Macrophages in brain tumours induced transplacentally by N-ethyl-N-nitrosourea in rats: an electron-microscope study

The fine structure of macrophages has been studied in experimental brain tumours induced transplacentally in BD-IX rats by a single intravenous injection of 30 mg of N-ethyl-N-nitrosourea per kg of body weight on the 15th day of gestation. The tumours, depending on their localisation and size, caused various lesions in the brain, namely axonal degeneration, loss of myelin, oedema, haemorrhage and cell necrosis. The tumours and the resulting alterations elicited a strong reaction by macrophages: activation of microglial cells in situ and infiltration of the brain by leucocytes, chiefly by monocytes. Since both microglial cells and monocytes underwent morphological changes, it was difficult, or impossible, to establish the origin of these reacting cells. In a few cases, however, microglial cells and monocytes could be distinguished; this indicated that microglial cells were still being activated and leucocytes were still entering the brain. Various stages of activity of macrophages have been described: the number of lysosomes and cytoplasmic inclusions were thought to indicate activation, phagocytosis and repletion. Activation is characterised by an increase of lysosomes and frequent cell divisions. Phagocytic activity is accompanied by the appearance of inclusions which varied in different lesions: protein-like material in oedema, remnants of erythrocytes in haemorrhages and myelin-lamellae with lipid droplets in demyelination. These various inclusions were frequently present in the same cell, since the different lesions not uncommonly occurred at the same time. In the stage of repletion macrophages contained mainly lipid droplets and unidentifiable debris in their abundant cytoplasm and thus corresponded to the compound granular corpuscles.