Development of rodlet cells in the gut of turbot (Psetta maxima L.): relationship between their morphology and S100 protein immunoreactivity.

Rodlet cells are an enigmatic cell type described in tissues of both marine and freshwater teleosts. Although their structure is well established, up to date their function remains subject of debate. However, there is consensus among the majority of researchers that rodlet cells play an important role within immune system, and this function is probably related with the release of rodlets due to contractile capability of their fibrous layer. Regulation of the contraction mechanism would require proteins that modulate Ca(++) intracellular concentration to be expressed in rodlet cells. We performed a morphological and immunohistochemical study at light and electron microscopy levels to assess S100 protein immunoreactivity in developing rodlet cells. Immature stages did not exhibit immunoreactive signal; however, immunoreactivity was observed in the fibrous layer of both transitional and mature rodlet cells. The latter stage also showed immunosignal within the rodlets. These findings suggest a clear association between S100 protein expression and rodlet cell development that could be linked to the regulation of rodlet activity and contractile property of their fibrous layer. Furthermore, S100 protein antibody constitutes a novel marker for rodlet cells that could be used in future studies of this particular cell type.