A man's job: A surprise delivery from sperm to egg

Abstract

and demanded that they produce a semen sample to prove that they could provide her with grandchildren. May the best sperm win, joked one of the nervous suitors. In true reality, it can be hard to figure out which sperm can fertilize an egg and which ones can't. When a couple struggling to conceive seeks help from an infertility specialist, the abundance, motility, and shape ofthe man's sperm come under close scrutiny. In more advanced tests, physicians may gauge how well the man's sperm penetrates the protective barrier around a woman's egg or binds to it. They may even subject the sperm to chromosomal analysis. Such testing may reveal a problem but not explain it. If a man comes in with a very poor sperm count or a poorly motile sperm, we can describe that and say that it's not normal, but we don't know why it's abnormal, notes reproductive biologist David Miller of the University Of Leeds in England. Moreover, many infertile men have no sperm defects that show up in current assays. Says Miller, Outwardly, the sperm may look normal. It's inside sperm, therefore, that many secrets of male infertility hide. And Miller may have begun to expose some of them. To the surprise of many biologists, he and his colleagues have found that mature human sperm contain several thousand different strands of RNA, the cell's directives for protein manufacturing. For most cells, such an RNA payload is the norm. Cells synthesize RNA, a chemical relative of DNA, when they read the protein-building recipe encoded within a gene. Finding RNA inside mature human sperm, however, is startling because most scientists have assumed that newly made sperm cells shed almost everything as they mature. They obviously carry the father's set of chromosomes into an egg, but it was thought that these streamlined cells had little room or need for RNA. The discovery of so many RNA strands in sperm could have an impact on several areas of biology and medicine. Miller and his colleagues are establishing an RNA fingerprint of sperm from fertile men and plan to compare it with sperm from infertile men. The RNAs may also reveal the genes that sperm cells use for their own development; some of those genes may be targets for male contraceptives. Furthermore, sperm RNAs could give researchers a new way to evaluate the safety of environmental chemicals. says David Dix of the U.S. Environmental Protection Agency (EPA) in Research Triangle Park, N.C., one of Miller's collaborators (see box, p. 217). Finally, and most controversially, some of the RNAs inside sperm cells may play a role in the early growth of an embryo. This expansion of sperm responsibility could explain why cloning, a process in which an egg develops without a sperm fertilizing it, is proving to be so difficult. Speaking for his gender, reproductive biologist Stephen Krawetz of Wayne State University in Detroit boasts, We're delivering more [to the egg] than we thought we were.