Epithelial Tissue Overview

Epithelial Tissue

• Avascular tissue that covers external body surfaces and lines internal cavities.
• Forms glands and sensory receptors.
• Composed of cells arranged in sheets.
• Cells have specialized junctions (junctional complexes).
• Distinct apical (free), lateral, and basal surface domains.
• Cells rest on a non-cellular basal lamina (protein-polysaccharide-rich layer).

Epithelial Functions

• Transport: Moves particles and mucus via cilia (e.g., trachea, bronchi).
• Sensory reception: Receives stimuli (e.g., taste buds, retina).
• Lubrication: Secretion of mucous and serous fluids.
• Excretion: Filters blood, producing urine, and sweat.
• Reproductive: Supports sperm and egg production.

Epithelial Classification

• Classified by cell arrangement (simple or stratified) and cell shape (squamous, cuboidal, columnar).
• Not classified by function!

Simple Epithelia - Cell Shapes

• Squamous: Width is greater than height.
  • Simple Squamous: Single layer; lines blood vessels, Bowman's capsule, respiratory spaces.
  • Function: Exchange, barrier (central nervous system), lubrication, barrier between compartments.
• Cuboidal: Width, depth, and height are approximately the same.
  • Simple Cuboidal: Single layer; found in thyroid follicle walls, kidney tubules, ovary surface, and small ducts of exocrine glands.
• Columnar: Height greatly exceeds width.
  • Simple Columnar: Single layer; lines the intestinal tract (stomach to rectum), uterus, and cervix, and gall bladder.
  • Function: Absorption, secretion, and barrier.
• Pseudostratified: Appears stratified but all cells rest on the basal lamina; a simple epithelium..
  • Found in the respiratory tract (trachea, bronchi), epididymis, and ductus deferens; frequently ciliated or with stereocilia.
• Transitional: Stratified epithelium that accommodates distension; found in the urinary bladder, part of the urethra, and ureter, and renal calyces.
• Stratified Squamous: Multiple layers; superficial layer is squamous. Functions as a barrier; protects the body.
  • Keratinized: Epidermis (skin).
  • Non-keratinized: Lining of oral cavity, vagina, and esophagus.

Glands

• Specialized epithelial cells for secretion.
  • Exocrine: Secrete onto an epithelial surface via ducts (sweat, earwax, saliva, digestive enzymes).
  • Endocrine: Secrete hormones into the bloodstream.
  • Paracrine: Secrete into the extracellular space.

Exocrine Gland Classification

• Unicellular: Composed of a single cell (e.g., goblet cell).
• Multicellular: Composed of multiple cells.
  • Simple: Duct does not branch.
  • Compound: Duct branches.
  • Shape: Acinar/alveolar (sac-like), tubule (straight, coiled, branched).

Gland Secretion Methods

• Merocrine: Most glands; watery secretions (saliva, digestive enzymes, watery sweat).
• Apocrine: Smelly sweat (e.g., sweat glands).
• Holocrine: Cells die and rupture to release products (e.g., sebaceous glands).

Epithelial Regeneration

• Continuously recycled (e.g., epidermis, intestinal epithelium).
• Abnormal cell division can lead to tumors.
• Epithelial cancers are called carcinomas.
• Glandular cancers are called adenocarcinomas.

This quiz focuses on the characteristics and functions of epithelial tissue, which covers body surfaces and lines internal cavities. It covers classification based on cell arrangement and shape, as well as specific roles such as transport, sensory reception, and lubrication. Test your knowledge on this essential tissue type!