A prolonged cartilaginous phase in newt forelimb skeletal regeneration

The morphologic events leading to the regrowth of the amphibian limb have long been known, yet it is poorly appreciated that even when the limb appears outwardly restored, its skeletal parts may be formed entirely of cartilage. To determine when forelimb skeletal ossification begins and is completed in the spotted newt Notophthalmus viridescens, limbs from animals that had sustained midhumeral amputations were examined at intervals of up to 270 days. At this time, most regenerated skeletal parts were undergoing ossification, but those of the wrist remained entirely cartilaginous. Calcium content of regenerated bones was deficient, and X‐ray diffraction analysis disclosed the presence of apatite crystals with minimally discernible orientation.