Necrosis and apoptosis have been initially identified as two exclusive pathways for cell death. In acute brain lesions, such as focal ischemia, this binary scheme is challenged by demonstrations of mixed morphological and biochemical characteristics of both apoptosis and necrosis in single cells. The resulting difficulty in defining the nature of cell death that is triggered by severe insults has dramatically impeded the development of therapeutic strategies. We show that in the early stages of cerebral infarction, neurons of the so-called "necrotic" core display a number of morphological, physiological, and biochemical features of early apoptosis, which include cytoplasmic and nuclear condensations and specific caspase activation cascades. Early activation cascades involve the death receptor pathway linked to caspase-8 and the caspase-1 pathway. They are not associated with alterations of mitochondrial respiration or activation of caspase-9. In contrast, pathways that are activated during the secondary expansion of the lesion in the penumbral area include caspase-9. In agreement with its downstream position in both mitochondria-dependent and -independent pathways, activation of caspase-3 displays a biphasic time course. We suggest that apoptosis is the first commitment to death after acute cerebral ischemia and that the final morphological features observed results from abortion of the process because of severe energy depletion in the core. In contrast, energy-dependent caspase activation cascades are observed in the penumbra in which apoptosis can fully develop because of residual blood supply.

The generally accepted concept that astrocytes are highly resistant to hypoxic/ischemic conditions has been challenged by an increasing amount of data. Considering the differences in functional implications of protoplasmic versus fibrous astrocytes, the authors have investigated the possibility that those discrepancies come from specific behaviors of the two cell types. The reactivity and fate of protoplasmic and fibrous astrocytes were observed after permanent occlusion of the medial cerebral artery in mice. A specific loss of glial fibrillary acidic protein (GFAP) immunolabeling in protoplasmic astrocytes occurred within minutes in the area with total depletion of regional CBF (rCBF) levels, whereas "classical" astrogliosis was observed in areas with remaining rCBF. Severe disturbance of cell function, as suggested by decreased GFAP content and increased permeability of the blood-brain barrier to macromolecules, was rapidly followed by necrotic cell death, as assessed by ultrastructure and by the lack of activation of the apoptotic protease caspase-3. In contrast to the response of protoplasmic astrocytes, fibrous astrocytes located at the brain surface and in deep cortical layers displayed a transient and limited hypertrophy, with no conspicuous cell death. These results point to a differential sensitivity of protoplasmic versus fibrous cortical astrocytes to blood deprivation, with a rapid demise of the former, adding to the suggestion that protoplasmic astrocytes play a crucial role in the pathogenesis of ischemic injury.