squamous

Squamous refers to a type of epithelial cell that is flat, thin, and scale-like in appearance. In histology, squamous cells are typically broad and flattened, with the nucleus often flattened as the cells mature. The term also describes the epithelium formed by these cells, known as squamous epithelium, which serves as a protective barrier and, in some locations, a surface for diffusion or filtration.

There are two main organizational patterns of squamous epithelium. Simple squamous epithelium consists of a single layer of flat cells and lines surfaces where rapid diffusion or exchange is needed, such as the alveoli of the lungs, the lining of blood vessels (endothelium), and body cavities (mesothelium). Stratified squamous epithelium comprises multiple cell layers and provides robust protection against abrasion. It forms the outer layer of the skin (keratinized stratified squamous epithelium) and lines areas subject to abrasion such as the oral cavity, esophagus, and vagina. Some stratified squamous epithelia are non-keratinized, retaining living cells at the surface in moist environments.

Keratinization is a process in which outermost cells accumulate keratin, become flattened, die, and form a tough, protective, water-resistant layer typical of the skin. In non-keratinized stratified squamous epithelium, the surface remains moist and the cells stay alive.

Clinical relevance includes squamous cell carcinoma, a malignant tumor arising from squamous epithelium. It is commonly associated with sun exposure in the skin and with tobacco use or HPV infection in mucosal sites such as the head and neck, lungs, and esophagus. Diagnosis relies on biopsy and histopathology; treatment varies with stage and location.