Migration of intraperitoneally injected thyroid cells in the Amazon molly, Poecilia formosa.

We have previously reported the development of an extensive invasive growth of the thyroid gland of the gynogenetic teleost, Poecilia formosa (the Amazon molly), following i.p. injection of UV- or gamma-irradiated thyroid cells. This result was surprising by comparison with mammalian work, in which the thyroid is rarely the site for tumor metastases, but the anatomy of the circulation of fish is different from mammals, and in fish the gills and thyroid gland would be among the first tissues in which injected cells might be arrested. Techniques using a fluorescent dye, 125I membrane label, or [3H]thymidine label were used to follow the distribution of i.p. injected cells in the Amazon molly. Fish sampled as soon as 30 min after injection had some labeled cells dispersed in the connective tissue around the ventral aorta and in the bases of the gills, and by 1 to 4 hr large numbers of cells had moved into the thyroid region. A few cells still persisted there 200 hr later. Experiments on the distribution of heat-killed cells indicated that the initial distribution of the cells was largely governed by mechanical factors. Injected cells would appear to be disseminated in fish by mechanisms similar to those in mammals.