Light micrograph showing squamous cell carcinoma (cancer) of the human bronchus, one type of lung cancer. Squamous cell cancer, the most common primary cancer that affects the lung, usually arises in the main bronchi (the major airways serving the lungs). The tumors tend to invade the local lung tissue as well as causing obstructions in the airways, and spread (metastasize) via the lymphatic system. Squamous cell cancers vary in their degrees of differentiation - their histological resemblance to original cell type. Here, poorly-differentiated cancer cells (stained blue and purple) show no evidence of their squamous cell origins at this magnification. Figure 11.12b, "Basic Histopathology".