Epithelial Tissue Overview

Epithelial Tissue

• Epithelial tissue is one of four basic tissue types in the human body, along with connective, muscular, and nervous tissues.
• These tissues are composed of cells and extracellular matrix.
• Epithelial tissues are not standalone units, but are in association with one another.
• Epithelial tissues are characterized by having a free surface (apical surface), a basement membrane (basal lamina), and closely packed cells with little intercellular substance (matrix).
• Epithelial tissues are avascular (lack blood vessels), and their nourishment depends on diffusion from underlying connective tissues.
• Epithelial tissue is renewed continuously through mitotic activity.
• The function of epithelial tissue includes protection, secretion, absorption, sensation, and contractility.
• Epithelial tissues range from simple to stratified.

Types of Epithelial Tissue

• Covering and lining epithelium:
  • Simple squamous epithelium: single layer of flat cells, suited for diffusion and filtration (e.g., lungs, lining of blood vessels).
  • Simple cuboidal epithelium: single layer of cube-shaped cells, involved in secretion and absorption (e.g., glands, kidney tubules).
  • Simple columnar epithelium: single layer of tall, column-shaped cells, involved in secretion and absorption; can be ciliated or non-ciliated (e.g., digestive tract, lining of the uterus).
  • Pseudostratified columnar epithelium: appears multilayered but is one layer, cells vary in height, often ciliated and involved in secretion and movement (e.g., respiratory tract).
  • Stratified squamous epithelium: multiple layers of cells with flattened surface cells, provide protection against abrasion (e.g., skin, lining of the mouth).
  • Stratified cuboidal epithelium: multiple layers of cube-shaped cells; involved in secretion and protection (e.g., sweat glands).
  • Stratified columnar epithelium: multiple layers of columnar cells; involved in secretion and protection (e.g., large glands).
• Transitional epithelium: specialized cells allowing for stretching (e.g., urinary bladder).
• Glandular epithelium: forms glands, specialized for secretion (e.g., endocrine and exocrine glands).
• Neuroepithelium: modified epithelial cells in sensory organs (e.g. retina, nasal epithelium).
• Myoepithelial Cells: specialized cells that have both contractile and secretory elements (e.g., glands).

Epithelial Tissue Characteristics

• Origin: Derived from all three embryonic germ layers.
• Structure: Cells are closely packed with minimal intercellular substance.
• Basement membrane: Thin membrane separating epithelium from underlying connective tissue.
• Avascular: Epithelial tissues do not contain blood vessels.
• Renewal: Continuously renewed through mitosis.