Morphologic studies of connective tissue resorption in the tail fin of metamorphosing bullfrog tadpole

Abstract

The sequential morphologic changes in bullfrog tadpole tail fin during thyroxine-induced metamorphosis were observed by phase contrast and electron microscopy. The earliest change was fraying and separation of the collagenous layers and fibrils of the basement lamella, soon followed by invasion of the lamella from below by mesenchymal cells. The cytoplasm of these cells filled with bundles of collagen fibrils suggesting phagocytosis of the fibrils. The lamella swelled tenfold from a compact 6–7 μ layer to a diffuse fragmented region filled with large mesenchymal cells. The extracellular collagen fibrils showed great changes in diameter and density, the amount of filamentous material increasing as the fibrils diminished in number. Changes in epithelial cells and adepidermal membrane are also described. The morphologic sequences are correlated with knowledge concerning degradative enzyme systems found in tail fin, resulting in a speculation on the mechanism of connective tissue resorption during metamorphosis.