Development of the Ability to Bind to Zonae Pellucidae During Epididymal Maturation: Reversible Immobilization of Mouse Spermatozoa by Lanthanum

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Abstract
Mouse spermatozoa recovered from the caput, corpus, or cauda epididymidis were examined for their ability to bind the vitro to zonae pellucidae. Since spermatozoa from the caput epididymidis do not display progressive motility as compared with more mature spermatozoa, direct comparison of the different sperm populations may not measure zona binding ability validly. To equalize the motile properties of the spermatozoa, a method was developed to immobilize vigorously motile corpus and cauda spermatozoa. Reversible immobilization was achieved by incubation in 25 microM La3+ which resulted in a twitching, nonprogressive type of motility. La3+ incubation did not appear to affect the spermatozoa adversely, since vigorous motility (equivalent to the controls) of corpus and cauda sperm was displayed upon subsequent incubation in standard La3+-free culture medium. Moreover, cauda spermatozoa preincubated for 90 min in La3+ displayed levels of fertilization in vitro equivalent to their control counterparts. Using this La3+-immobilization technique, the zona binding ability of the different sperm population could be assayed. Gamete collision was insured under these conditions by shaking the gamete-containing dishes at 100 cycles/min. Regardless of the extent of sperm motility, a similar zona-binding pattern emerged: cauda sperm bound in high numbers, corpus sperm bound at some intermediate level (an average of 24% of cauda binding level), and caput sperm bound rarely (2% of cauda binding level). Thus it appears that, for mouse spermatozoa, the onset of fertility during epididymal transit parallels the ability to bind to zonae pellucidae. Unlike the interaction between spermatozoa and zonae, La3+ was unable to support sperm binding in to egg plasma membrane, supporting the view that mouse sperm may have different sites for interaction with the zonae pellucida and the egg plasma membrane.