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.