Sputum analysis is a common procedure used to diagnose various respiratory conditions, from infections to malignancies. Sputum is the material expelled from the lower respiratory tract—the lungs, bronchi, and trachea—by coughing, which clears the airways of mucus, debris, and cells. Examining the cellular content of this material, known as sputum cytology, provides a direct look at the environment within the lungs. Squamous epithelial cells (SECs) are frequently observed in sputum samples, and their presence acts primarily as an indicator of sample quality or, rarely, points toward a serious disease process.
Structure and Normal Origin of Squamous Epithelial Cells
Squamous epithelial cells are specialized cells characterized by their distinctive flattened, scale-like shape, derived from the Latin word squama. Under a microscope, these cells appear thin, broad, and polygonal, featuring a small, typically oval nucleus surrounded by abundant cytoplasm. These cells form stratified squamous epithelium, a multi-layered protective tissue designed to withstand abrasion. This robust lining is found in the skin, vagina, oral cavity, and pharynx. Since these cells are constantly shed, sputum often picks them up as it passes over the mouth and throat lining during collection.
Interpreting Sputum Sample Quality
The primary role of squamous epithelial cells in sputum analysis is as an indicator of sample quality. A sputum sample must reflect the cellular environment of the lower respiratory tract, where the disease is located. If the sample is heavily contaminated with saliva from the mouth, the results may only reflect the normal flora of the upper airway, potentially leading to an inaccurate diagnosis.
Laboratories use the number of SECs observed to determine if the sample represents the lower lungs or is merely contaminated saliva. Standard criteria for an acceptable, high-quality sputum sample require a low number of SECs, typically fewer than 10 or 25 per low-power field. Conversely, a high count of SECs strongly suggests the sample is predominantly saliva or oropharyngeal secretions and is likely not useful for diagnosis.
To ensure the specimen is truly from the lower respiratory tract, the number of SECs is often compared to the number of inflammatory cells, specifically polymorphonuclear leukocytes (PMNs). An ideal sample has a high number of PMNs, indicating an active inflammatory process deeper in the lungs, and a low number of SECs. Specimens that fail this quality screen are often rejected, and a repeat collection is requested to obtain a more diagnostic sample.
When Squamous Cells Indicate Disease
While SECs typically signal contamination, their appearance can occasionally represent a serious pathology originating within the lung. The lower respiratory tract is normally lined by pseudostratified columnar epithelium, distinct from the flat squamous cells of the mouth. Under chronic irritation, such as from tobacco smoke or inflammation, the columnar cells can transform into more resilient squamous cells.
This change is called squamous metaplasia, a protective response where a delicate lining is replaced by tougher squamous epithelium. Squamous metaplasia is considered a precancerous condition, as it can progress to dysplasia and malignant transformation. The detection of metaplastic squamous cells with cellular atypia—an abnormal appearance—in the sputum is a significant finding that requires further investigation.
The most serious pathology involving squamous cells is Squamous Cell Carcinoma (SCC) of the lung. SCC usually originates in the larger, central bronchi, allowing malignant cells to easily shed into airway secretions. Sputum cytology is a non-invasive tool that detects these malignant cells, which exhibit features like enlarged, irregular nuclei and a high nuclear-to-cytoplasm ratio. The cytopathology review is essential to differentiate benign oral contamination from potentially life-threatening malignant cells originating from the lung tissue.