cells or groups of cells, forming an intraepithelial keratin pearl. There may be an increase in the number of basal cells, and they may lose their cellular orientation (losing their polarity and long axis). Alteration of the normal epithelial-connective tissue architecture - the rete pegs may become "drop shaped". wider at their base than more superficially. Generally, dysplasia is subjectively graded by pathologists into mild, moderate or severe dysplasia. This requires experience as it is a difficult skill to learn. It has been shown that there is high degree of inter-observer variation and poor reproducibility in how dysplasia is graded. Severe dysplasia is synonymous with the term carcinoma in situ, denoting the presence of neoplastic cells which have not yet penetrated the basement membrane and invaded other tissues. Differential diagnosis There are many known conditions that present with a white lesion of the oral mucosa, but the majority of oral white patches have no known cause. These are termed leukoplakia once other likely possibilities have been ruled out. There are also few recognized subtypes of leukoplakia, described according to the clinical appearance of the lesion. Almost all oral white patches are usually the result of keratosis. For this reason, oral white patches are sometimes generally described as keratoses, although a minority of oral white lesions are not related to hyperkeratosis, e.g. epithelial necrosis and ulceration caused by a chemical burn (see: Oral ulceration#Chemical injury). In keratosis, the thickened keratin layer absorbs water from saliva in the mouth and appears white in comparison with normal mucosa. Normal oral mucosa is a red-pink color due to the underlying vasculature in the lamina propria showing through the thin layer of epithelium. Melanin produced in the oral mucosa also influences the color, with a darker appearance being created by higher levels of melanin in the tissues (associated with racial/physiologic pigmentation, or with disorders causing melanin overproduction such as Addison's disease). Other endogenous pigments can be overproduced to influence the color, e.g. bilirubin in hyperbilirubinemia or hemosiderin in hemochromatosis, or exogenous pigments such as heavy metals can be introduced into the mucosa, e.g. in an amalgam tattoo. Almost all white patches are benign, i.e. non-malignant. The differential diagnosis of a white lesion in the mouth can be considered according to a surgical sieve (see table). Leukoplakia cannot be rubbed off the mucosa, distinguishing it readily from white patches such as pseudomembraneous candidiasis, where a white