During 4NQO carcinogenesis basal lamina between epithelia and connective tissue is progressively destroyed (collagen IV immunohistochemistry): Histological sections of control, healthy tongue (a), mild (b), moderate (c), and severe (d) dysplasia as well as OSCC (e) were immunostained for collagen IV (arrows). Control and mild dysplasia samples show a strong immunoreactivity for collagen IV, which is weaker in moderate and severe dysplasia and patchy or absent in OSCC, particularly in areas of connective tissue invasion (arrowhead). Bar scale: 10 μm.