Biological behaviour and morphological characteristics of a transplantable tumour derived from a uterine smooth muscle tumour in the F344 rat.

A subcutaneously transplantable tumour (SMT-Y) was established from a smooth muscle tumour arising from the uterus of a female F344 rat. SMT-Y was serially passaged by subcutaneous implantation into syngeneic female rats up to the 15th generation, but transplantation failed in males. The rat with the primary uterine tumour also had mononuclear cell leukaemia (MCL), and MCL cells grew concurrently in implanted rats. At passage five, MCL cells were eliminated from transplants by implanting the central part of an SMT-Y nodule, consisting only of neoplastic smooth muscle cells. SMT-Y at passages six to 15 was examined biologically and morphologically. The primary tumour and SMT-Y tumours consisted mainly of interlacing fascicles of elongated and fusiform neoplastic smooth muscle cells with abundant cytoplasm. Occasional cells showed nuclear atypia. Mitosis counts per 10 high-power microscopic fields in the primary tumour and SMT-Y ranged from 11 to 36. Neoplastic cells reacted positively for desmin, muscle actin and myosin, but not for myoglobin. Electron microscopy revealed cytoplasmic myofilaments with oval dense bodies. These findings suggested a smooth muscle origin of SMT-Y and it was regarded as a leiomyosarcoma by the criteria for human uterine smooth muscle tumours. Despite the malignant histological features, SMT-Y grew slowly into a large nodule, with an average diameter of 5 cm and average weight of 81 g, 24 weeks after transplantation. Neither invasive tumour growth nor metastases were observed in SMT-Y-bearing rats.