Metastasis

Metastasis has been reported for certain types of fish neoplasms, including nephroblastomas (Masahito et al., 1992), pigment cell neoplasms (Okihiro et al., 1992) and lymphomas (Nigrelli, 1947). Melanomas commonly metastasize in some fish (Fig. 3.1), although this may not occur in all species. There are several reports of metastasis of hepatic neoplasms; these and other metastatic neoplasms of fish were reviewed by Machotka et al. (1989). Overall, metastasis in fish may be less common than in mammals because several common metastatic primary tumours in mammals (lung, breast, cervix, prostate and uterus) and some of the most frequent sites of metastases (lungs, lymph nodes and bone marrow) are not present in fish. Many common neoplasms of fish are relatively well differentiated, and this could also be related to their weakly malignant behaviour. Other reasons for the less frequent occurrence of metastasis in fish have been proposed, including differences in the 'lymphatic system' (Haddow and Blake, 1933; Machotka et al., 1989) and lower body temperature of fish (Hendricks et al., 1984b). The 'lymphatic system' of fish is better described as a

Fig. 3.1. Melanoma in the skin of a channel catfish. This fish had multiple, black, slightly raised lesions scattered over the body. Bar = 25 mm. (RTLA Accession No. 5202; specimen contributed by Rodney W. Horner and L. Durham.)

secondary vascular system, which differs from the lymphatic system of tetrapods by receiving fluid from arteries (Steffensen and Lomholt, 1992). Further study is needed to determine how the lack of a lymphatic system in fish affects metastasis of neoplasms. Protocols used for experimental exposure of fish to carcinogens typically involve necropsy of the fish soon after neoplasms are present; if the fish were allowed to live longer, metastasis of experimentally induced neoplasms might be more common (Hendricks et al., 1984b).

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