Classification of epithelium
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classification of epithelium histology simple stratified pseudostratified transitional authoritative
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Classification of epithelium
Epithelium is classified mainly by:
- Number of cell layers
- Shape of the surface cells
- Sometimes, apical specialization, such as cilia or keratin
Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 321-322.
1. Based on number of layers
| Type | Definition |
|---|---|
| Simple epithelium | A single layer of cells. All cells rest on the basement membrane. |
| Stratified epithelium | Two or more layers of cells. It is named according to the shape of the most superficial cells. |
| Pseudostratified epithelium | Appears multilayered because nuclei lie at different levels, but every cell contacts the basement membrane. It is actually a simple epithelium. |
| Transitional epithelium (urothelium) | A specialized stratified epithelium adapted for stretching in the urinary tract. |
2. Based on shape of cells
| Cell type | Shape |
|---|---|
| Squamous | Flat cells, width greater than height |
| Cuboidal | Height, width, and depth are approximately equal |
| Columnar | Height is greater than width |
Types, sites, and functions
| Type of epithelium | Main sites | Major function |
|---|---|---|
| Simple squamous | Alveoli of lungs, endothelium of blood vessels, mesothelium of serous cavities, Bowman capsule | Rapid diffusion, filtration, smooth lining |
| Simple cuboidal | Kidney tubules, thyroid follicles, small ducts of glands, surface of ovary | Secretion and absorption |
| Simple columnar, non-ciliated | Stomach, intestine, gallbladder | Absorption and secretion |
| Simple columnar, ciliated | Uterine tubes, small bronchi | Movement of ovum or mucus |
| Pseudostratified ciliated columnar | Trachea and most of upper respiratory tract | Mucociliary clearance |
| Pseudostratified columnar with stereocilia | Epididymis, ductus deferens | Absorption and secretion |
| Stratified squamous, keratinized | Epidermis of skin | Protection from abrasion, dehydration, and infection |
| Stratified squamous, non-keratinized | Oral cavity, pharynx, esophagus, vagina, anal canal | Protection against abrasion on moist surfaces |
| Stratified cuboidal | Ducts of sweat glands; larger ducts of some exocrine glands | Protection and limited secretion |
| Stratified columnar | Large ducts of glands, parts of male urethra, conjunctiva | Protection and secretion |
| Transitional epithelium / urothelium | Renal calyces, renal pelvis, ureters, urinary bladder, proximal urethra | Distension while maintaining a urine barrier |
Key exam points
- Stratified epithelia are named by the shape of their superficial cells, not the basal cells.
- Pseudostratified epithelium is simple, because all cells touch the basement membrane.
- Transitional epithelium is stratified and has dome-shaped superficial umbrella cells when relaxed.
- Apical modifications may be included in the name:
- Ciliated simple columnar epithelium
- Keratinized stratified squamous epithelium
A simple way to remember:
- Simple = absorption, secretion, diffusion
- Stratified = protection
- Transitional = stretching
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