Figure 1. Examples of methods for evaluating mitochondrial morphology, subcellular localization and functional status.

(a) Each panel shows a single live embryonic rat hippocampal neuron at a different stage of development in culture (1, 5 and 14 days). Mitochondria in the neurons were stained with the fluorescent probe MitoTracker Red. Note that the immature neuron has elaborated short processes, and that the vast majority of mitochondria are clustered in a perinuclear location. At 5 days in culture the neuron exhibits longer neurites that contain multiple mitochondria, and perinuclear mitochondria remain abundant. Mitochondrial numbers have increased, indicating that biogenesis has occurred. At 14 days in culture the neuron has elaborated an extensive neuritic network with each neurite containing multiple mitochondria. Relative numbers of mitochondria in a perinuclear location are reduced. (b) Example of the results of an experiment in which oxygen consumption, mitochondrial membrane potential and cytoplasmic Ca²⁺ levels were monitored in a single cultured embryonic rat hippocampal neuron before and during exposure to the excitatory neurotransmitter glutamate. In response to glutamate receptor activation, oxygen consumption increased and then slowly returned towards baseline levels, intracellular Ca²⁺ levels rose rapidly and remained elevated, whereas mitochondrial membrane potential declined progressively. Adapted from Gleichmann et al. (2009).