Anatomy chapters
Epithelium and cell junctions
The types of covering epithelium and where each is found, glands, surface specialisations, and the junctions that hold epithelial cells together.
- Epithelium is named by number of layers and shape of the top layer.
- It sits on a basement membrane and has no blood vessels.
- Five junctions to know: tight, adherens, desmosome, hemidesmosome and gap.
On this page
Overview
Epithelium covers body surfaces, lines cavities and tubes, and forms glands.
Epithelial cells sit close together with very little between them. They rest on a basement membrane and have no blood vessels; they get nutrients by diffusion from the connective tissue below.
Naming epithelium
Two features give the name:
- Number of layers: simple (one layer), stratified (many layers), or pseudostratified (one layer that looks like many).
- Shape of the surface cells: squamous (flat), cuboidal (cube) or columnar (tall).
Simple epithelia
| Type | Function | Sites |
|---|---|---|
| Simple squamous | Easy diffusion and filtration | Lung alveoli, lining of blood vessels (endothelium), lining of body cavities (mesothelium), Bowman's capsule |
| Simple cuboidal | Secretion and absorption | Kidney tubules, thyroid follicles, ducts of glands, surface of the ovary |
| Simple columnar | Absorption and secretion | Stomach, small and large intestine, gallbladder |
| Simple columnar ciliated | Moves fluid or cells | Uterine tubes, small bronchi |
| Pseudostratified columnar ciliated with goblet cells | Traps and clears mucus | Trachea, bronchi, nasal cavity ("respiratory epithelium") |
In pseudostratified epithelium every cell touches the basement membrane, even though the nuclei lie at different levels.
Stratified epithelia
| Type | Function | Sites |
|---|---|---|
| Stratified squamous keratinised | Protection against drying and wear | Skin (epidermis) |
| Stratified squamous non-keratinised | Protection against wear, kept moist | Mouth, oesophagus, vagina, cornea, ectocervix |
| Stratified cuboidal | Protection | Ducts of sweat glands |
| Stratified columnar | Protection | Part of the male urethra, large ducts, conjunctiva |
| Transitional (urothelium) | Stretches without tearing | Renal pelvis, ureter, bladder, upper urethra |
Transitional epithelium lines the urinary tract. Its large dome-shaped surface cells flatten when the bladder fills.
Surface specialisations
- Microvilli: tiny finger-like folds that increase absorptive area. They form the brush border of the small intestine and the proximal kidney tubule.
- Cilia: longer motile projections with a core of microtubules in a 9 + 2 pattern. They beat to move mucus (airways) or the oocyte (uterine tube).
- Stereocilia: very long microvilli that do not move. Found in the epididymis and the hair cells of the inner ear.
Stereocilia are not true cilia. They are long microvilli and do not beat.
Glands
Glands are made of epithelial cells that secrete.
- Exocrine glands pour their secretion into a duct (sweat glands, salivary glands, pancreas).
- Endocrine glands have no ducts; they release hormones into the blood (thyroid, adrenal).
Exocrine glands are also grouped by how they release their product:
| Type | How it secretes | Example |
|---|---|---|
| Merocrine (eccrine) | By exocytosis, cell stays intact | Most sweat glands, salivary glands, pancreas |
| Apocrine | Top part of the cell is pinched off | Mammary gland (fat part), apocrine sweat glands of axilla |
| Holocrine | Whole cell breaks down | Sebaceous glands |
Cell junctions
| Junction | Main protein | Job |
|---|---|---|
| Tight junction (zonula occludens) | Claudins and occludin | Seals the gap between cells, so substances must pass through cells |
| Adherens junction (zonula adherens) | Cadherins, linked to actin | Belt that holds cells together |
| Desmosome (macula adherens) | Cadherins (desmogleins), linked to keratin filaments | Strong spot weld between cells; resists pulling |
| Hemidesmosome | Integrins | Anchors the cell to the basement membrane |
| Gap junction | Connexins forming connexons | Channels that let ions and small molecules pass, so cells communicate |
Gap junctions let electrical signals spread between cardiac muscle cells and smooth muscle cells.
A hemidesmosome joins a cell to the basement membrane, not to another cell.
Basement membrane
The basement membrane is a thin sheet under all epithelia. It contains type IV collagen, laminin and proteoglycans.
It supports the epithelium and acts as a filter (in the kidney glomerulus). Cancer that breaks through it is called invasive.
Clinical relevance
Metaplasia
How it happens: long-term irritation makes one adult epithelium change to another, tougher type.
What you see: in smokers, the ciliated respiratory epithelium of the bronchi becomes stratified squamous. In Barrett oesophagus, acid reflux turns the lower oesophageal squamous lining into columnar epithelium with goblet cells.
Pemphigus vulgaris
How it happens: antibodies attack desmogleins in desmosomes.
What you see: fragile blisters within the epidermis that break easily, and mouth ulcers.
Bullous pemphigoid
How it happens: antibodies attack hemidesmosomes.
What you see: tense blisters under the epidermis, usually in older people.
Kartagener syndrome
How it happens: a defect in the dynein arms of cilia.
What you see: chronic sinus and chest infections, male infertility and sometimes organs on the wrong side.
Questions give a site and ask for the epithelium (bladder, trachea, oesophagus, alveoli), or give a disease and ask which junction is affected (pemphigus: desmosome; pemphigoid: hemidesmosome). Metaplasia in smokers and in Barrett oesophagus is a common applied angle.
Quick revision
- Simple squamous: alveoli, endothelium, mesothelium.
- Pseudostratified ciliated columnar with goblet cells: trachea and bronchi.
- Non-keratinised stratified squamous: mouth, oesophagus, vagina, cornea.
- Transitional: urinary tract.
- Stereocilia: epididymis; not motile.
- Pemphigus: desmosomes. Pemphigoid: hemidesmosomes.
- Gap junctions are made of connexins.
Written by The House Job from standard histology references (Junqueira's Basic Histology).
FAQ
What epithelium lines the trachea?
Pseudostratified ciliated columnar epithelium with goblet cells.
What epithelium lines the urinary bladder?
Transitional epithelium (urothelium).
What is the difference between a desmosome and a hemidesmosome?
A desmosome joins two cells. A hemidesmosome joins a cell to the basement membrane.
What happens to bronchial epithelium in smokers?
It can change to stratified squamous epithelium (squamous metaplasia).