Squamous epithelium

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In anatomy, squamous epithelium (from Latin squama, "scale") is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell. Epithelium may possess only one layer of these cells, in which case it is referred to as simple squamous epithelium; or it may possess multiple layers, referred to then as stratified squamous epithelium. Both types perform differing functions, ranging from nutrient exchange to protection.

Simple squamous epithelia

A simple squamous epithelium is characterized by the presence of squamous cells that are all in contact with the basement membrane. The cells are irregularly shaped and very flat, so flat that the cell nucleus sometimes creates a bump in the surface of the cell. Gases and other substances can easily diffuse across squamous cells to the underlying basement membrane, and, because of their smooth surface, liquids can quickly flow over them. As such, simple squamous epithelia are seen lining body cavities and capillaries to reduce friction, as well as lining the alveoli of the lung to facilitate gas exchange.

Stratified squamous epithelia

A stratified squamous epithelium consists of squamous (flattened) epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity. Although this epithelium is referred to as squamous, many cells within the layers may not be flattened; this is due to the convention of naming epithelia according to the cell type at the surface.

This type of epithelium is well suited to areas in the body subject to constant abrasion, as the layers can be sequentially sloughed off and replaced before the basement membrane is exposed.

Stratified squamous epithelium is further classified by the presence or absence of keratin at the apical surface. Non-keratinised surfaces must be kept moist by bodily secretions to prevent them drying out and dying, whereas keratinised surfaces are kept hydrated and protected by keratin.

• Non-keratinizing stratified squamous epithelium (SSNKE): cornea (see also corneal epithelium), oral cavity, esophagus, rectum, vagina, and the internal portion of the lips
• Keratinizing stratified squamous epithelium (SSKE): skin, tongue (partially keratinised), and the external portion of the lips

See also

• Squamous cell carcinoma