The localization of actin filaments in abalone sperm was investigated using 7‐nitrobenz‐2‐oxa‐1,3,‐diazole‐phallacidin (NBD‐phallacidin) as a specific probe for actin filaments. Fluorescence staining of intact sperm revealed the localization of actin filaments in the axial rod. After the acrosome reaction the fluorescent rod elongated to form the acrosomal process, which signified the elongation of actin filaments. More definite identification of the bundle of filaments in the axial rod as filamentous actin was indicated by decoration with myosin subfragment 1 (S1). The actin filaments in both the unreacted and reacted sperm all appeared decorated with S1, and the polarity was found to be unidirectional with the arrowheads pointing towards the nucleus. When Triton X‐100‐treated unreacted sperm were incubated in exogenous G‐actin, actin monomers assembled on the “barbed end” of the actin filaments of the axial rod. These results suggest that the elongation of the acrosomal process occurs via polymerization of endogenous G‐actin at the tip (the barbed end) of the axial rod.
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Shiroya et al. (1986) studied this question.
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