Rodlet cells in teleosts: a new insight into their nature and functions

The nature of rodlet cells (RCs) and their functions is subject to a number of different interpretations. This review provides a detailed analysis of the parasitic and endogenous origin of these cells. Two new functional aspects of RCs are considered in detail. The possible function of RCs as immune cells was derived from studies that reported an increase in the number of RCs in fish infected with protozoan and metazoan parasites, particularly at the site of the pathogen infection and/or attachment. Accordingly, RCs represent inflammatory cells, with a similar role to eosinophile granule cells, epithelioid cells and mesothelial cells. Rodlet cells may potentially act as biomarkers. Experimental studies that examined the response of RCs in fish exposed to chemical substances such as metals and herbicides reported an increase in the number of RCs in the tissues of the fish. Fish exposed to these substances expressed myelinic figures in the cytoplasm of the RCs and various degrees of rodlet degeneration and high vacuolization of RC cytoplasm were often noticed. Further lines of research are suggested that might elucidate the true function of these enigmatic cells.