A human mesothelioma spheroid formed ex vivo in three-dimensional tissue culture and visualized by scanning electron microscopy. Cancer cell spheroids allow study of the tumor microenvironment and how it affects the ability of drugs to penetrate and kill tumor cells. Mesothelioma (or, more precisely, malignant mesothelioma) is a rare form of cancer that develops from cells of the mesothelium, the protective lining that covers many of the internal organs of the body. Mesothelioma is most commonly caused by exposure to asbestos.