Mitochondria are flexible, oblong shaped organelles that are located in the cytoplasm of every eukaryotic cell. They occur in varying numbers, depending on the cell and its function, but are particularly abundant in animal cells. A single liver cell, for instance, may contain as many as 2,000 of the important organelles, which convert oxygen and nutrients into adenosine triphosphate (ATP), the stable storage form of energy that can be utilized to power the cell's metabolic activities. Each mitochondrion in a cell is comprised of an outer, smooth membrane and an inner membrane that is heavily folded into cristae. Mitochondria are distinct from other organelles due to the fact that they possess their own DNA and reproduce independently of the cell in which they are housed. Mitochondrial DNA has been utilized in a broad array of inheritance studies, and has been of particular interest to evolutionary scientists since it is believed to change at a slower rate than nuclear DNA. Mitochondrial DNA has also been implicated in a number of genetic diseases, including Alzheimer's disease and diabetes.

The mitochondria present in the culture of MRC-5 fibroblasts featured in the digital image above were targeted with MitoTracker Red CMXRos, a derivative of X-rosamine. In addition, the culture was labeled for nuclear DNA and the cytoskeletal F-actin network with the ultraviolet-absorbing probe DAPI and Alexa Fluor 488 conjugated to phalloidin, respectively. Images were recorded in grayscale with a QImaging Retiga Fast-EXi camera system coupled to an Olympus BX-51 microscope equipped with bandpass emission fluorescence filter optical blocks provided by Omega Optical. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.