Joint Formation in the Caudal Fin Rays of Danio rerio (Zebrafish) During Fin Regeneration

Abstract

Danio rerio, more commonly called zebrafish, possess the ability to regenerate many of their organs and appendages including complete regeneration of the caudal fin back to its original size. The caudal fin consists of 16-18 fin rays (lepidotrichia) separated by interray tissue. These fin rays consist of bone segments that are partitioned by joints. Each bone segment consists of two oval shaped hemirays that form a tube-like structure that houses a multitude of tissues. During regeneration, the fin ray elongates through the distal addition of bone segments separated by newly formed joints. This is all regulated by a highly proliferative group of cells called the blastema located in the most distal portion of the regenerating fin ray. Our lab has been able to distinguish different stages of joint maturation during the regenerative process which has led to an interest in the effects of joint cell ablation during regeneration. A transgenic line of zebrafish that contained lnta11 regulatory elements was found to give expression of a reporter in the joints of adult zebrafish. We used this regulatory element to drive expression of nitroreductase (NTR) in joint cells. In the presence of NTR, metronidazole (MTZ) induces ablation through initiation of the caspase dependent apoptosis cascade. Previous joint cell photoblation results in our lab indicated that joints may not be able to regenerate, we wanted to determine the results of complete ablation of joint cells during regeneration using the NTR/MTZ inducible targeted ablation technique. Surprisingly treatment with MTZ did not completely eliminate the joints during regeneration. However, there was a reduction in segment length and overall fin length of fish treated with MTZ compared to controls. Furthermore, as the concentration of MTZ is increased there is a proportional reduction in segment length of the fin regenerate. Although we failed to eliminate joints, the phenotype observed provided support for our proposed model of joint formation during regeneration. The creation of transgenic fish lines are underway to allow the separate ectopic expression of two transcription factors; sp7 (a known marker of osteoblast commitment) and hoxa13a (a hypothesized joint commitment factor) during regeneration.