Like a deadbeat dad, sperm were thought to give eggs little more than the initial bang that gets things started. But a new study suggests that sperm do more than deliver DNA and spur an egg to develop. Researchers have shown for the first time that sperm also carry RNA, some of which may provide important signals to the developing embryo.
Some of the first growth signals that are activated after an egg and sperm mingle their DNA come from stored messenger RNAs (mRNAs) in the egg. The mRNAs spawn proteins that set the embryo on the road to a squalling baby. About half of failed fertilizations can be traced to sperm, but in most cases the sperm appear to be healthy and vigorous. In the mid-1990s, two independent groups gathered hints that sperm might bear RNA--a controversial finding because most of a cell's RNA-containing cytoplasm is extruded during sperm production.
Reasoning that such RNA, if it exists, could hold the key to paternal potency, molecular geneticist Stephen Krawetz of Wayne State University in Detroit, Michigan, and colleagues used microarray analysis to create an mRNA "fingerprint" of healthy sperm. The team purified sperm from nine men and screened for mRNA with a microarray that can register matches to some 30,000 gene fragments. They calculated that healthy sperm contain between 2682 to 2886 different mRNA messages. To determine whether these protein templates might be involved in early embryo development--a job previously considered the domain of the egg's RNA stash--the researchers compared the mRNA from sperm, unfertilized eggs, and fertilized embryos just beginning to develop. They found 10 mRNAs in the developing embryo that were present in sperm but not eggs, turning the egg-centric view of RNA on its head.
An mRNA fingerprint of healthy sperm could help doctors identify men whose sperm aren't up to snuff, says developmental biologist Gerald Schatten of the Magee-Womens Research Institute in Pittsburgh, Pennsylvania. "The men's sperm might look fine, but from an mRNA standpoint, they're shooting blanks." Surprised at the number of mRNAs Krawetz's team found, he speculates that missing sperm-donated RNA might underlie some of the difficulties researchers have had cloning animals.