Sheets of tightly packed cells on a basement membrane, with no blood vessels of their own
Epithelium — one of the four tissue types.
Anatomy · Cells & tissues
Epithelium
The lining and covering tissue of the body
Basic tissue · surfaces, tubes and glands
One of the four basic tissues of the body: sheets of tightly packed cells with very little material between them, resting on a thin basement membrane. It covers the outside of the body and lines the gut, airways, blood vessels and body cavities, and forms the secretory cells of glands. It has no blood vessels of its own and takes nutrients by diffusion from the connective tissue below. Classified by the shape of its cells (squamous, cuboidal, columnar) and the number of layers (simple, stratified, pseudostratified), plus the special transitional type of the bladder.
How to spot them
- ▸A sheet of cells packed edge to edge with almost no gaps, sitting on a thin basement membrane
- ▸Cells have a free (apical) surface facing a space and a base fixed to the tissue beneath
- ▸Flat scale-like cells: squamous; cube-like cells: cuboidal; tall pillar-like cells: columnar
- ▸One layer means simple; many layers means stratified; a false layered look means pseudostratified
- ▸Mucus-secreting goblet cells scattered among columnar cells are a mark of the gut and airways
What to know
A single layer of extremely flat cells lining the air sacs of the lung and the inside of blood vessels
Epithelium — simple squamous.
Layers of flattened cells in the mouth, gullet and outer surface of the body, toughened for wear and tear
Epithelium — stratified squamous.
A single layer of tall cells with a brush border and goblet cells lining the small intestine
Epithelium — simple columnar, specialised for absorption.
Looks layered but every cell touches the basement membrane; ciliated and lining the windpipe
Epithelium — pseudostratified columnar ciliated.
A tissue whose surface cells balloon and flatten as the bladder fills
Epithelium — transitional epithelium, or urothelium.
The seat of most cancers, the carcinomas
Epithelium — the majority of human cancers arise in it.
Key ideas
- Cells are held together by tight junctions (sealing), adherens junctions and desmosomes (strength) and gap junctions (communication), so the sheet acts as a selective barrier.
- Where the surface is exposed to wear, the tissue turns over fast: the lining of the small intestine is renewed roughly every few days.
Quick-fire facts
- •Its four functions are protection, secretion, absorption and filtration
- •Simple squamous tissue lines the alveoli, vessels (as endothelium) and body cavities (as mesothelium)
- •Simple cuboidal tissue forms kidney tubules and many glands; ciliated columnar tissue lines the uterine tubes
- •The skin's epidermis is a keratinised stratified squamous type
- •Glands are formed by epithelial cells that sink below the surface: exocrine glands keep a duct, endocrine glands lose it
- •The word comes from Greek epi (upon) and thele (nipple), coined in the early 18th century, usually credited to the Dutch anatomist Frederik Ruysch
- •Roughly 80–90% of human cancers are carcinomas, arising in epithelial cells
- •Epithelia are avascular but richly supplied with nerves, and are attached below by a basement membrane
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