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Updated: June 8, 2025
"But," as a very high authority declares, "the limitations in this change in type are strictly drawn, so that one type can assume only the characters of another which is closely related to it. This change of one form of closely related tissue into another is called metaplasia....
"When differentiation has advanced so that such distinct types of tissue have been formed as connective tissue, epithelium, muscle, nerve, these do not again merge through metaplasia. There is no evidence that mesoblastic tissues can be converted into those of the epiblastic or hypoblastic type, or vice versa."
Again, we have the following from one of the foremost pathologists, as to the strict and rather narrow limits of even pathologic change: "Epithelium and gland cells ... never become converted into bone or cartilage, or vice versa; while, again, it may be laid down that among epiblastic and hypoblastic tissues, on the one hand, and mesoblastic tissues on the other, there is no new development or metaplasia of the most highly specialized tissues from less specialized tissues; a simple epithelium cannot in the vertebrate give rise to more complex glandular tissue, or to nerve cells; in regeneration of epithelium there is no new formation of hair roots or cutaneous glands.
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