Figure 1 Typically, our view is that the mitochondrion is an elongated structure, often described as a sausage-like entity (1) (Figure 1). However, this is typical for cells that have a high dependency on mitochondria for the generation of cellular energy, namely ATP. This is the case for mature cells such as muscle cells and neurons, which characteristically have large numbers of mitochondria within their cytoplasm (2) (Figure 2). However, oocytes (3, 4), embryonic cells (5) and embryonic stem cells (6, 7) have naïve, oval or round mitochondria that are often not prime producers of ATP and, consequently, preserve their mitochondria for later events when these cells give rise to mature cells with specific functions (Figure 2) (6). There is a certain amount of confusion surrounding oocytes and their ability to generate ATP through their mitochondria. This arises as they have large numbers of mitochondria, more so than somatic cells that utilise mitochondria for ATP production (cf. Figure 2B and C). However, it should be remembered that their mitochondria are naïve, structurally and functionally quiescent and need to mature in order to generate ATP, and, therefore, generate ATP through other means, such as the adenosine salvage pathway (8). Primarily, oocyte mitochondria are punctate and do not form networks (9, 10).

Figure 2 The outer shell of the mitochondrion consists of a double membrane that acts as a barrier, which regulates the influx and efflux of molecules into and out of the mitochondrion (1) (Figure 1). The inner membrane of the mitochondria houses many of the internal structures required to generate energy, and that this inner membrane is highly folded into cristae to provide a large surface area to harbour many of these structures (11). Indeed, the density of the cristae within the matrix is indicative of a cell’s ability to generate ATP (12). When the cristae are denser, as frequently found in the elongated sausage-like structures typical of somatic cells, these mitochondria have a greater propensity to generate ATP (12). The mitochondria of oocytes and embryonic cells and embryonic stem cells have an opaque matrix that is indicative of their being naive and unlikely to be major contributors to the generation of ATP (6-8). They primarily rely on glycolysis for the generation of ATP and salvage pathways, which take place in the cell’s cytoplasm.

Figure 3The double mitochondrial membrane provides a barrier for the influx of molecules that would harm the mitochondria or perturb their functions (1) (Figure 1). It also allows the influx of molecules that contribute to and help assemble many of the structures associated with cellular function and the generation of ATP. The membrane also regulates the influx of other factors such as calcium, as mitochondria often act as a calcium store (13). A functional double membrane is maintained by the mitochondrial membrane potential that provides an electrical type barrier or fence that regulates the flux of factors into and out of the mitochondria (1). The mitochondrial membrane potential is generated by hydrogen ions (H+) that pass through the complexes of the electron transfer chain as part of the process of generating cellular energy through the biochemical process of oxidative phosphorylation (OXPHOS) (Figure 3).