图示：嗜烟者喉部呼吸道上皮化生。长期的慢性刺激使一种上皮（右侧的是正常的呼吸道粘膜上皮）转变为另一种上皮（左侧的是鳞状上皮）。化生不是生理过程，而可能是肿瘤形成的先兆。
Metaplasia of laryngeal respiratory epithelium has occurred here in a smoker. The chronic irritation has led to an exchanging of one type of epithelium (the normal respiratory epithelium at the right) for another (the more resilient squamous epithelium at the left). Metaplasia is not a normal physiologic process and may be the first step toward neoplasia.