The fate of larval chondrocytes during the metamorphosis of the epibranchial in the salamander, Eurycea bislineata

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Abstract
The metamorphosis of the epibranchial cartilage, a skeletal component of the hyobranchial apparatus, in the salamander Eurycea bislineata entails a combination of the reabsorption of a larval cartilaginous element with the simultaneous genesis of an adult cartilage in the same place. In this study we focus on the fate of the larval chondrocytes. Two hypotheses are considered: one, larval cells simply die off during metamorphosis, or, alternatively, they dedifferentiate and participate in the formation of the adult element. Thyroxine treatment and experimental tissue manipulation coupled with measurements of thyroxine levels using radioimmunoassay show that, within 24 h after T4 treatment, larval chondrocytes in the epibranchials exhibit large autophagocytic vacuoles, disruption of the rough endoplasmic reticulum, abnormally shaped mitochondria, abundance of lysosomes and nuclear degeneration, all symptoms of the onset of cell death. In conclusion, evidence from light microscopy, TEM and SEM show that the larval chondrocytes in response to rising levels of thyroid hormones undergo a process of lysosomal autophagocytosis and do not participate in the formation of adult structures.