Examples of the geometrical features and their dynamics that together generate the overall morphology of mitochondrial networks. Mitochondrial networks in budding yeast are used as a model system to highlight these features but the features are all equally applicable to other organisms and cell types. Mitochondria are labeled by a matrix-targeted fluorescent protein  and cell boundaries are shown with thin yellow lines as in Figure 1c. The specific condition or mutation generating each example image is not specified because a variety of mutations can alter each of these geometric features and the purpose of the figure is to illustrate the features themselves in a more general way. Size: example cells with a larger and a smaller mitochondrial network arising from growth in respiratory and non-respiratory conditions, respectively. Shape: example cells with mitochondria lacking their normal underlying tubular structure or exhibiting swollen irregular tubules due to changes in internal membrane organization are shown. Mitochondria in the third cell display normal tubular structure but instead the network contains many fewer branch points than normal. These tubules lie parallel to each other without connections between them. This network is also less uniformly distributed within the cell, highlighting the interdependence between features (depicted by the large gray arrows). Position: example cells with a more uniform versus more asymmetric distribution within the cell. Note the large area of the cell devoid of any mitochondria in the latter case. Dynamics: example cells displaying the resultant over-fragmented and over-fused mitochondrial networks generated when either fusion or fission dynamics are reduced. The final topology of these networks is also greatly altered as a result, once again emphasizing the interdependence of these features. As described in the text, the topology itself may affect the local fission and fusion dynamics as well as the ability of sub-regions of the network to move around within the cell. This may explain the less uniform distribution of the over-fused networks and re-emphasizes the importance of considering both directions of interdependence between features.
Rafelski BMC Biology 2013 11:71 doi:10.1186/1741-7007-11-71