Studies of hyperthyroidism IV. The behavior of the epidermal mitochondria and the pigment in frog tadpoles under conditions of thyroid‐accelerated metamorphosis and of regeneration following wound infliction

In the epidermal cells of green‐frog tadpoles (Rana clamitans) there are present coarse, conspicuous mitochondrial threads. In the dorsal body regions, epidermal cells of the middle layers also contain pigment granules, grouped in crescentic masses in the distal portions of the cells. Administration of thyroid extract results in permanent disappearance of the mitochondrial threads and disappearance to a large extent of the epidermal pigment granules. Processes of cellular dedifferentiation and proliferation occur rapidly over widespread areas, the mitochondria undergoing intracellular resorption. A new type of epidermis is developed containing many cutaneous glands, adapted for the approaching terrestrial life. Wound infliction induces similar processes of cellular dedifferentiation and proliferation in the epidermis in the immediate vicinity of the wound. Cells in this region lose their mitochondrial threads by intracellular resorption, and there is also some disappearance of pigment granules. The new epidermal cells in the early stages of regeneration produce neither mitochondrial threads nor pigment. This condition is not permanent, however, and in the later stages the larval characteristics appear again, both mitochondrial threads and pigment being redifferentiated. In the hyperthyroid animals there is also a significant mobilization of mesenchymal chromatophores, correlated to some extent with the loss of epidermal pigment. The significance of epidermal changes is discussed with reference to cutaneous abnormalities associated with hyperthyroid and hypothyroid conditions.