been crucial in changing the way ischaemic stroke has begun being viewed as a potentially “treatable disease” rather than a “preventable catastrophe” (RAMOS-CABRER et al. 2011). Hence, the goal of any circulation restoring strategy is to salvage the at-risk tissue surrounding the infarct core. Within the ischaemic penumbra, several mechanisms have been identified on a molecular level that irreversibly damage brain tissue. In the area with the most severe hypoperfusion, synthesis of adenosine-triphosphate (ATP) drops, while its utilization remains high, leading to acidosis and the dysfunction of energy-dependent ion transporters. As a result, cells swell and membranes rupture. However, ischaemic tissue death is not only caused by the limited availability of ATP, but is a result of a complex interplay between numerous biochemical and cellular events that lead to acute cell death (Fig. 2). Fig. 2 Biochemical and cellular mechanisms leading to cell death in ischaemic brain tissue. Excitotoxicity The metabolic depletion of energy can result in inappropriate release as well as failure of the reuptake mechanisms of excitatory amino acid neurotransmitters, such as glutamate, in depolarizing or dying neurons. Its accumulation in the extracellular space initiates cell damage through prolonged stimulation of α-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, which dramatically increases the Emmrich, J. V., Knauss, S., Endres, M., Current advances, challenges, and opportunities in stroke research, management, and care NAL-live 2021.2, v1.0, doi:10.34714/leopoldina_NAL-live_0002_01000 5 influx of calcium, sodium, and water into neurons (ANKARCRONA et al. 1995). This elevation of cellular calcium ions can, in turn, trigger a wide range of harmful processes, including inappropriate activation of catabolic processes, generation of free radicals, and mitochondrial injury. Oxidative and Nitrosative Stress Oxidative and nitrosative stress occur during ischaemia, when the production of free radicals overwhelms the limited scavenging capacity of the brain’s endogenous antioxidants. Free radicals, such as oxygen-derived free radicals and nitric oxide, are molecules with one or more unpaired electrons. They are highly reactive, causing damage and dysfunction to DNA, proteins, and lipids, and potentially lead to cell death (CHAN 2001). Brain tissue damage following the restoration of circulation after a period of ischaemia, a so-called reperfusion injury, is mainly caused by damage from free radicals (SUN et al. 2018). Cortical Spreading Depolarizations Electrophysiological studies and imaging data show that cortical spreading depolarization (CSD) occurs spontaneously in the surrounding tissue of developing infarcts which is metabolically and functionally compromised but has not yet died – i.e. the penumbra (MIES et al. 1993, WOITZIK et al. 2013). CSD, triggered by high levels of extracellular potassium and glutamate that occur during ischaemia, is a highly energy-consuming phenomenon characterised by slowly propagating waves of depolarization of neurons and neuroglia for minutes to days after the onset of stroke (DREIER and REIFFURTH 2015). Expansion of the infarct core is caused by a mismatch of a drastically increased energy demand of activated ion transporters to support membrane repolarization and insufficient delivery of oxygen and glucose to the tissue. This results in neuronal death in energy-deprived tissue (ibid.). Interestingly, stimulation of the brain in critical peri-infarct “hot zones” may cause additional spreading of injury depolarizations via supply-demand mismatch transients and contribute to additional brain injury (BORNSTÄDT et al. 2015). Mechanisms of Cell Death Necrosis and apoptosis are the main mechanisms of cell death following ischaemic injury (DIRNAGL et al. 1999). Necrosis and apoptosis can be induced by multiple triggers, including oxidative and nitrosative stress and calcium overload. Mild ischaemic injury preferentially induces delayed neuronal cell death via apoptotic mechanisms, such as activation of caspases, leading to subsequent phagocytosis of the apoptotic bodies (ENDRES et al. 1998). In contrast, necrosis, the passive death of a cell due to severe ischaemia and energy depletion, leads to an uncontrolled release of inflammatory cellular content and other debris (DIRNAGL et al. 1999). In some cases, neuronal cell death with morphological features of necrosis can be induced by death receptor signalling after ischaemic injury. This receptor-dependent mechanism of cell death has become known as necroptosis (DEGTEREV et al. 2005). Other, more recently identified mechanisms contributing to neuronal death following ischaemic injury include autophagy, a catabolic pathway leading to the self-eating of cellular components by autophagosomes, and phagoptosis, the phagocytosis of eat-me signals which expose stressed but viable neurons (NEHER et al. 2011, WANG et al. 2018). Inflammation Inflammation following brain injury is a key contributor to the pathophysiology of stroke. Under conditions of hypoxia, high shear stress, endothelial cell damage, and the generation of reactive oxygen species, the inflammatory process begins to unfold in the vascular compartment within minutes after occlusion of an artery. Postischaemic