Epithelial tissue forms continuous sheets of cells that cover all external and internal body surfaces, lining cavities and hollow organs. This tissue acts as a selective barrier, regulating the passage of substances and providing physical protection. Squamous cells are one of the three primary shapes used to classify epithelial cells, alongside cuboidal and columnar types. They are important in forming both highly permeable linings and robust protective coverings, depending on their structural organization.
The Unique Shape of Squamous Cells
The term “squamous” comes from the Latin word squāma, meaning “scale,” which describes their physical appearance. Squamous cells are distinctively thin, flat, and sheet-like, often appearing polygonal when viewed from the top. This flattened structure means their nucleus is also horizontally compressed, typically appearing as a thin oval shape occupying the widest part of the cell.
These cells are densely packed together, forming a continuous layer with very little space between them. The thinness of the squamous cell minimizes the distance that any substance must travel to pass through the cell. This morphology creates a smooth, low-friction surface, which is beneficial in areas where fluids need to move easily across the tissue.
How Squamous Cells Are Organized in the Body
Squamous epithelial cells are organized into two configurations based on the number of layers they form. Simple squamous epithelium consists of a single layer of flattened cells resting on a basement membrane. This single-layer type is found lining the internal surfaces of blood vessels (endothelium), in the air sacs of the lungs (alveoli), and lining body cavities (mesothelium).
The second configuration is stratified squamous epithelium, composed of multiple layers of cells stacked upon one another. Only the deepest layer, resting on the basement membrane, may contain cells that are cuboidal or columnar in shape. The cells progressively flatten out as they move toward the surface, exhibiting the characteristic squamous appearance in the outermost layers. This stratified tissue lines surfaces subjected to mechanical stress and abrasion, such as the skin, mouth, esophagus, and vagina.
Essential Roles and Functions
The function of squamous cells is directly related to their organization, optimizing the tissue for either transport or protection. Simple squamous epithelium, with its minimal thickness, is specialized for rapid diffusion and filtration. In the lungs’ alveoli, this thin barrier allows for efficient gas exchange, letting oxygen quickly enter the bloodstream and carbon dioxide exit. In capillaries, this structure facilitates the swift exchange of nutrients, waste, and fluids between the blood and surrounding tissues.
In contrast, stratified squamous epithelium functions primarily as a barrier against external damage. The multiple layers provide protection against mechanical trauma, abrasion, and the invasion of microorganisms. The tissue maintains its integrity because the outermost layers can be shed without compromising the layer beneath. This organization also helps prevent water loss, particularly in the keratinized form found on the surface of the skin.
What Happens When Squamous Cells Become Abnormal
Abnormal changes in squamous cells are medically significant and often detected through common screening procedures. One type of change is metaplasia, a noncancerous process where one mature cell type is replaced by another. This occurs when columnar cells in the respiratory tract or cervix change into squamous cells in response to chronic irritation. Squamous metaplasia is often considered a normal change in the cervix, but it is also where most cervical cancers begin.
A more serious change is dysplasia, which refers to the abnormal growth and maturation of cells, often considered a pre-cancerous condition. Dysplastic squamous cells show changes in their appearance and organization, which can be graded based on severity. If left untreated, severe dysplasia can progress to squamous cell carcinoma, cancer originating in the squamous cells. The Papanicolaou (Pap) smear is a routine test designed to examine squamous cells from the cervix to detect these dysplastic changes before they become malignant.