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(1) A permanent transformation of one variety of tissue into another that differs from it morphologically and functionally but preserves its basic species affiliation.
In animals and man, metaplasia is seen only in epithelial and connective tissues, such as in the transformation of cylindrical mucosal epithelium (respiratory, digestive, uterine) into flat, multilayered, cornifying epithelium similar to epidermis, or in the transformation of fibrous connective tissue into fatty, cartilaginous, or bony tissue. Other examples include the ossification of connective-tissue cicatricial adhesions or of the capsules around caseous tubercular foci.
A distinction is made between direct metaplasia, in which one tissue is transformed into another by means of a change in its structural elements, such as the transformation of fibrocytes into osteocytes, and indirect metaplasia, in which the development of new tissue is accomplished by the proliferation of undifferentiated cells, which only later differentiate. Indirect metaplasia most often occurs with regeneration.
The causes of metaplasia are changes in the surrounding medium and in the state of the body tissues (prolonged inflammatory processes, infectious diseases, avitaminosis A, diseases of the hematopoietic organs, hormonal shifts). Metaplasia disrupts normal tissue function and makes possible a subsequent transformation into a rudimentary tumor (anaplasia). The range of phenomena embraced by the concept of metaplasia is strictly defined by some histologists, who include only changes in differentiation on the cellular level (transformation of iris cells into lens; conversion of cells of pigmented retinal epithelium into neural retina, accompanying regeneration of the eye in adult newts).
REFERENCESEliseev, V. G. SoediniteVnaia tkan’ Gistofiziologicheskie ocherki. Moscow, 1961.
Metaplaziia tkanei. Moscow, 1970. (Collection of articles.)
Strukov, A. I. Patologicheskaia anatomiia, 2nd ed. Moscow, 1971.
(2) Metaplasia, or more commonly in English, metaplasis, is the period of the developmental prime in individual organisms (sexual maturity) or in the history of a group of organisms (expressed in intensive variability and abundance of individuals).