Epithelial tissue
Textus epithelialis
Also known as: epithelium, epithelia, covering and lining epithelium
Sheets of closely packed, polarised cells with little extracellular matrix that cover body surfaces, line cavities and ducts, and form glands; avascular and resting on a basement membrane.
Overview
Defining features: (1) cellularity — cells are tightly packed and joined by junctions, with minimal intercellular material; (2) polarity — an apical (free) surface, often with microvilli or cilia, and a basolateral surface with different membrane proteins; (3) attachment to a basement membrane; (4) avascularity — nutrients diffuse from capillaries in the underlying connective tissue (but epithelia are innervated); (5) high capacity for regeneration from basal or stem cells.
Covering epithelia are classified by number of layers — simple (one layer; absorption, secretion, filtration), stratified (two or more; protection) or pseudostratified (one layer that looks stratified) — and by the shape of the cells in the surface layer: squamous (flat), cuboidal (as tall as wide) or columnar (taller than wide). Stratified epithelia are named after their surface cells; transitional epithelium (urothelium) is a special stratified type that changes shape with distension. Surface specialisations (keratin, cilia, microvilli) and the presence of goblet cells complete the description.
Glandular epithelium forms exocrine glands (which secrete onto surfaces via ducts) and endocrine glands (ductless, secreting into the blood). Epithelial cells are identified by cytokeratin intermediate filaments and E-cadherin junctions.
Parts & subdivisions
- Diffusion and filtration.
- Secretion and absorption.
- Absorption and secretion.
- Respiratory epithelium.
- Protection from abrasion.
- Ducts of glands.
- Rare; large ducts, conjunctiva.
- Urinary tract; distensible.
- Exocrine and endocrine glands.
Anatomy
- Development
Derived from all three germ layers: ectoderm (epidermis, corneal epithelium, glands of skin), endoderm (lining of gut and airways, liver, pancreas, thyroid) and mesoderm (endothelium, mesothelium, renal tubules, reproductive ducts).
Function
- Protection (mechanical, chemical, microbial, desiccation).
- Absorption (intestine, kidney).
- Secretion (glands, goblet cells).
- Filtration and diffusion (glomerulus, alveoli).
- Selective barrier and transport (tight junctions, transporters).
- Sensory reception (taste buds, olfactory and retinal epithelia).
Clinical significance
Carcinoma
Most human cancers arise from epithelium. Breach of the basement membrane distinguishes invasive carcinoma from carcinoma in situ. Epithelial–mesenchymal transition (loss of E-cadherin, gain of vimentin) enables invasion.
Metaplasia
Epithelia adapt to chronic stress by changing type, e.g. squamous metaplasia of bronchial epithelium in smokers, columnar metaplasia of the oesophagus in reflux (Barrett).
Test yourself
The epithelium lining the urinary bladder, which can change from about six cell layers to two or three as it fills, is:
- Stratified squamous non-keratinised
- Transitional epithelium (urothelium)
- Pseudostratified columnar
- Stratified cuboidal
Answer: Transitional epithelium (urothelium)
Urothelium has large dome-shaped umbrella cells with uroplakin plaques and tight junctions that make an impermeable blood–urine barrier and flatten when the bladder is distended.
The trachea is lined by:
- Simple columnar epithelium with microvilli
- Stratified squamous non-keratinised epithelium
- Pseudostratified ciliated columnar epithelium with goblet cells
- Simple cuboidal epithelium
Answer: Pseudostratified ciliated columnar epithelium with goblet cells
"Respiratory epithelium": all cells touch the basement membrane but not all reach the lumen, so nuclei lie at different levels. Cilia and goblet-cell mucus form the mucociliary escalator.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.