Ultrastructure of cultured mouse S91 melanoma cells with special emphasis on viral particles

Mouse S91 melanoma cells which grow as a monolayer in culture were studied by transmission and scanning electron microscopy after fixation in situ. The cells were spindle-shaped and tended to grow in dense colonies. Melanosomes at all stages of melanization (stages I-IV) were observed. Other cell organelles reflected high cellular activity. Both A-type and C-type viral particles were abundantly observed. A-type particles were 60-90 nm in diameter. They had an electron-lucent center surrounded by a double shell. They were abundantly observed in the cisternae of the endoplasmic reticulum but never extracellularly. C-type retrovirus particles were larger, 80-110 nm in diameter. They had an electron-dense core surrounded by an electron-lucent area and a limiting membrane. They were seen to bud from the plasma membrane into intracellular vacuoles or into the intercellular space.