Epithelial Tissue Overview

Epithelial Tissue

• One of the four basic tissue types in the body (along with connective tissue, muscle, and nervous tissue).
• Forms glands and lines the inside and outside of the body.
• Epithelial cells are anchored to a basement membrane.
• Epithelial tissue lacks extracellular matrix, meaning the cells are packed closely together.
• Avascular, meaning it does not contain blood vessels. Receives nutrients and oxygen through diffusion from capillaries in the underlying connective tissue.

Classifications of Epithelium

• Simple Squamous Epithelium:
  • Single layer of flat, scale-like cells.
  • Functions:
    • Diffusion
    • Filtration
    • Lubrication
  • Locations:
    • Alveoli (for gas exchange)
    • Glomerulus (in the kidney, for filtration)
    • Lining of blood vessels (endothelial cells)
• Simple Cuboidal Epithelium:
  • Single layer of cube-shaped cells.
  • Functions:
    • Absorption
    • Secretion
  • Locations:
    • Nephron (tubules in the kidney)
    • Glands (e.g., sweat glands, liver, pancreas)
• Simple Columnar Epithelium:
  • Single layer of tall, thin cells.
  • Functions:
    • Absorption
    • Secretion
    • May have cilia for moving fluid across apical surface
  • Locations:
    • Gastrointestinal tract (stomach to anus)
    • Glands
    • Airways
    • Uterus and uterine tube
• Stratified Squamous Epithelium:
  • Multiple layers of cells, with the apical cells being flat.
  • Function: Protection against abrasion
  • Locations:
    • Epidermis (skin)
    • Esophagus
    • Vagina
  • Two types:
    • Keratinized: Contains keratin, a tough protein, for added protection against water; apical cells lack nuclei.
    • Non-keratinized: Does not contain keratin; apical cells have nuclei.
• Transitional Epithelium (Urothelium):
  • Specialized stratified epithelium found only in the urinary system.
  • Apical cells are dome-shaped and can stretch and distend.
  • Function: Allows for the stretching and distension of the urinary system (e.g., bladder).
  • Locations:
    • Urinary system (ureter, bladder, proximal urethra)
• Pseudostratified Ciliated Columnar Epithelium (Respiratory Epithelium):
  • Appears stratified, but all cells are anchored to the basement membrane.
  • Ciliated, with cilia projecting from the apical surface.
  • Function:
    • Secretion (mainly mucus)
    • Propulsion of mucus by cilia
  • Location:
    • Respiratory system (trachea, proximal bronchi)

Cilia and Microvilli

• Cilia:
  • Hair-like projections located on the apical surface of cells.
  • Function: Move liquid over the surface of the cell (like a crowd surfing at a concert)
• Microvilli:
  • Finger-like projections located on the apical surface of cells.
  • Function: Increase surface area for absorption.

Surface Area & Absorption

• Microvilli increase surface area for absorption in the jejunum
• Brush border is a term for the appearance of the microvilli which resemble a hairbrush
• The jejunum is where amino acids, simple sugars, and fats are absorbed

Epithelial Tissues

• Simple squamous epithelium is one layer of flat cells (e.g., Bowman's capsule from the glomerulus)
• Simple cuboidal epithelium is one layer of cube-shaped cells (the nucleus rounds out the cell)
• Simple columnar epithelium is one layer of tall, thin cells (the tall nuclei are a key identifier)
• Pseudostratified ciliated columnar epithelium is one layer of cells that appears stratified. The key identifier is the presence of cilia projecting from the cell surface (e.g., Respiratory epithelium)
• Stratified squamous keratinized epithelium is multiple layers of cells with the top layer flat and no nuclei
• Stratified squamous non-keratinized epithelium is multiple layers of cells with the top layer containing nuclei.
• Nucleus is located typically in the center of cells.