title: The apical structure in Perophora annectens (tunicate) spermatozoa: fine structure, differentiation and possible role in fertilization.
abstract: The apical structure in Perophora annectens spermatozoa is approximately 4 micron in length and it is helically coiled. Its major component is a striated structure, which may be analogous to a perforatorium. The plasmalemma enclosing the anterior quarter of the apical structure is covered by extracellular materials, the anterior ornaments. During spermiogenesis, the apical structure is first recognized as a small blister of the plasmalemma at the apex of the young spermatid. It develops into a conical protrusion and then into a finger-like process (approximately 1 micron in length). This process is transformed into an elongated process (approximately 4 micron in length) with electron-dense material in its core. Finally, the elongated process is helically coiled to form an apical structure in which electron-dense material forms dense striations. Vesicles (50-70 nm in diameter), presumably derived from the Golgi apparatus, have been recognized in the blisters of younger spermatids, and can be followed through to the finger-like process. In the finger-like process these vesicles are transformed into smaller vesicles (20-30 nm in diameter), which probably fuse with the anterior plasmalemma of the finger-like process. This suggests that chorion lysin(s) is associated with the anterior membrane enclosing the apical structure in these spermatozoa.
mesh_terms: D000818:Animals; D005306:Fertilization; D008297:Male; D008854:Microscopy, Electron; D013077:Sperm Head / Q000254:growth & development / Q000648:ultrastructure*; D013087:Spermatids / Q000254:growth & development / Q000648:ultrastructure; D013091:Spermatogenesis; D013094:Spermatozoa / Q000648:ultrastructure*; D014561:Urochordata / Q000648:ultrastructure*