title: Adhesion between plasma membrane and mitochondria with linking filaments in relation to migration of cytoplasmic droplet during epididymal maturation in guinea pig spermatozoa.
abstract: High-resolution microscopy has been used to investigate the mechanism of the migration of cytoplasmic droplets during epididymal maturation of guinea pig spermatozoa. On testicular spermatozoa, droplets are located at the neck and, after passage through the middle cauda epididymidis, migrate only as far as the center of the midpiece. Initially, the space between the plasma membrane and outer mitochondrial membranes outside the droplet is 30.8+/-11.0 nm, whereas on mature spermatozoa, it significantly (P<0.01) narrows to a more consistent 15.9+/-1.3 nm. This is accompanied by the appearance of thin filaments cross-linking the two membranes above and below the droplet. Changes also occur in the arrangement of intramembranous particles (IMPs) in the plasma membrane overlying the midpiece. At the spermatid stage, linear arrays of IMPs are absent but appear on immature spermatozoa, where they are short with an irregular orientation, in the epididymis. On mature spermatozoa, numerous parallel linear arrays are present at the region where the plasma membrane adheres to the mitochondria. The membrane adhesion process can thus be observed two-dimensionally. The initial migration of the droplet from the neck is probably attributable to diffusion, with the formation of cross-linking filaments between the two membranes in the proximal midpiece preventing any backward flow and squeezing the droplet distally until it is arrested at the central midpiece by the filaments formed in the distal midpiece. The filaments might also stabilize the flagellum against hypo-osmotic stress encountered during ejaculation and within the female tract.
authors.affiliation: Department of Anatomy and Developmental Biology, Graduate School of Medicine, Chiba University, Chiba 260-8670, Japan. fstoyota@cnc.jp
authors.forename: Fumie
authors.initials: F
authors.lastname: Suzuki-Toyota
authors.forename: Chizuru
authors.initials: C
authors.lastname: Ito
authors.forename: Mamiko
authors.initials: M
authors.lastname: Maekawa
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Toyama
authors.forename: Kiyotaka
authors.initials: K
authors.lastname: Toshimori
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000502:physiology* / Q000648:ultrastructure; D003593:Cytoplasm / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D003594:Cytoplasmic Granules / Q000378:metabolism* / Q000502:physiology; D003595:Cytoplasmic Streaming / Q000502:physiology; D003599:Cytoskeleton / Q000502:physiology / Q000648:ultrastructure; D004822:Epididymis / Q000378:metabolism / Q000502:physiology*; D006168:Guinea Pigs; D008297:Male; D008561:Membrane Fusion / Q000502:physiology; D046529:Microscopy, Electron, Transmission; D008928:Mitochondria / Q000502:physiology* / Q000648:ultrastructure; D008954:Models, Biological; D009068:Movement / Q000502:physiology; D013079:Sperm Maturation / Q000502:physiology*; D013094:Spermatozoa / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure