Calmodulin during human sperm incorporation into hamster oocyte: An immunogold electron microscope study

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Abstract
In the present study, immunogold labeling of ultrathin sections of human sperm, before and after incorporation into hamster oocyte, was used to obtain insight into the ultrastructural localization and possible function of calmodulin during fertilization. In heads of ejaculated, capacitated, and acrosome‐reacted fixed human sperm, calmodulin was mainly found in two compartments, the subacrosomal layer and the postacrosome. After sperm‐egg fusion, the subacrosomal calmodulin was unaltered and surrounded by the fertilization cone in which actin was abundant. There was no co‐localization of calmodulin and actin. In contrast, postacrosomal calmodulin disappeared as soon as the sperm head was incorporated into egg cytoplasm. These unique localizations and redistributions are in agreement with the concept of a calmodulin targeting from acrosome toward postacrosome through the subacrosomal layer during spermatogenesis (Weinman et al., 1986b: J Histochem Cytochem 34:118). Moreover, they strongly suggest a role for calmodulin both in sperm‐egg fusion and in the initial pulse of Ca2+ occurring during fertilization.