STUDIES ON THE MORPHOGENESIS OF THE TUMORS EXPERIMENTALLY INDUCED BY ETHYLNITROSOUREA WITH SPECIAL REFERENCE TO THE LIGHT AND ELECTRON MICROSCOPY OF MICROTUMORS

Abstract

Tumors of the nervous system were induced in rats of Donryu strain by a single injection of 50mg/kg.b.w. of ethylnitrosourea (ENU) through the mothers. A total of 126 tumors were produced in 92 of 117 treated offsprings. The survival time of animals with tumor development ranged from 53 to 607 days. The neural tumors amounted 104, comprising 82.5% of the total number of neoplasms. Besides grossly visible tumors, histological examination of the brain and spinal cord disclosed a total of 166 foci of glial cell hyperplasia or tumors of microscopic size. On histological sections 86 of these lesions measured less than lmm in diameter, 61 between 1 and 2mm, and 19 more than 2mm. The predominant cell type of the smaller lesions was either astrocytic or oligodendroglial. With increasing tumor size, a number of microtumors with a mixed cellular composition became frequent. Sites of predilection for such microtumors were the subcortical white matter of the brain, the hippocampus, the paraventricular areas, the basal ganglia, and the spinal cord. It would seem that grossly visible tumors are formed by expansion and fusion of these lesions of microscopic size and by development of anaplasia of constituent glial cells.Light and electron microscopic studies were also made on the brain and the trigeminal nerve of rats treated transplacentally with ENU at intervals of 10, 35, 80, 120, 150, 200, 250, 300 and 400 days after birth. A total of 48 microtumors were found in the brain of rats which had survived over 120 days. Electron microscopy disclosed these lesions to be largely composed of cells with ultrastructural features of astrocyte and oligodendroglia. The presence of a large amount of extracellular matrix was considered in part to account for the honeycombed structure typical of oligodendroglial microtumors in light microscopy. Observations of the trigeminal nerve disclosed occasional foci of diffuse endoneurial cell proliferation which started to arise from the 80th day. At the ultrastructural level, the proliferating cells were found to retain a typical Schwann cell appearance and relationship to axon. The nerve fiber was often surrounded by numerous cell processes, forming onion bulb-like structure.The third experiment was undertaken to investigate the acute cytotoxic effect of ENU on the nervous system. A single dose of 50 or 100mg ENU was given to rats perinatally and the nervous system was examined with light and electron microscopy at intervals from 6 to 48 hours after injection. A considerable proportion of cells constituting subependymal matrix layer of the lateral ventricle and extragranular layer of the cerebellum were found undergoing degeneration with disappearance of mitotic figures after 6 to 12 hours. Schwann cells in the trigeminal nerve and glial cells of the cerebral white matter were also found similarly involved. Neurons seemed to be less affected.From these observations it was concluded that oligodendroglia, astrocytes and Schwann cells are the most susceptible targets in the brain and nerve to carcinogenic nitrosourea compounds.