Autolysis and heterolysis of the epidermal cells in anuran tadpole tail regression

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Abstract
Autolysis and heterolysis of the degenerating epidermis of the tail fin of Rana japonica tadpoles during spontaneous metamorphosis were observed by transmission and scanning electron microscopy. In the early climactic stages of metamorphosis (st. 19–20), the outermost epidermal cells developed vacuoles that were acid phosphatase positive and showed apparent breakdown of the cell membrane. The cells shrunk, perhaps due to the rupture of the cell membrane, and sloughed off without typical cornification. As tail resorption proceeded, autolysis of the epidermal cells spread towards the inner layers, in which some epidermal cells lost desmosomal junctions. They also displayed atrophic figures with condensed cytoplasm, breakdown of the cell membrane, and pycnotic nuclei. Lymphocytes, neutrophils and macrophages were already present in the basal layers of the premetamorphic epidermis (st. 10). Based on ultrastructural observation, blood cells could be distinguished from autolysing epidermal cells. Only a few blood cells were found in the early climactic stages of metamorphosis (st. 19–20), but the number of the blood cells, especially macrophages, greatly increased during the final stages of metamorphosis (st. 23–24). During the final stages, many macrophages were observed to phagocytose the autolysing epidermal cells by projecting slender pseudopodia into the inner epidermis. Macrophages also were observed to pass through the degraded basal lamella. These results suggest that not only autophagy but also heterophagy of the epidermal cells by the macrophages is a major process in the regression of the tail fin epidermis.