Ultrastructure of human colorectal tumour xenografts maintained serially in immunosuppressed mice

Five colorectal human tumour xenograft lines maintained by serial transplantation in immunosuppressed mice were studied by light and electron microscopy. Three had adenomatous while two had a mixed (adenomatous and mucinous) structure in the surgical specimens. After xenotransplantation in the mixed tumours the mucinous areas became dominant. Beside this there were no striking differences in the fine structure of the primarily implanted tumour compared with serially transplanted ones or between the different passages. The cells in adenomatous tumours formed acini with well-developed junctional complexes while the mucinous areas of mixed tumours were composed of single or loosely attached cells. In the former ones the cells had irregular microvilli on the apical surface and were separated from the stroma by a continuous basal lamina missing. The partially preserved functional activity was indicated by small vesicles in adenomatous, and mucin-droplets in mucinous tumours, showing intimate relationship with the rough endoplasmic reticulum and well-developed Golgi region. Some cells had features characteristic of both adenomatous and mucinous tumours. The ultrastructure of the studied tumours called attention to the possibility that malignant transformation may occur at different levels of cell differentiation.