Genesis and regression of the figures of Eberth and occurrence of cytokeratin aggregates in the epidermis of anuran larvae.

In tadpoles of Rana temporaria, Bufo bufo and Xenopus laevis the development of the massive skeins of tonofilaments (cytokeratin intermediate filaments) that form the figures of Eberth follows a similar sequence in all species studied. Increase in the number of filaments is preceded by increase in polyribosomes and rER in the basal layer cells of the epidermis. A filamentous zone develops proximally in the cells concurrently with hemidesmosomes, which assume the typical larval bobbin form as the skeins occupy more of the cytoplasm. The figures of Eberth are at maximum development throughout mid-larval life. The first signs of regression appear before the climax stages, when aggregates of cytokeratin material are found among the filaments of the skeins. Such cytokeratin aggregates have previously been reported only from mitotic mammalian cultured or tumour cells, or in experimental situations. At metamorphic climax they are numerous in the basal layer cells and in some other cells of the epidermis, even when not dividing. This condition persists into the postmetamorphic stage when the figures of Eberth and the bobbin-type hemidesmosomes have gone. The formation of cytokeratin aggregates can be enhanced by chilling the animals before fixation, but does not depend on a drop in temperature, and takes place even if fixation is carried out at room temperature.