Epithelial Tissue Overview

Defining Epithelial Tissue

• Epithelial tissue is a sheet-like tissue that covers body surfaces, lines body cavities and forms glands.
• It is characterized by closely packed cells with minimal extracellular matrix.
• It acts as a barrier, protecting underlying tissues from mechanical injury, dehydration, and harmful substances.

Functions of Epithelial Tissues

• Protection: Forms a barrier against pathogens, abrasion, and dehydration.
• Secretion: Specialized epithelial cells produce and release substances such as hormones, mucus, and enzymes.
• Absorption: Epithelial cells lining the digestive tract absorb nutrients.
• Excretion: Removal of waste products.
• Filtration: Selective passage of substances across the epithelium.
• Diffusion: Movement of substances across the epithelium.
• Sensory reception: Specialized epithelial cells detect stimuli, such as taste, smell, and touch.

Characteristics of Epithelial Tissues

• Cellularity: Composed primarily of cells with minimal extracellular material.
• Specialized contacts: Cells are closely joined by specialized junctions (tight junctions, adherens junctions, desmosomes, and gap junctions) to form continuous sheets.
• Polarity: Apical (free) and basal (attached) surfaces with distinct structures and functions.
• Supported by connective tissue: Attached to a basement membrane, a specialized layer of extracellular matrix.
• Avascular: Lacks blood vessels, relying on diffusion from underlying connective tissue for nutrients and waste removal.
• Regeneration: High capacity for renewal and repair due to rapid cell division.

Classification of Epithelial Tissues

• Based on cell shape and arrangement in layers, epithelial tissues are categorized into different types:
  • Squamous (flattened)
  • Cuboidal (cube-shaped)
  • Columnar (column-shaped)
• Based on the number of layers:
  • Simple (single layer) e.g., simple squamous epithelium
  • Stratified (multiple layers) e.g., stratified squamous epithelium
• Some special types:
  • Pseudostratified columnar: Appears layered but all cells contact the basement membrane.

Example Tissues and their functions

• Simple squamous epithelium: Lines blood vessels and air sacs of lungs, facilitating diffusion.
• Stratified squamous epithelium: Forms the epidermis of the skin (protects against abrasion/drying).
• Simple cuboidal epithelium: Lines kidney tubules, involved in secretion and absorption.
• Stratified cuboidal epithelium: Found in sweat glands.
• Simple columnar epithelium: Lines the digestive tract, involved in absorption and secretion.
• Pseudostratified columnar epithelium: Found in respiratory tract, involved in secretion and movement of mucus.

Glands

• Glands are derived from epithelial tissue specialized for secretion.
• Classified as either exocrine (secrete products into ducts; e.g., sweat glands, salivary glands) or endocrine (secrete products directly into the bloodstream; e.g., thyroid gland).

Epithelial Tissue and Disease

• Aberrant proliferation and differentiation of epithelial cells contribute to a variety of diseases including cancers such as skin cancer, lung cancer, and colon cancer.

Summary

• Epithelial tissues perform essential functions in covering surfaces, lining cavities, and forming glands.
• The structure, consisting of tightly packed cells attached to a basement membrane, dictates the diverse roles played by this essential tissue type.
• Their highly regenerative nature is crucial for repairing damaged tissues and maintaining overall health.