Formation and structure of the acrosomal filament in the sperm of Sicyonia ingentis

The sperm of the natantian Sicyonia ingentis are nonmotile, unistellate cells. During sperm‐egg interaction, they undergo a biphasic acrosome reaction (exocytosis of an acrosomal vesicle and formation of an acrosomal filament). Formation of the acrosomal filament is temporally separated from acrosomal exocytosis by 10–20 min in vivo and 30–45 min in vitro. The acrosomal filament is composed of 1) the filament proper and 2) an extended saucer/petal complex. The filament proper is 0.3–0.5 μm in diameter, about 10 μm in length, and contains tubular‐like structures that do not resemble microtubules. The acrosomal filament terminates anteriorly in the extended saucer/petal complex, a dense conical structure that flares out into 12–15 petal‐like extensions. The extended saucer/petal complex is present in the subacrosome prior to filament formation and during elongation of the filament proper (which occurs over a time span of 3–5 min) is pulled/pushed away from the main body of the sperm.