832 



SPERM, OVA, AND PREGNANCY 



is completed (approximately 6 hours after 

 ovulation), penetration of spermatozoa no 

 longer occurs, despite the presence of ade- 

 quate numbers in the environs. It is im- 

 portant to remember that the deposition of 

 the mucous coat in the rabbit ovum may 

 limit its fertilizable life (Pincus, 1930; 

 Hammond, 1934). The actual time after 

 ovulation that mucous deposition begins 

 has been variously reported as 5, 6, 8, and 14 

 hours (Pincus, 1930; Hammond, 1934; 

 Chang, 1951b; Braden, 1952). It remains 

 to be determined whether failure of sperm 

 penetration into the rabbit egg after 6 

 hours' sojourn in the ampulla is related to 

 the loss of the cumulus, the deposition of 

 the mucous coat, or to a specific change in 

 the physical characteristics of the zona 

 pellucida itself. 



B. THE ZONA PELLUCIDA AND 

 SPERM PENETRATION 



The general appearance and pi'operties 

 of the zona pellucida were described earlier. 

 The manner whereby spermatozoa pene- 

 trate the zona pellucida and the conditions 

 influencing this process are poorly under- 

 stood. Despite the numerous attempts to 

 fertilize mammalian ova in vitro, only a 

 few investigators have described isolated 

 stages in the process of sperm penetration 

 through the zona pellucida or into the vitel- 

 lus. Shettles (1953) described in some detail 

 the behavior of a human spermatozoon 

 passing through the zona pellucida of an 

 isolated follicular ovum. As the spermato- 

 zoon became attached to the zona it ro- 

 tated on its longitudinal axis. As the head 

 was observed in focus in the equatorial 

 plane, the rate of rotation decreased until, 

 by the time the tip of the head was midway 

 in the zona, the front and side views of 

 the head could be seen to alternate. The 

 progression of the head through the zona 

 pellucida was intermittent until only the 

 tail lay within it. The head and body then 

 underwent several intermittent side-to-side, 

 jerky movements and finally slipped into 

 the peri vitelline space. It required 18 min- 

 utes for a spermatozoon to traverse the 

 zona pellucida. Duryee (1954) described 

 the consistency of the zona pellucida of the 

 human follicular egg as jelly-like, much less 



tough and resilient than the tubal egg. It 

 would be interesting to know whether these 

 differences in the physical properties of 

 the zonae of ovarian and tubal eggs in the 

 human affect the manner of spermatozoon 

 penetration. 



On two occasions Pincus (1930) found 

 rabbit ova with the heads of spermatozoa 

 partially embedded within the zonae, and 

 described the slow yet perceptible forward 

 progress until the heads penetrated the 

 vitelli. Pincus believed that the flagellae 

 did not enter the ooplasm but were left be- 

 hind in the zonae pellucidae. 



There is no sound evidence of a prede- 

 termined pathway or "micropyle" in the 

 zona pellucida of mammals. In the few 

 instances where attention has been paid to 

 this matter, spermatozoa seem to be able to 

 penetrate the zona at any point on its sur- 

 face. A small elliptical slit with the sperm 

 tail partially projecting through it has been 

 noted in the zona pellucida of living ferti- 

 lized eggs of the rat, guinea pig, and Libyan 

 jird (Austin, 1951b; Austin and Bishop, 

 1958). The slits in the zona are not seen 

 in eggs which do not contain spermatozoa. 

 It is usually possible to discern as many 

 slits as there are sperm within the peri- 

 vitelline space. The general appearance of 

 the slit and the manner in which the per- 

 foratorium of the sperm head attacks the 

 zona pellucida in vitro creates the impres- 

 sion that the zona may be fractured by the 

 spermatozoon. Similar slits can be made by 

 fracturing rat zonae with tungsten needles 

 sharpened electrolytically to several micra 

 in thickness. 



Recently Austin and Bishop (1958) have 

 presented observations suggesting that the 

 acrosome is lost as the sperm passes through 

 the female reproductive tract and postulate 

 that the perforatorium elaborates an en- 

 zyme which depolymerizes the zona pel- 

 lucida in a very restricted zone as the sperm 

 moves through it. 



Discussions on the mechani.sms involved 

 in sperm penetration of the zona have im- 

 plicated a variety of conditions and sub- 

 stances as being of importance in changing 

 the physical characteristics of the zona in 

 the localized area of contact. As mentioned 

 earlier, the zona pellucida can be softened 



