Fig 008 Epithelium

Epithelial Tissue Classification

Epithelial tissues are classified using three morphological criteria - number of layers, cell shape, and surface specializations - that relate to their location in the body and specific functions.

The number of layers is combined with the cell shape to give the full classification (e.g., simple squamous, stratified squamous, simple columnar). The presence of keratin and goblet cells is usually included when present (e.g., keratinized stratified squamous).

• Number of Cell Layers:
  • Simple Epithelium: Single layer of cells optimized for rapid exchange between compartments
    • Excels at diffusion, filtration, and absorption because substances only need to cross a single cell
    • Offers minimal mechanical protection
  • Stratified Epithelium: Two or more layers that prioritize protection against abrasion and physical stress
    • Sacrifices rapid exchange for toughness
  • Pseudostratified Epithelium: Type of simple epithelium that appears layered because the nuclei of its cells are at different heights, but every cell maintains direct contact with the basement membrane
    • Frequently referred to as the respiratory epithelium because it is found in much of the respiratory tract (the nasal cavity, trachea, and bronchi)

• Cell Shape: Assessed at the free (apical) surface and describes the outermost layer in stratified tissues
  • Squamous Cells: Flattened cells typically with a thin nucleus
  • Cuboidal Cells: Roughly as tall as they are wide, with a central, spherical nucleus
  • Columnar Cells: Taller than wide, with the nucleus usually located near the base
    • Extra height accommodates transport machinery, making them common in secretory or absorptive surfaces

• Surface Specializations: Modifications to meet specific functional demands
  • Cilia: Hair-like, motile projections on the apical surface that move substances (like mucus) across the surface
  • Keratin: Protective, water-insoluble protein layer found in the surface cells of skin
    • Dead, keratin-packed cells form a tough outer barrier that resists abrasion
    • Prevents water loss and pathogen entry
  • Goblet Cells: Specialized columnar cells that secrete lubricating mucus
    • Distributed throughout many simple columnar and pseudostratified epithelia
    • Secretes mucins which, when hydrated, form mucus that lubricates and protects the epithelial lining
    • Mucus traps debris and pathogens so cilia can clear them
  • Microvilli: Tightly packed, short, finger-like extensions that dramatically increase the apical surface area for absorption
    • Form the "brush border” visible under a light microscope
  • Stereocilia: Long, non-motile mechanosensory projections on the apical surface closely related to microvilli
    • Detect sound waves and balance in the inner ear
    • Assist in fluid absorption in the male reproductive tract (epididymis)

Specialized Epithelial Tissue

Specialized epithelial tissues are epithelial cells that have developed specific structural and functional adaptations beyond the basic barrier, absorptive, or secretory roles.

• Transitional Epithelium: Highly specialized stratified epithelium found exclusively in the urinary system, lining the bladder, ureters, and proximal urethra
  • Unique ability to stretch as these organs fill
  • When relaxed, the large dome-shaped “umbrella cells” cover the surface
  • As the organ distends, the epithelium thins and the cells flatten to accommodate the increased volume
• Pigmented Epithelium: Stratified squamous epithelium of skin contains melanocytes interspersed in the deepest layer, sitting directly on the basement membrane
  • Dark, melanin granules are transferred to other cells in the epithelium
  • Granules form a protective cap over the nucleus, providing protection from ultraviolet (UV) light before it can damage DNA
  • Responsible for skin color

Regeneration

Because epithelial surfaces are subject to continuous wear, most epithelia maintain a resident population of stem cells in the basal layer that proliferate and differentiate to replenish lost cells. The rate of turnover varies considerably: intestinal epithelium renews itself approximately every four to five days, while the epidermis turns over roughly every two to four weeks.

This regenerative capacity also underlies the ability of epithelial wounds to heal, though it means that epithelial cells are among the most mitotically active in the body and, consequently, among the most common sources of cancers (carcinomas).