Fine structure of cellular and vascular reaction in brain tuberculomas: a model for phagocytosis in the CNS.

The fine structure of cells of the mononuclear phagocyte series (MPS) and a few other cells with phagocytic capacity, has been critically evaluated, mainly from an electronmicroscopic examination of the reactive border zone of 11 human brain tuberculomas, which provide ideal material for the study of macrophages. Most of them appeared to be blood monocyte-derived epithelioid cells of various forms and stages. The cytoplasm of these cells showed either more rough ER representing protein synthesising activity; or more frequently, phagosomes, phagolysosomes, dense bodies or empty vacuoles, representing various stages of ingestion and digestion of necrotic material. Often such material, which was more or less osmiophilic, was seen abundantly between the cells. These actively phagocytic cells occasionally undergoing, mitosis, are referred to as "epithelioid macrophages" and were morphologically similar to the "activated microglia" described in other conditions. They also showed a tendency to be closely adjacent to each other and occasionally fuse to form giant cells. There were also a number of lymphocytes and plasma cells. The latter showed various stages of active and granular or depleted and distended rough ER tubules, phagocytic activity and tendency to fuse. Expected vasculitis and small vessel necrosis formed part of this granulomatous reaction. Constituents of oedematous or necrosed brain tissue were seen immediately around the reactive zone of these tuberculomas, the most frequent being reactive astrocytes, many of which showed membrane-bound vacuoles. It is conceivable that the excessive pleomorphic cellular, vascular and necrotic reaction in these brain tuberculomas could have resulted from a delayed type of hypersensitivity to a very small quentity of antigenic tuberculoprotein, which probably initiates the chain of immunologic responses.