Calretinin and S-100 expression in goldfish, Carassius auratus (L.), schwannoma.

Schwannoma (neurilemmoma) and neurofibroma are neoplastic diseases belonging to the peripheral nerve sheath tumours (PNST) in humans and in higher vertebrates. It can be very difficult to distinguish these two tumours and in particular to differentiate both of them from fibromas. In this regard, to distinguish schwannoma from neurofibroma remains a diagnostic problem, and some authors prefer to consider them as a single morphological entity. Twelve adult goldfish (7–10 cm in length), with multiple external nerve sheath tumours, were sent to the laboratory for examination. The specimens were routinely processed for histopathology, histochemistry and immunohistochemistry. Sections were incubated overnight with rabbit polyclonal antibody against S-100 and calretinin. Immunohistochemistry showed a strong positive reaction in neoplastic cells when tested with S-100 (data not shown) and calretinin, with the same pattern in all fish. Calretinin immunostaining was homogeneously spread in neoplastic areas, whereas no reaction was detected in the dermis. At high magnification the immunostaining was evident in the cytoplasm of all tumour cells. Histological features of the nerve sheath neoplasm in the present case, such as the presence of Antoni type A and B tissues (characteristic of the human schwannoma) as well as the presence of Verocay bodies, strongly support a Schwann cell origin of this tumour. This diagnosis is also supported by positive staining of tumour cells with calretinin, a specific marker used to distinguish schwannoma from neurofibroma in PNST in humans and here reported for the first time in fish.