Squamous epithelium is the scientific name for a type of cell found in the human body. These cells make up the outer layer of skin, as well as lining the internal organs. They are called "squamous," or "scaly," for their distinctive thin, flat shape.
There are many different kinds of epithelial cells, but they all share various purposes in common: protection, absorption, secretion, filtration, and sensory reception. Squamous cells differentiate themselves from other kinds of barrier cells in that they are flat. With their irregular edges and off-center nucleus, they often resemble a fried egg. They are also known as "pavement cells" since they fit together like irregular pavement stones seen from above.
Squamous epithelium is further divided by thickness into two subcategories. Simple tissue is only one cell thick, and serves as a lining and porous barrier. Stratified tissue has multiple layers, to better serve as a protective covering.
The cell barrier formed by simple squamous epithelium is smooth and flexible. This makes it an ideal lining for the heart muscles, arteries, the abdominal cavity, and the thoracic cavity or chest. These parts of the body undergo constant rhythmic movements and contractions, and so it is important for the tissues making them up to be smooth and as frictionless as possible.
Simple tissue is also a diffusive cell barrier, allowing gasses and fluids to pass through gradually. These cells line the polyps inside the lungs where carbon dioxide is replaced with oxygen, and the insides of capillaries where oxygenated blood is diffused. The mouth, esophagus, anus, and vagina are also lined with it. The cells' porous nature allows for various fluids to seep through to aid in digestion and lubrication.
Stratified squamous epithelium, with its multiple layers of cells, forms the external layer of the skin. Where the simple epithelium cuts down on friction between body cavities and organs, the stratified type is designed to protect the outer tissues from scrapes and abrasions. The outermost layer is made up of a layer of dead protein cells called keratin, which is waterproof and stops moisture loss.
Despite its role as a protector, cancer can develop when stratified epithelium is exposed to high levels of UV rays. Basal cell carcinoma, which occurs in the lowest layer of skin, can appear as a light-colored sore. Squamous cell carcinoma, which happens in the middle layer of skin, can form a scaly bump. These skin disorders are among the most common types of cancer in the US.