Epithelium Notes

Epithelium

Lecture Overview

  • Definitions, functions, and relevance of epithelium.
  • Characteristics of epithelium.
  • Types and classification of epithelia.

Definitions

  • Cells > Tissues > Organs > Systems
  • Tissues = aggregation of cells and extracellular substances with a common purpose.
  • Organs are made up of four basic tissue types:
    • Epithelial tissue
    • Connective tissue
    • Muscular tissue
    • Nervous tissue

Three Types of Epithelium

  • Lining (surface) epithelia: Form sheets that cover body surfaces or line luminal organs, tubular structures, and body cavities.
  • Glandular epithelia: Have specialized cells that synthesize, store, and release various products.
  • Special epithelia: Contain sensory nerve endings; found in the skin, ears, and on the tongue; modified neurons are in the nose and eyes.

Functions of Epithelial Tissues

  • Protection: The epidermis protects from mechanical abrasion/injury, harmful chemicals, invading microbes, and excessive water loss.
  • Friction Reduction: Smooth endothelial cells line the entire circulatory system to reduce friction between blood and walls of blood vessels and heart chambers.
  • Cleaning: Ciliated respiratory epithelium assists in removing dust particles/foreign bodies from air passages.
  • Diffusion: Endothelium of capillaries promotes the diffusion of gases, liquids, and nutrients.
  • Absorption: Certain epithelial cells lining the small intestine absorb nutrients from digested food.
  • Secretion: Epithelial tissue secretes chemical substances such as enzymes, hormones, and lubricating fluids.
  • Sensation: Specialized epithelial tissue containing sensory nerve endings convey sensations from the skin, ears, and the tongue.

Relevance

  • Basic understanding of tissue organization, development, and functioning is essential.
  • Essential to understand:
    • Systemic histology: Many organs and all glands have an epithelial component.
    • Tumors / neoplasia: Terminology.

Characteristics of Epithelium

  • Supported by connective tissue (CT).
  • Avascular.
  • Basement membrane (BM) present.
  • Epithelial cells are cohesive.
  • Epithelial cells are polarized beneath CT.

Epithelial Cells are Polarized

Epithelial cells have:

  • Apical (free) surface
  • Lateral surfaces
  • Basal surface

Classification of Lining Epithelia

  • Simple (one layer of cells):
    • Squamous
    • Cuboidal
    • Columnar
  • Stratified (multiple layers of cells):
    • Squamous:
      • Keratinized
      • Non-keratinized
    • Cuboidal/columnar - rare
    • Transitional/urothelium
  • Pseudostratified columnar ciliated

Lining Epithelia: Simple Squamous Epithelium

  • Composed of flat, elongated cells with a round to oval nucleus, often centrally located.
  • Common locations:
    • Lining of body cavities, generally called mesothelium (pleural, pericardial, and peritoneal).
    • Alveolar walls in lungs.
    • Inner lining of blood vessels and lymphatic vessels, called endothelium.

Stratified Squamous Epithelium

  • Epithelium composed of several layers of cells.
  • Superficial cells determine the name; the superficial cells have a squamous shape.
  • Two types:
    • Keratinized (cornified)
    • Non-keratinized

Lining Epithelia: Simple Columnar Epithelium

  • Tall, narrow cells with ovoid nucleus located near the base of the cell.
  • Examples:
    • Lining the luminal surface of the stomach, small and large intestine, and gall bladder.

Simple Columnar Ciliated Epithelium

  • Examples:
    • Lining of oviduct.

Pseudostratified Epithelium

  • Composed of a single layer of irregularly shaped and sized cells.
  • All cells touch the basement membrane, but not all of them reach the apical surface.
  • Can be ciliated (motile).
  • Examples:
    • Nasal cavity, upper respiratory tract.

Transitional Epithelium (Urothelium)

  • Lines urinary passages (e.g., urinary bladder).
  • The superficial cells have a specialized plasma membrane providing an osmotic barrier between urine and tissue fluids.
  • In an empty urinary bladder, the superficial cells are cuboidal with a dome-shaped surface.
  • When the urinary bladder is full, the volume of urine has stretched the lining to such a degree that the epithelium appears flattened, and more like a stratified squamous epithelium.

Epithelial Repair

  • Epithelial cells are constantly lost and replaced.
  • Stem cells are present which have high mitotic potential.
  • Location of stem cell varies depending on epithelial type and function.
  • Stem cells are located in a basal cell layer (stratum basale).
  • Cells are pushed by the new layers away from the BM and are shed off in stratified squamous keratinized epithelium.

Exfoliative Cytology

  • Harvesting cells shed from surface tissues, from mucous membranes, or found in body liquids and examining them under a microscope.
  • Examples:
    • Skin surface cytology (differential diagnosis of dermatitis)
    • Urine sediment analysis (screening of lesions or neoplasia - e.g., transitional cell carcinoma)
    • Vaginal smear analysis (bitches) – the type of epithelial cells observed indicates phase of the estrus cycle
    • Fine needle biopsy of masses

Classification of Glandular Epithelia

  • Number of cells in the gland (unicellular or multicellular).
  • Shape of secretory units (adenomere).
  • Type of product.

Unicellular Glands

  • Found in epithelial lining and glands of intestine and in the epithelial lining of the respiratory tract.
  • Example: Goblet cells have a “cup” – goblet shape due to presence of abundant mucinogen granules in the apical part of the cell.

Multicellular Glands

  • Modified epithelia:
    • TUBULAR: straight or coiled (sweat gland, stomach glands, intestinal glands)
    • ACINAR: pie-shaped, small lumen (pancreas, salivary glands)
    • ALVEOLAR: larger luminal space (mammary gland, prostate, sebaceous glands)

Acinar vs. Alveolar Shape

  • Acinar shape corresponds to a rounded secretory unit.
  • Alveolar shape corresponds to a bigger luminal space.

Parenchyma and Stroma

  • The collective of secretory units and ducts of a compound gland is termed parenchyma; the connective tissue elements comprise the stroma.
  • Large glands are divided into lobes. These are further subdivided by connective tissue into lobules.

Classification According to Product

  • Serous: Watery product, contains enzymes (e.g., sweat).
  • Mucous: Slick, viscous secretion (e.g., tracheal mucus).
  • Mixed: A gland producing both mucus and serous secretion (e.g., saliva).
  • Sebaceous: Oily secretion, often known as sebum.