Part of “CHAPTER 107 – CONCEPTION, IMPLANTATION, AND EARLY DEVELOPMENT“

The relatively thick and rigid structure that invests the mammalian egg, called the zona pellucida, has necessitated some
changes in the physiology of fertilization, particularly with respect to the sperm.

Mammalian sperm require the occurrence of two events before they can fertilize an oocyte. The first, known as capacitation, is the process by which sperm become competent for fertilization, an act they are not able to accomplish before an appropriate, species-dependent incubation time within the female reproductive tract milieu or similar in vitro medium.11,12 and 13 During this time, the sperm not only mature but also attain a state of hyperactivated motility that is necessary for them to move through the length of the female reproductive tract and to generate the force necessary to pierce through the cumulus oophorus and the zona pellucida of the oocyte. In addition, certain incompletely defined factors known as decapacitation factors must be removed from the sperm before they become competent for fertilization. Presumably, these factors are macromolecules that are blocking certain receptor sites necessary for this functional change to occur, and there is evidence that removal of these factors increases the response of the sperm to extracellular Ca²⁺.11

Once the sperm are capacitated, the acrosome reaction can begin, and it is through this process that the sperm can ultimately fuse with the oocyte. The morphology of the sperm head is such that an inner acrosomal membrane is immediately adjacent to the nuclear membrane of the cell, whereas an outer acrosomal membrane and the plasma membrane act as the limiting membrane of the acrosome.14 The acrosome itself contains proteases, such as acrosin, and other enzymes necessary for the sperm to navigate the interstices of the corona radiata. The outer acrosomal membrane possesses specific molecules for attachment to the zona before penetration of the egg, including a receptor that binds to a glycoprotein named ZP3 of the zona pellucida of the oocyte and a galactosyltransferase that recognizes N-acetylglucosamine residues.15,16,17 and 18 This morphology necessitates some interesting adaptations during the fusion of the sperm to the oocyte. Because the surface molecules necessary for attachment to the zona must be retained, the outer membrane must remain intact after the release of enzymes. The spermatozoon joins with the egg by membrane fusion of a mid-portion membrane, the equatorial region of the sperm head. The acrosome reaction, then, seems designed to create the structural alterations required for these various constraints to be overcome. First, the sperm-limiting membrane changes to allow influx of calcium, presumably along an electrochemical gradient. Immediately thereafter, the acrosomal membrane becomes fenestrated, appearing to allow the acrosomal contents to be released while leaving the acrosomal membrane, with its putative zona attachment elements, largely intact. The equatorial portion of the membrane is left intact for fusion with the oolemma, the limiting membrane of the unfertilized egg. Once fusion has occurred, the sperm head swells rapidly and forms the male pronucleus, leaving the sperm midpiece visible within the fertilized egg.19,20,21 and 22