The Acrosome and Its Differentiation during Spermiogenesis of a Compound Ascidian Botryllus schlosseri (Ascidiacea, Tunicata)

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The acrosome differentiation in a compound ascidian Botryllus schlosseri has been observed as the prerequisite step for morphological studies on the internal fertilization in compound ascidians. The spermatozoa of B. schlosseri has architectural features characteristic of ascidian spermatozoa that have been previously described. They have an elongated head (approximately 10 mm in length) and a single mitochondrion which is closely applied laterally to the nucleus as a lateral body. They lack a midpiece. An acrosome is present at the apex of the sperm head. The acrosome of B. schlosseri is a depressed sphere, approximately 50 x 80 x 40 nm with an electron-dense plate in its central region. At an early stage of spermiogenesis, two small vesicles appear in a blister at the apex of each spermatid. At a late stage of spermiogenesis, these vesicles fuse each other to form a single horseshoe-shaped vesicle. At the completion of spermiogenesis, the horseshoe-shaped vesicle transforms to a round flattened acrosome.