An ultrastructural study of spontaneous cell death in a mouse mastocytoma with particular reference to dark cells

The morphology and frequency of the various types of cell death occurring spontaneously in a mouse mastocytoma growing as intramuscular nodules were investigated. In addition to necrosis and apoptosis, which have been well documented in malignant neoplasms in the past, a third morphological pattern of cell death, the formation of dark cells, was observed. Necrosis first appeared in confluent patches about 5 days after tumour inoculation, and these increased in size as the tumours grew. Apoptosis, on the other hand, was present at all stages of tumour growth, and remained at a reasonably constant level. Dark cells were first observed at about 6 days, and increased in numbers thereafter. Dark cells were characterised by overall cellular condensation and gross swelling of mitochondria. Their cytoplasm became squeezed out between adjacent cells and then fragmented. Remnants of dark cells were eventually phagocytosed and degraded by macrophages. The distribution of dark cells in the tumours suggested that crowding and compression may contribute to their development.