Mitochondria are the cell's energy-producing organelles and they contain their own DNA (mtDNA). Scientists have long known that offspring inherit the vast majority of their mitochondria from their mother. They believed this was simply because the egg contains many more of the organelles than the sperm does. But new research in fish reveals that sperm mitochondria are actually completely destroyed within the first two hours of fertilization. This may in fact protect the offspring, the researchers note, because the stresses of sperm production can impair the sperm mtDNA. The mechanism of this elimination of the sperm mitochondria--whether they self-destruct or the egg extinguishes them--has yet to be determined. In the image above, the red area shows the mitochondria in the sperm. The findings are being published online this week by the Proceedings of the National Academy of Sciences.