metaplasia, in zoology, the conversion of one type of living cell or group of cells into another as a means of regeneration. For example, the damaged or removed lens of a salamander eye is replaced through the transformation of nearby pigmented iris cells into lens cells. The regeneration of brain tissue from epidermis in annelid worms is another well-documented example of metaplasia. The term metaplasia also refers to the abnormal replacement of cells of one type by those of another type.