4-3 Classifications of Epithelial Cells

Cell Shape and Classification of Epithelia

• Cell shape and number of layers determine the classification of epithelia.
  • Learning Outcome: Describe the relationship between structure and function for each type of epithelium.
  • Epithelial tissue includes epithelia and glands.
  • Glands are either attached to or derived from epithelia.

Classification of Epithelia

• Epithelia are sorted into categories based on:
  1. Cell shape
  2. Number of cell layers between the basement membrane and the exposed surface of the epithelium.
• Using these two criteria, almost every epithelium in the body can be described.

Basic Shapes of Epithelial Cells

• Epithelial cells have three basic shapes:
  • Squamous: Thin and flat like scales.
  • Cuboidal: Resemble little boxes.
  • Columnar: Tall and slender rectangles.
• To classify by shape, examine the superficial cells in a section perpendicular to both the apical surface and the basement membrane.

Definitions of Direction from Epithelial Cells:

• Apical surface: The upper, exposed surface of the epithelium.
• Basement membrane: The underlying layer that supports the epithelium.

Number of Cell Layers in Epithelia

• There are two options for the number of cell layers:
  • Simple Epithelium: One layer of cells covering the basement membrane.
  • Characteristics:
    • Thin and fragile.
    • All cells have the same polarity (the distance from the nucleus to the basement membrane does not change among cells).
    • Cannot provide much mechanical protection, so it is located in protected areas of the body.
    • Examples include:
    • Internal Compartment Linings: e.g., pleural, pericardial, and peritoneal cavities, heart chambers, and blood vessels.
    • Functional Areas: Lining of intestines and gas exchange surfaces of the lungs (where thinness aids absorption/diffusion).
  • Stratified Epithelium: Multiple layers of cells covering the basement membrane.
  • Generally found in areas exposed to mechanical or chemical stresses, such as the skin and lining of the mouth.

Squamous Epithelia

• Simple Squamous Epithelium:

  • Most delicate type of epithelium, located in protected regions for absorption or diffusion.
  • Examples:
  • Respiratory exchange surfaces (alveoli of the lungs).
  • Lining of thoracic and abdominal cavities.
  • Lining of heart and blood vessels (smooth linings are vital to prevent blood clotting).
  • Mesothelium: Simple squamous epithelium lining body cavities; includes thoracic (pleura), pericardial, and peritoneal cavities.
  • Endothelium: Simple squamous epithelium lining the heart and blood vessels.

• Stratified Squamous Epithelium:

  • Protects against physical and chemical attacks, resembles layers of plywood.
  • Found on the surface of the skin, lining of the mouth, esophagus, and anus.
  • Can be keratinized (water-resistant, tough) or non-keratinized (moist, resists abrasion).
  • Keratinized: Surface cells packed with keratin.
  • Non-keratinized: Must be kept moist (found in oral cavity, pharynx, esophagus, anus, vagina).

Cuboidal Epithelia

• Simple Cuboidal Epithelium:

  • Cells appear square in typical sectional views, spherical nuclei are near the center.
  • Provides limited protection; found where secretion or absorption occurs (e.g., glands and kidney tubules).

• Stratified Cuboidal Epithelium:

  • Rare, located along the ducts of sweat glands and larger ducts of mammary glands.

Transitional Epithelia

• Unusual stratified epithelium that changes shape from squamous to cuboidal.
• Tolerates repeated cycles of stretching without damage and is found in the urinary system (e.g., urinary bladder).
  • Appearance changes based on fullness of the bladder: plump and cuboidal when empty, flattened when full.

Columnar Epithelia

• Simple Columnar Epithelium:

  • Cells typically found where absorption or secretion occurs (e.g., small intestine).
  • Secretions protect against chemical stresses (stomach and large intestine).

• Pseudostratified Columnar Epithelium:

  • Appears layered due to varying cell heights but is not truly stratified (all cells contact the basement membrane).
  • Contains cilia and is found lining respiratory pathways (nasal cavity, trachea, bronchi) and male reproductive tract.

• Stratified Columnar Epithelium:

  • Relatively rare, provides protection in portions of the pharynx, epiglottis, anus, urethra, and large excretory ducts.

Glandular Epithelia

• Contain specialized cells capable of secretion; classified into two types:
  • Endocrine Glands: Secrete hormones into blood, regulating activities of tissues and organs.
  • Called ductless glands; examples include thyroid, thymus, and pituitary glands.
  • Exocrine Glands: Secrete substances onto epithelial surfaces via ducts (e.g., sweat, digestive enzymes, mucus).

Classification of Exocrine Glands

1. Gland Structure:
   • Unicellular Glands: Scattered secretory cells, e.g., goblet cells that secrete mucin.
   • Multicellular Glands: Aggregations of glandular cells that produce secretions, categorized based on:
     1. Duct Structure: Simple (single duct), Compound (divided ducts).
     2. Shape of Secretory Portion: Tubular (straight/coiled), Alveolar (blind pockets), Tubuloalveolar (both).
     3. Duct-Gland Relationship: Branching indicates multiple areas sharing a duct.

Methods of Secretion in Exocrine Glands

• Merocrine Secretion: Product released via exocytosis (e.g., salivary glands).
• Apocrine Secretion: Involves loss of apical cytoplasm along with the product (e.g., mammary glands).
• Holocrine Secretion: Destroys gland cells (e.g., sebaceous glands); relies on stem cells for replacement.

Types of Exocrine Secretions

• Serous Glands: Produce watery solutions with enzymes (e.g., parotid salivary glands).
• Mucous Glands: Produce mucins that hydrate to form mucus (e.g., sublingual salivary glands).
• Mixed Glands: Produce both serous and mucous secretions (e.g., submandibular salivary glands).

Clinical Note: Exfoliative Cytology

• The study of cells shed from epithelial surfaces for diagnostic purposes (e.g., Pap test, amniocentesis).
• Useful in identifying cancer and genetic screening for chromosomal abnormalities.