Zebrafish neuromast development: a target for endocrine disrupting chemicals?

In response to increasing evidence of human and environmental health impacts of endocrine disrupting chemicals (EDCs), screening and testing programs for EDC assessment are being developed, requiring characterization of potential adverse health effects. The development of the lateral line (LL), a neurosensory system in fish, has been suggested as a potential toxicological target of EDCs. The LL contains neuromasts with hair cells (HCs) which convert mechanical stimuli into neural signals. An exploratory study was performed to assess whether neuromast development is affected by a selection of model EDCs with different modes of action. Zebrafish embryos were exposed to EDCs, targeting estrogen and thyroid pathways, immediately after fertilization. The number of HCs in four neuromasts was counted at 120 h post fertilization. Methimazole and resorcinol (thyroid hormone [TH] synthesis inhibitors) elicited the strongest response, characterized by a reduction in HC numbers, while fulvestrant (anti-estrogen) slightly increased HC numbers. Further investigation confirmed a reduction of HCs and neuromasts after exposure to methimazole during late embryonic development, when TH synthesis is active. Gene transcript level analysis revealed a decreased marker for HC activity and increased markers of support cells, essential for HC regeneration. Taken together, neuromast development appears to be affected by certain EDCs in zebrafish embryos, at concentrations similar to those causing other effects (e.g., impaired swim bladder inflation). However, variability in the responses complicates characterization using the FM1-43 method. Further research, including rescue experiments and more sensitive or functionally relevant methods, is needed to clarify the mechanisms underlying EDC-induced HC disruption.