The spermatozoa and spermiogenesis of Perophora formosana (ascidia) with special reference to the striated apical structure and the filamentous structures in the mitochondrion

Abstract

The spermatozoon of Perophora formosana is approximately 140 μ m long. It contains an extremely elongated filiform nucleus (ca. 90 μ m in length) and a single long mitochondrion (ca. 80 μ m in length) which is loosely wound around the nucleus as a lateral body. The anterior third of the head is coiled in a helical configuration. Filamentous structures approximately 10 nm thick are observed exclusively in the mitochondrial matrix. They are arranged parallel to one another along the long axis of the mitochondrion (lateral body). During spermiogenesis, they appear in the mitochondria of spermatids as the mitochondrion begins to elongate into the lateral body. They occur regularly aligned in a row in the matrix just beneath the inner membrane in the elongating region of the mitochondrion. They are first recognized as an accumulation of moderately electron-dense material. Possible functions for these filamentous structures in producing and maintaining mitochondrial shape changes are discussed. A striated apical structure, about 2 μ m in length, is found at the anterior end of the spermatozoon. It is composed of an electron-opaque substance in the form of dense striations. Each striation is approximately 7 nm thick. During spermiogenesis, an electron-dense material is deposited to form striated structures in the finger-like process at the apex of spermatids at almost the same stage that the filamentous structures appear in the mitochondrion. The role of the apical structure in the process of fertilization has not been determined.