MT 30 (LEC): Epithelial cells

  • The human body consists of four basic tissue types:

    • Epithelial tissue

    • Connective tissue

    • Muscular tissue

    • Nervous tissue

Epithelial Tissues

  • Made up of cells and extracellular matrix molecules

  • Form sheets that cover the body's surfaces and line cavities

  • Main functions:

    • Protection (e.g., skin)

    • Absorption (e.g., intestines)

    • Secretion (e.g., glands)

    • Contractility (e.g., myoepithelial glands)

  • Some epithelial cells are specialized for sensation (e.g., tongue, nasal lining)

  • Everything entering or leaving the body must pass through an epithelial sheet

Types of Epithelial Tissue

Divided into two main groups:

  1. Covering (Lining) Epithelium

  2. Glandular Epithelium

Covering or Lining Epithelium

  • Arranged in layers covering or lining the body

  • Classified based on:

    • Number of cell layers:

      • Simple epithelium: Single layer

      • Stratified epithelium: Multiple layers

    • Shape of surface cells:

      • Squamous (thin and flat)

      • Cuboidal (cube-shaped)

      • Columnar (taller than wide)

Simple Epithelium

  • Simple Squamous Epithelium:

    • Single thin layer

    • Found in blood vessels and areas requiring permeability

  • Simple Cuboidal Epithelium:

    • Cube-shaped cells, energy-intensive

    • Found in kidney tubules and glands

  • Simple Columnar Epithelium:

    • Taller, absorption-specialized

    • Found in intestines, gall bladder, oviduct

    • May have cilia or microvilli for better function

Stratified Epithelium

  • Stratified Squamous Epithelium:

    • Found in skin (keratinized) and moist cavities (non-keratinized, e.g., mouth, esophagus, vagina)

    • Protects from water loss and microorganisms

  • Stratified Cuboidal and Columnar Epithelium:

    • Rare, found in gland ducts (sweat, salivary glands, conjunctiva of the eye)

  • Transitional Epithelium (Urothelium):

    • Found in urinary bladder, ureter, proximal urethra

    • Dome-shaped umbrella cells protect against urine acidity

    • Stretchable—layers appear fewer when bladder is full

Pseudostratified Epithelium

  • Appears layered but all cells touch the basement membrane

  • Found in upper respiratory tract (ciliated pseudostratified columnar epithelium)

Glandular Epithelium

  • Specialized in secretion

  • Cells store and release proteins, lipids, or carbohydrate-protein complexes

  • Examples:

    • Pancreas (proteins)

    • Sebaceous glands (lipids)

    • Salivary glands (carbohydrates & proteins)

  • Types of glands:

    • Unicellular (single cells, e.g., goblet cells)

    • Multicellular (clusters, e.g., salivary glands, pancreas)

Examples of Epithelial Tissue Locations

  • Skin → Keratinized stratified squamous epithelium

  • Tongue → Squamous epithelium over connective tissue

  • Kidney → Simple cuboidal epithelium (in tubules and glands)

  • Urinary bladder → Transitional epithelium (urothelium)

  • Respiratory tract → Ciliated pseudostratified columnar epithelium

Epithelial Tissue and Disease

  • Neoplasia: Uncontrolled epithelial growth (may or may not lead to cancer)

  • Metaplasia: One type of epithelium transforms into another

    • Example: In smokers, ciliated epithelium in bronchi may turn into stratified squamous epithelium

    • Example: Vitamin A deficiency can lead to metaplasia in bronchi and urinary bladder