呼吸道正常上皮及鳞状上皮化生

肿瘤形成首先是细胞发生变化，图示是右侧正常喉部呼吸上皮向鳞状上皮化生，左侧化生是由于慢性抽烟损伤。 这两类细胞变化潜在可逆，也可能向肿瘤发展。
化生：正常上皮转变成另一类上皮细胞。当刺激撤消后，化生可逆
非典型增生：上皮的异常增生和分化，如果刺激因子去除后，仍然可逆。

The first step toward neoplasia（肿瘤形成） is cellular transformation（变化、转化、转换）. Here, there is metaplasia （化生）of normal respiratory（呼吸的） laryngeal（喉的） epithelium on the right to squamous（鳞状的） epithelium on the left in response to chronic irritation of smoking. The two forms of cellular transformation that are potentially reversible, but may be steps toward a neoplasm, are:

• Metaplasia(化生): the exchange of normal epithelium for another type of epithelium. Metaplasia is reversible when the stimulus for it is taken away.
• Dysplasia（非典型增生）: a disordered growth and maturation of an epithelium, which is still reversible if the factors driving it are eliminated.