Morphologic and microspectrophotometric studies on spontaneous melanomas in Xiphophorus helleri

Melanomas developed in both sexes of a strain of Tuxedo variety of the swordtail (Xiphophorus helleri) at a relative frequency of 10-15%. They did not metastasize. However, the tumor margin had infiltratitive growth and subsequently ulcerated. This feature, together with the histologic and cytologic features and apparent heteroploidy of the tumors, as revealed by their DNA content, indicated that the tumors were indeed neoplastic. Electron microscopic findings on the melanosomes in these melanomas at various stages of development were comparable with those on the Harding-Passey mouse melanoma, which contains granular premelanosomes.