IDENTIFICATION OF SOURCES OF JOINT CELLS AND OSTEOBLASTS DURING FIN REGENERATION IN MEDAKA

Abstract

Teleost fish can regenerate their fins within two weeks of an amputation. Identification of the cellular sources of different cell types in the regenerating fin is critical for understanding mechanisms underlying fin regeneration. It has been shown by others that upon amputation, osterix expressing differentiated osteoblasts dedifferentiate and contribute to the pool of new osteoblasts in the regenerating fin. However, ablation of these osteoblasts does not affect the rate and extent of fin regeneration. This suggests that an alternative source of osteoblasts must exist. Our lab has previously shown that in the axial skeleton, col10a1 expressing osteoblast progenitors are precursors of osterix expressing osteoblasts and could possibly represent a novel source for osteoblasts in the regenerating fin. Expression analysis in osterix and col10a1 transgenic reporter lines showed that osterix is expressed in differentiated osteoblasts along the bony segments while col10a1 expressing cells were present in segments as well as intersegmental regions. To determine the effect of cell ablation of col10a1 cells and double ablation of both osterix and col10a1 positive osteoblasts on fin regeneration, I generated medaka transgenic lines expressing Nitroreductase under osterix and col10a1 promoters. As reported in zebrafish by another lab, ablation of osterix cells did not affect fin regeneration. However, simultaneous ablation of both, osterix and col10a1 cells, resulted in a slight reduction of fin outgrowth without affecting the recruitment of osteoblasts in the regenerating fin. Next, to determine the role of col10a1-positive/osterix-negative cells, I followed genetically labelled col10a1 cells after osterix ablation. The labelled col10a1 cells contributed to the joint cells in the intersegmental region. Thus, I conclude that the col10a1-positive/osterix-negative cells do not serve as an exclusive secondary source of osteoblasts but instead represent a source of joint cells in the regenerating fin. To test if osteoblasts in the regenerating fin could arise by transdifferentiation of cells from a non-osteoblast lineage or osteoblast progenitors, I labelled different cell types in the developing medaka fin. These different cell types can be lineage traced to determine their role as a secondary source of osteoblasts by transdifferentiation induced after ablation of osterix expressing osteoblasts.