Ultrastructural Changes of the “Truncated Cone” during the Acrosome Reaction in Japanese Abalone Spermatozoa

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Abstract
An electron-dense structure termed the truncated cone covers the apical surface of the acrosomal contents except for the trigger region in H. discus spermatozoa. The truncated cone, having a slant height of 0.3 .mu.m and diameters of circular top and base of 0.3 and 0.6 .mu.m, shows striations with a periodicity of 6.6-8.0 nm. During the acrosome reaction, the truncated cone elongates simultaneously with the protrusion of the acrosomal process through the truncated cone. As the growth of the acrosomal process further proceeds, the truncated cone transforms into a cylindrical shape and eventually reaches 1 .mu.m in length and 0.2 .mu.m in diameter. The elongated truncated cone is characterized by regularly helical striations with a periodicity of 19-21 nm with an inclination of 40-46.degree.. Apparently, the truncated cone is composed of coiled filaments, which coil up further during the acrosome reaction causing the truncated cone to slenderize and elongate. The elongation is also achieved by streching of the coil. In H. discus hannai Ino, structural changes in the truncated cone show close homology to those in H. discus. No such morphologically unique organelle was found in other species thus far.