts or directly onto a free surface
• What do endocrine glands secrete?


  Hormones into the blood
• What are examples of exocrine glands?


  1. Sweat glands
  2. Ceruminous glands
  3. Sebaceous glands
  4. Mammary glands
  5. Digestive glands (pancreas)
• What are examples of endocrine glands?

  1. Pituitary gland
  2. Hypothalamus
  3. Adrenal gland
  4. Pancreas
  5. Ovaries/testes
  6. Thyroid
  7. Parathyroid
• What are the structural classifications of glandular epithelium?


  1. Unicellular
  2. Multicellular
• What is an example of a unicellular gland?


  Goblet cell or mucous cell
• What are the three types of multicellular glands?

  1. Secretory epithelial sheet
  2. Intraepithelial gland
  3. Glands with ducts
• What are the classifications of exocrine glands with ducts according to morphology?


  1. Simple (single unbranched duct)
  2. Compound (branching ducts)
• What are the subtypes of compound glands according to shapes of secretory portion?


  1. Tubular gland
  2. Alveolar or acinus gland
  3. Tubuloacinar gland
  4. Coiled
  5. Branched
• What are the classifications of exocrine glands according to mode of secretion?


  1. Merocrine
  2. Holocrine
  3. Apocrine
• What is the mode of secretion for merocrine glands?


  • Secretory granule fuses with cell membrane
  • Releases secretion by exocytosis
  • No loss of cell material
• What is the mode of secretion for holocrine glands?


  • Entire cells are released with product
  • Cells die as product is released
• What is the mode of secretion for apocrine glands?


  • Apical part of cell pinches off
  • Cell repairs itself afterwards
• What are the types of glands according to the nature of secretion?

  1. Mucous
  2. Serous
• What are the characteristics of mucous glands?

  • Produce viscous secretion
  • Flat and basally located nucleus
  • Secretory granules occupy most of the cell
  • Pale-staining cytoplasm
• What are the characteristics of serous glands?


  • Produce thin, watery secretion
  • Columnar cells
  • Spherical and basally located nucleus
  • Secretory granules mostly in the apical region
• What are the classifications of covering/lining epithelium according to the number of cell layers?


  1. Simple
  2. Stratified
• What are the classifications of covering/lining epithelium according to cell shapes?


  1. Squamous
  2. Cuboidal
  3. Columnar
• What are the surface modifications of epithelial cells?

  1. Microvilli
  2. Cilia
  3. Flagella
  4. Stereocilia
• What are the characteristics of microvilli?


  • Short, fine, fingerlike processes
  • Protrude from apical surface
  • Consist of actin filaments
  • Function: increase surface area
• What are the characteristics of cilia?


  • Motile, fingerlike extensions
  • Longer and thicker than microvilli
  • Beat in one direction
  • Consist of microtubules
  • Function: transport over surface of epithelium
• What are the characteristics of stereocilia?


  • Microvilli that are as long as cilia
  • Non-motile
  • Consist of actin filaments
  • Examples: hair cells in inner ear; vas epididymis
• What are the characteristics of flagella?


  • Same structure as cilia but longer
  • Present only in sperm cells (humans)
  • Consist of microtubules
• What are the modifications on the lateral surface of epithelial cells?

  1. Tight junction or Zonula occludens
  2. Adherens junction or Zonula adherens
  3. Desmosome
  4. Gap junction
• What is a tight junction?


  • AKA "zonula occludens"
  • Forms a band around the epithelial cell
  • Most apically situated
  • Membranes closely stick to each other; sometimes fuse
• What is an adherens junction?


  • AKA "zonula adherens"
  • Forms a band that encircles the epithelial cell
  • Membranes are close but do not adhere or fuse
  • Separated by intercellular space
• What is a gap junction?


  • Broad areas of plasma membrane closely apposed but not fused
  • Cells separated by intercellular space (3 nm wide)
  • Connexons allow exchange of ions and molecules between cells
• What is a desmosome?


  • Forms button-like or rivet-like adhesions
  • Consists of an ovoid protein disc split in halves
  • Each half is attached to each cell membrane
  • Intercellular space (30 nm) separates the cells
• What are the modifications on the basal surface of epithelial cells?

  1. Hemidesmosome
  2. Basal infoldings of plasmalemma
• What is a hemidesmosome?


  • Structurally identical to half of a desmosome
  • Anchors epithelial cells to underlying basal lamina
  • Example: stratum basale of epidermis
• What are the types of stratified epithelium?

  1. Stratified squamous
  2. Stratified cuboidal
  3. Stratified columnar
  4. Transitional