Rodlet cells (RCs), and other inflammatory cells, namely eosinophile granule cells (EGCs), eosinophilic granulocytes and epithelioid cells in the liver, pancreas and peritoneal serosa of uninfected and naturally parasitized minnow, Phoxinus phoxinus (Linnaeus, 1758) collected by electrofishing from a stream in province of Padua (North Italy), were studied by light and electron microscopy. Forty-eight minnows were examined and in 18 fishes encysted larvae of nematode Raphidascaris acus (Bloch, 1779) were encountered, mainly in the pancreas. The number of larvae in the latter organ ranged from 2 to 46. Nematode larvae were encapsulated by epithelioid granulomata and these cells displayed typical epithelial characteristics such as desmosomes and tonofilaments. EGCs and RCs characteristically surrounded the reactive foci and were suggestive of an integrated inflammatory network involving both cell types. In many instances RCs were noticed at the periphery of the pancreas, beneath the peritoneal serosa, partially or entirely surrounded by mesothelial cells. In the latter situation partially damaged RCs were presented in the splancnic cavity entirely surrounded, but not truly phagocytized (no phagolysosome occurred) by mesothelial cells, which shared the same ultrastructural features of epithelioid cells. This phenomenon has never been previously described and may represent a peculiar RCs' turn-over in the pancreas likely is related to the high sensitivity to damages. A significant difference (p<0.01) in the number of rodlet cells (RCs) between uninfected and parasitized fish was noticed in the liver and pancreas. The data suggest that rodlet cells represent an inflammatory cell type closely linked to other piscine inflammatory cells, such as EGCs, epithelioid cells as well as mesothelial cells
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