Structural and endocytotic differences of fibroblasts and macrophages in the tail fin of amphibian larvae during metamorphosis.

Different features in the fibroblasts and the macrophages, which are prominent cell types in the dermis of the dorsal tail fin of the larval axolotl, Ambystoma mexicanum, and the tadpole, Rana japonica, were examined by light and electron microscopy. At the non-metamorphic stages, the cytoplasm of the macrophage, loaded with numerous lysosomes, is generally located in the cell periphery. Outstanding was the presence of many ruffles or microvillous projections of different shapes and sizes in the plasma membrane. In contrast, the fibroblasts are spindle-shaped and possess less numerous microvillous projections compared with the macrophages, and extracellular spaces neighboring the fibroblasts are loaded with collagen fibers. The fibroblasts are located superficially and sometimes contact each other by the desmosomes. The macrophages are situated relatively deep in the dermis. At the metamorphic stages, both fibroblasts and macrophages contain many phagolysosomes in common, but the desmosomes still remain between the fibroblasts, and therefore the fibroblasts are distinguished from the macrophages. The macrophages are characterized by the their phagolysosomes which contain degenerating cells or cell debris of fibroblasts, myelinated nerve fibers and neutrophils. The macrophages also exhibit a higher endocytotic activity than the fibroblasts for the injected foreign particles (FITC-dextran or latex beads). On the other hand, the phagolysosomes (or autophagolysosomes) of the fibroblasts are characterized by intracellular collagen fibers. These different phagolysosomes between the macrophages and the fibroblasts mean that they may play different roles in the degeneration of the cellular or the extracellular components, respectively, of the tail fins during metamorphosis.