Debris of “dark” (compacted) neurones are removed from an otherwise undamaged environment mainly by astrocytes via blood vessels

Abstract

By means of a condenser-discharge electric shock paradigm, “dark” granule neurones were momentarily produced in a sporadic distribution among normal ones in the otherwise undamaged (non-necrotic, non-excitotoxic, non-inflammatory or non-contused) hippocampal dentate gyri of the rat brain. In the electron microscope, the ultrastructural elements of the affected neurones remained undamaged but turned markedly electron-dense and the distances between them became strikingly reduced (compaction). A proportion of such neurones recovered in 1 day while others died. During the first week of survival, the dead “dark” granule neurones retained the compacted and electron-dense ultrastructure, but underwent cytoplasmic convolution and fragmentation. The fragments were enclosed by membranes and separated from each other and from the intact neuropil by astrocytic processes containing an excess of glycogen particles. Neither proliferation of microglial cells nor infiltration of haematogenous macrophages was observed. A few fragments were taken over by resting microglial cells, while the majority was engulfed by astrocytes. The latter transported the engulfed fragments, either unchanged or digested to various degrees, to capillaries, arterioles and venules. Thereafter, the astrocyte-engulfed neuronal fragments, as well as their partly or completely digested remnants, were either transferred to phagocytotic pericytes or discharged into vascular lumina.