Differentiation of melanomas occurring in platyfish-swordtail hybrids of different ages: an ultrastructural study.

The degree of differentiation of two types of hereditary melanomas of interspecific hybrids between the spotted variety of the platyfish (Xiphophorus maculatus) and the swordtail (Xiphophorus helleri) was examined electron microscopically. Melanomas of the fry consisted mainly of pigmented cells containing intermediately matured melanosomes and few other cytoplasmic structures. Melanomas of the adult consisted of lightly pigmented cells having a small number of immature melanosomes; numerous polysomes; well-developed, rough-surfaced endoplasmic reticulum (RER) and Golgi systems; and irregularly shaped nuclei. Thus the fry and adult melanomas appear to represent relatively well- and poorly differentiated states of fish melanophores, respectively. Amelanotic melanomas in the adult melanotic F1 generation were also characterized. Virus-like particle were often found in the nuclei of both melanotic and amelanotic melanoma cells in the adult fish.