Repair of fractured lower jaws in the spotted salamander: Do amphibians form secondary cartilage?

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Abstract
Secondary cartilage forms on avian and mammalian dermal bones, both during normal ontogeny and during repair of fractures, but it has not been observed in any other vertebrate class. We fractured the left lower jaws of adult spotted salamanders, Ambystoma maculatum, to see whether secondary cartilage would form during fracture repair. It did not. Instead, periosteal hyperplasia produced a callus from which new dermal bone formed to bridge the fracture. Meckel's cartilage underwent superficial dissolution but showed a minimal chondrogenic response. A large callus cartilage did form, but it appeared to arise by metaplasia from connective tissue adjacent to the bone. Thus, the environment within the fracture is conducive to chondrogenesis but the periostea of the dermal bones either are not able to respond to that environment or are unable to synthesize cartilage‐specific products. Among recent vertebrates, the ability to form secondary cartilage is limited to birds and mammals and is not a primitive property of the periostea of dermal bones shared by “lower” vertebrate classes.