Epithelium in Histology

Introduction

• Lecturer: Dr. Morton, a noted anatomist.
• Topic: Epithelium, one of the four basic tissue types in the body.
• Overview Questions:
  • What is epithelium?
  • What are the different classifications, functions, and locations of epithelium?
  • What are cilia and microvilli?

What is Epithelium?

• One of the four basic tissue types (
  • Connective tissue
  • Muscle
  • Nervous tissue
• Functions:
  • Forms glands (e.g., liver, pancreas, etc.).
  • Lines the inside and outside of the body.
  • Example: Lines stomach, tubular organs, thoracic cavity, and skin (epidermis).
• Structure:
  • Consists of epithelial cells anchored to a basement membrane.
  • No extracellular matrix (cells packed closely like bricks).
  • Avascular (no blood vessels) – relies on nutrients from nearby loose connective tissue.

Classifications of Epithelium

• Criteria: Histologists classify epithelium based on cell layers, cell shape, and specializations.
• Examples:

Simple Squamous Epithelium

• Definition: Single layer of flat cells.
• Functions: Diffusion, filtration, lubrication.
• Locations: Alveolus, glomerulus, blood vessels.
• Histological Features:
  • Hematoxylin and Eosin (H&E) stain: Nuclei stain dark purple, cytoplasm pink.

Simple Cuboidal Epithelium

• Definition: Single layer of cube-shaped cells.
• Functions: Absorption, secretion.
• Locations: Nephron tubules, glands (e.g., sweat glands).
• Histological Features:
  • Nuclei are round/oval, cytoplasm is pink.

Simple Columnar Epithelium

• Definition: Single layer of tall, column-shaped cells.
• Functions: Absorption, secretion.
• Locations: GI tract, glands, airways, uterus.
• Modification: Can have cilia.
• Histological Features:
  • Long, narrow nuclei, often with apical microvilli (brush border).
  • Cilia can be present to move fluid.

Stratified Squamous Epithelium

• Definition: Multiple layers of cells, topmost being flat.
• Functions: Protection against abrasion.
• Locations: Epidermis, esophagus, vagina.
  • Subtypes: Keratinized (no nuclei in top layer) and non-keratinized (nuclei present).
• Histological Features:
  • Basal cells are cuboidal, apical cells are squamous.

Transitional (Urinary) Epithelium

• Definition: Multiple layers, apical layer cells are dome-shaped.
• Functions: Permits stretching and distension.
• Locations: Urinary bladder, ureter.
• Histological Features:
  • Epithelial cells close together, showing domed apical layer.

Pseudostratified Ciliated Columnar Epithelium

• Definition: Appears stratified but all cells touch the basement membrane.
• Functions: Secretion (mainly mucus), propulsion (cilia).
• Locations: Trachea, proximal bronchi.
• Histological Features:
  • Presence of cilia and goblet cells, looks stratified but is not.

Cilia and Microvilli

• Cilia:

  • Located on the apical surface.
  • Moves liquid over the cell surface (e.g., respiratory tract).
  • Analogy: Like arms in a crowd surf.

• Microvilli:

  • Located on the apical surface.
  • Increases surface area for absorption.
  • Analogy: Like the bristles of a brush.

Practice Session

Match-the-tissue:

1. Simple Squamous Epithelium
2. Simple Cuboidal Epithelium
3. Simple Columnar Epithelium
4. Pseudostratified Ciliated Columnar Epithelium (Respiratory Epithelium)
5. Stratified Squamous Keratinized Epithelium
6. Stratified Squamous Non-Keratinized Epithelium

• Summary:
  • Simple tissues: Single layer, different shapes (squamous, cuboidal, columnar).
  • Stratified tissues: Multiple layers, apical layer's shape significant.
  • Special tissues: Transitional for urinary systems, pseudostratified for respiratory with cilia.

Conclusion

• Epithelium is vital for forming glands and lining organs.
• Classifications aid in understanding functions and locations.
• Special structures like cilia and microvilli enhance function.