Better intracytoplasmic sperm injection without sperm membranes and acrosome

Millions of years of evolution leading to a well orchestrated series of events for sperm transport and selection in the female tract and sperm–egg interaction during fertilization can be bypassed by the direct microinjection of spermatozoa into oocytes to alleviate infertility or for the experimental manipulation of model animals (1). Since its introduction in humans (2), many thousands of children have been born by using intracytoplasmic sperm injection (ICSI). ICSI may represent on average approximately one-half of all assisted reproductive techniques used in Europe (3), with considerable differences between countries. Germany and the United Kingdom have many more in vitro fertilizations (IVFs) than ICSIs. France has equal numbers, and Belgium and Spain have each twice as many ICSI cycles than IVF cycles. The widespread use of ICSI relies, in part, on the belief that it is not necessary to pay attention to aspects of sperm (dys)function because they become irrelevant if a spermatozoon is directly microinjected into the oocyte. The article by Morozumi et al. (4) in this issue of PNAS demonstrates that this belief constitutes an oversimplification and that preparation of sperm for ICSI and an understanding of the underlying mechanisms of sperm function during fertilization are important for sperm microinjection. Interestingly, sperm preparation for ICSI does not seem to be of the utmost importance for the initial steps taking place after sperm penetration and leading to the first cell division, but it does have profound effects on subsequent development. The links between early changes after sperm microinjection and mechanisms of embryo and fetal development are not clear; the contribution by Morozumi et al. (4) suggests that efforts to investigate them may be rewarding.

Fertilization is a complex process. Many steps are involved to bring a sperm and an egg together to allow for a dialogue between gametes that …

*E-mail: roldane{at}mncn.csic.es