Mammalian Fertilization Misread? Sperm Penetration of the Eutherian Zona Pellucida Is Unlikely to be a Lytic Event

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Abstract
Spermatozoa and oocytes of many eutherian (placental) mammals, including the human, are now manipulated successfully in a variety of ways for reasons having to do particularly with human infertility and domestic animal production, as well as basic research. Even so, several evolutionarily novel features of their function and form largely defy explanation still [1]. Most obvious in the male is the absolute dependence of eutherian spermatozoa on the environment of the upper epididymis for final maturation, and on an androgen- and usually a scrotal temperature-regulated system in the distal epididymis for their storage. Eutherian spermatozoa also have to undergo capacitation in the female tract before they can fertilize and, again for reasons that are unclear, the eutherian sperm head has adopted a novel design quite different from that in all other vertebrates. The equally unusual character of the minute egg is reflected in the ubiquitous cell mass of the cumulus oophorus around it and in a relatively prominent zona pellucida. By virtue of its thickness and resilience, this coat greatly outstrips the thin vestment around the eggs of other vertebrates and of invertebrates that have provided models for much of fertilization research. Finally, eutherian spermatozoa fuse with the oolemma by way of a segment of plasma membrane over the central (equatorial) region of the head, rather than by the apex of the inner acrosomal membrane (IAM), as in most other groups.