Generating, growing and transforming skeletal shape: insights from amphibian pharyngeal arch cartilages.

Amphibians that undergo a metamorphosis provide an unparalleled opportunity to investigate how skeletal shape is generated, preserved, and transformed in development. Their pharyngeal arch (PA) cartilages, which support breathing and feeding behaviors, form embryonically from cranial neural crest cells, grow isometrically at larval stages, and abruptly change shape during metamorphosis. Further, the shape changes occur in three different ways: some adult cartilages form de novo, others emerge from within resorbing larval cartilages and some larval cartilages reshape themselves at the cellular level. Isometric growth followed by abrupt shape change is unique to amphibian PA cartilages, which suggests that the origin and evolution of amphibian metamorphosis has been influenced by the tissue properties of cartilage. This essay reviews the functional role of the PA skeleton in frogs and salamanders and presents a mechanistic framework for understanding how its shape is generated, preserved, and transformed at the levels of cell behavior and specification mechanisms.