Contents · 8 sections
Epithelial tissue is the first tissue studied in histology courses and forms the foundation for understanding all organs. Epithelia cover all body surfaces, line cavities, and form glands. Epithelial classification is a mandatory question on every histology exam. This article provides a comprehensive systemization featuring tables, clinical examples, and mnemonics.
General properties of epithelial tissue
All types of epithelium share common characteristic features:
- Cells are densely packed with minimal extracellular matrix
- Polarity — apical (free) and basal poles differ in structure and function
- Basement membrane — separates the epithelium from the underlying connective tissue, consisting of type IV collagen, laminin, and proteoglycans
- Avascularity — nourished via diffusion from the underlying connective tissue
- High regenerative capacity — continuous renewal driven by stem cells
- Innervation — contains sensory nerve endings
Classification of surface epithelium
Based on two criteria: number of cell layers and shape of cells in the apical layer.
| Type | Structure | Localization | Function |
|---|---|---|---|
| Simple squamous | Single layer of flat cells | Vascular endothelium, mesothelium of serous membranes, lung alveoli (type I) | Gas exchange, transport, friction reduction |
| Simple cuboidal | Single layer of cube-shaped cells | Renal tubules, thyroid follicles, glandular ducts | Absorption, secretion |
| Simple columnar | Single layer of tall cells | Stomach, intestines, uterus | Secretion, absorption |
| Pseudostratified ciliated columnar | Cells of varying heights, nuclei at different levels, cilia | Respiratory tract, Fallopian tubes | Mucociliary transport |
| Stratified squamous non-keratinized | Multiple layers, superficial cells are flattened and living | Oral cavity, esophagus, vagina, cornea | Protection against mechanical damage |
| Stratified squamous keratinized | Multiple layers, superficial cells are dead keratinized scales | Skin (epidermis) | Barrier against desiccation, microbes, and mechanical damage |
| Transitional (urothelium) | Changes thickness upon stretching | Urinary tract: renal pelvis, ureters, urinary bladder | Distensibility, urine barrier |
Apical surface specializations
- Microvilli — increase absorptive surface area. Brush border of the intestine (enterocytes), striated border of proximal renal tubules
- Cilia (kinocilia) — motile projections for transport. Respiratory epithelium (mucociliary clearance), Fallopian tubes (transport of the oocyte)
- Stereocilia — long, non-motile microvilli. Epididymis, hair cells of the inner ear
Intercellular junctions
Types of junctions between epithelial cells:
| Contact | Function | Proteins |
|---|---|---|
| Tight junction (zonula occludens) | Barrier — prevents passage of molecules between cells | Claudins, occludin |
| Adherens junction (zonula adherens) | Mechanical attachment, connection to the cytoskeleton | E-cadherin, actin |
| Desosome (macula adherens) | Strong spot-like adhesion, resistance to mechanical stress | Desmoglein, desmocollin, intermediate filaments |
| Gap junction | Exchange of ions and small molecules | Connexins |
| Hemidesmosome | Attachment to the basement membrane | Integrins |
Glandular epithelium
Classification of glands:
- By secretion site: exocrine (have a duct) and endocrine (secrete into the blood)
- By secretion mechanism:
- Merocrine — exocytosis without cell damage (most glands)
- Apocrine — shedding of the apical part of the cell (mammary glands, axillary sweat glands)
- Holocrine — complete cell destruction (sebaceous glands)
- By secretion composition: serous (protein), mucous (mucin), mixed
- By terminal portion shape: tubular, alveolar, tubuloalveolar
Epithelial regeneration
Epithelium is one of the most actively renewing tissues. Stem cells are located in the basal layer (for stratified epithelium) or among regular cells (for simple epithelium).
- Epidermis — complete renewal in 28–30 days
- Intestinal epithelium — renewal in 3–5 days (Crypts of Lieberkühn)
- Hepatocytes — renewal in 200–300 days, but capable of rapid regeneration upon injury
Metaplasia — replacement of one type of epithelium with another. For example, ciliated pseudostratified columnar → stratified squamous in the bronchi of chronic smokers. This is a precancerous condition.
Clinical correlations
- Pemphigus — autoantibodies against desmoglein → destruction of desmosomes → blisters on the skin and mucous membranes
- Cystic fibrosis — CFTR chloride channel defect → thick mucus → impaired mucociliary clearance
- Celiac disease — autoimmune destruction of small intestine villi → atrophy of enterocyte microvilli → malabsorption
- Cervical dysplasia — impaired differentiation of stratified squamous epithelium, a precancerous condition
Frequently asked questions
How does pseudostratified epithelium differ from stratified epithelium?
In pseudostratified epithelium, all cells contact the basement membrane but vary in height — nuclei lie at different levels, creating a "pseudostratified" appearance. In stratified epithelium, only the basal layer of cells touches the membrane.
Why is urothelium called transitional?
Because its shape changes: when the bladder is empty, it is thick (5–6 layers); when full, it stretches to 2–3 layers. Surface (umbrella) cells are covered with special proteins called uroplakins, which make it impermeable to urine.
How to memorize all types of epithelium?
Use organ association: respiratory tract = ciliated, skin = keratinized, intestine = columnar with microvilli, bladder = transitional, esophagus = non-keratinized. Memorize "organ → epithelium", not the other way around.
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