Microglial activation. Formation of pleomorphic microglia and macrophages: The sequence of reactive changes in microglial cells is displayed in diagrammatic form. "Al" is a normal resting microglial cell. With metallic impregnation methods (as used for these sketches) this cell has branching processes. Under the stimulus of tissue necrosis, the resting microglial cell undergoes hypertrophy and hyperplasia, entering an intermediate phase often referred to as the "pleomorphic microglial response". Transitional forms in this pleomorphic microglial stage would include "A2a" and "A2b". It is important to remember that as soon as the resting microglial cell is activated it becomes an active mobile scavenger cell. If the necrosis is subtotal, the microglial cells may not be stimulated to evolve beyond this "pleomorphic" stage of response. If necrosis is complete, microglia proceed through further transitional forms, as shown on the diagram, to become rounded macrophages, "A6". The latter are also referred to as "compound granular corpuscles" or "gitter cells". It is this last stage which is seen in a well developed, organizing infarct. On the left side of the diagram, cells labeled "B1", "B2", and "B3" represent chronically activated microglial cells. "B3" is a markedly elongated cell, the nucleus in particular having a long, rod-shaped form. This rod cell is classically seen in general paresis (paretic neurosyphilis) but may also be seen in other chronic disease states of the brain.