Retinoic acid-induced cell death in the wound epidermis of regenerating zebrafish fins.

Previous work has shown that treatment with retinoic acid (RA) can induce malformations in regenerating pectoral and caudal fins. RA-treated regenerates are narrower than unamputated and regenerated control fins because of a decrease in the distance between rays, and either partial or total fusion of some of them. In order to tackle the issue of how RA induces its teratogenic effects on regenerating fins, and which cell types may be specifically affected by RA, we have examined the cellular changes occurring in early regenerates following treatment with retinoids. The work presented here shows for the first time that RA induces significant apoptosis in the wound epidermis, but not in the mesenchyme, of a regenerating appendage, besides inhibiting blastema development as reported in other species. We also show that RA does not retard regeneration by inhibiting accumulation of blastemal cells, but probably by impairing their ability to migrate distal to the amputation plane. This effect is rapidly reversed by discontinuing the treatment, and within 24 hr of removing the drug, blastema development is well advanced. By this time the teratogenic effects induced by RA are already apparent. A correlation between the length of the apical ectodermal ridge (AER) and the number of digits formed has been demonstrated in developing limb buds. We therefore suggest that RA-induced patterning abnormalities in regenerating fins are the consequence of a reduction in the size of the wound epidermis, due to increased cell death, which would affect patterning of the underlying mesenchyme.