An Ultrastructural Study of Melanoma in Xiphophorus

Melanotic melanoma (MM) of Xiphophorus (Te1eostei: Poeciliidae) was studied by conventional preparations and freeze-etch preparations for electron microscopy. MM of Xiphophorus exhibits tightly packed pigment cells with prominent dendritic processes and interdigitations of their plasma membranes. The most impressive feature of MM cells is the occurrence of Iarge lobulated nuclei with numerous nuclear pores and some nuclear pockets. Abundant spheroidal or ellipsoidal melanosomes (diameter 200-650 nm) and vesicular structures are distributed throughout the cellular dendrites, whereas the perinucJear cytoplasm is free of melanosomes. A further characteristic feature of melanoma cells in fish is the occurrence of melanosome complexes (i.e., .. compound melanosomes"). These melanosome complexes consist of a few to numerous melanosomes, which are enveloped by a separate rnembrane. Pinocytotic vesicJes couJd be demonstrated with distinct dif· ferences in frequency and distribution patterns, indicating differences in the metabolic activities of the cells in the same rnelanoma. Intercellular junctions are Jacking in the MM cells. The conventional TEM technique showed clear advantages in the demonstration of intemal architecture of organelles~ whereas FE bad considerable potential in respect to the visualization of membrane surface specializations.