Pathways of cytodifferentiation during the metamorphosis of the epibranchial cartilage in the salamander Eurycea bislineata

The metamorphosis of the epibranchial—a skeletal element of the hyobranchial apparatus—is characterized by the degeneration of the larval cartilaginous element coupled with the formation of an adult cartilage in the same position. By integrating histological evidence from serial sections, in toto clearing and staining, TEM, SEM, and ³H]thymidine autoradiography, we found that larval chondrocytes die and do not participate in the formation of the adult cartilage. Meanwhile, the adult element forms from a small group of cells located in the mid-ventral region of the larval perichondrium. At the onset of metamorphosis, this group of perichondral cells begins to proliferate and they assume a mesenchymal morphology to subsequently undergo chondrogenesis. Although we only document a specific case, we believe that this process of apportionment of prospective larval and adult cell development is characteristic of highly specialized metamorphic systems in all groups. In general, differentiation is an irreversible process and larval cells once part of a differentiated tissue cannot dedifferentiate and participate in the formation of a new adult structure. To circumvent this constraint during the evolution of divergent larval and adult morphologies, organisms have evolved compartmentalized metamorphic systems.