Describe the histological distinction between severe dysplasia and carcinoma in situ in the context of oral epithelial changes.

The histological distinction between severe dysplasia and carcinoma in situ in oral epithelial changes primarily lies in the extent of epithelial involvement by atypical cells, while both share common features of cellular abnormality. Both conditions represent non-invasive epithelial lesions, meaning the abnormal cells are confined to the epithelium and have not breached the basement membrane, which is the underlying boundary separating the epithelium from the connective tissue. This intact basement membrane is a defining feature distinguishing them from invasive carcinoma. Severe dysplasia and carcinoma in situ both exhibit a range of cellular changes, collectively termed atypia or dysplastic features. These include nuclear pleomorphism, which is variation in the size and shape of cell nuclei; hyperchromatism, meaning nuclei stain darker than normal due to increased DNA content; an increased nuclear-to-cytoplasmic ratio, where the nucleus occupies a larger proportion of the cell's volume; prominent nucleoli, which are small structures within the nucleus that become more visible; increased and often abnormal mitotic activity, indicating more cells are dividing and sometimes in unusual ways or locations; and a loss of cellular polarity, meaning the cells lose their normal orderly arrangement. The key differentiating factor is the vertical extent of these atypical changes within the epithelial layer. In severe dysplasia, these architectural and cytological (cellular) abnormalities involve the lower two-thirds or more of the epithelial thickness. However, a crucial aspect of severe dysplasia is that the superficial, or top, layer of the epithelium still retains some degree of maturation, even if it is minimal. This means that while the lower and middle layers are severely abnormal, the very surface cells still show some differentiation towards normal epithelial cells. In contrast, carcinoma in situ (CIS) is characterized by the presence of these same severe atypical changes involving the full thickness of the epithelium, from the basal layer (the deepest layer, resting on the basement membrane) all the way to the epithelial surface. There is a complete loss of epithelial maturation throughout all layers, meaning no normal or maturing cells are seen at the surface. Every cell from bottom to top displays the features of severe atypia, indicating a complete disorganization and loss of normal epithelial architecture.