Christopher S. Navara

Mitochondrial sheath movement and detachment in mammalian, but not nonmammalian, sperm induced by disulfide bond reduction

The successful completion of the fertilization process requires the properly choreographed unsheathing of the tightly packaged sperm once it has been fully incorporated into the eggs cytoplasm. The nuclear and accessory structures of mammalian sperm become stabilized by disulfide bonds (S‐S) during epididymal maturation. This stabilization is reversed during fertilization by the reduction of S‐S cross‐linking, but little is known about the effect of S‐S reduction on individual disulfide‐hardened structures such as the sperms connecting piece, fibrous sheath, and mitochondria. Here, we demonstrate the action of the S‐S‐reducing environment on the mitochondrial sheath of mammalian sperm, visualized by the vital fluorescent probe Mito Tracker and by electron microscopy.

The Implications of a Paternally Derived Centrosome During Human Fertilization: Consequences for Reproduction and the Treatment of Male Factor Infertility

PROBLEM: Successful fertilization in humans follows a complex series of events, including the completion of meiotic maturation of the oocyte with the extrusion of the second polar body, the decondensation of the sperm nucleus and the maternal chromosomes into male and female pronuclei, the restoration of the sperm centrosome, and the nucleation of microtubule‐mediated motility necessary to bring the male and female pronuclei into close apposition. These events occur after both fertilization in vitro and after intracytoplasmic sperm injection (ICSI), a new technique which is currently being applied in many clinics to overcome severe male infertility. Defects in any of the events leading to fertilization can be lethal to the zygote and may prove to be causes of infertility.

Mammalian model systems for exploring cytoskeletal dynamics during fertilization.

Publisher Summary This chapter compares and contrasts cytoskeletal features during fertilization among well-studied mammals. Particularly, it compares the data present in the most highly investigated system, the rodents (especially mice), with other mammalian species that have only recently been studied due to advances in in vitro maturation and in vitro fertilization techniques. The appropriate animal models for less tractable systems such as exotic species and humans have been speculated. In terms of cytoskeletal organization and dynamics during fertilization, rodents are atypical mammals and may perhaps be atypical animals. The differences between rodents and other mammals are summarized. The unfertilized rodent oocyte (with the exception of actin filament distribution in rat oocytes) has a polarized distribution of cortical granules, microvilli, and actin distribution. The nonrodent mammals that have been studied in this manner show no polarization of these three components. The meiotic spindle in rodent oocytes is arranged tangentially to the plasma membrane, while the meiotic spindle in nonrodent mammals is smaller and arranged radially with the plasma membrane. Numerous cytoplasmic asters are found in the unfertilized mouse oocyte. These asters elongate and with the cytoplasmic actin filaments are necessary for pronuclear migration. After fertilization in nonrodent mammals, a radial microtubule aster is formed in association with the incorporated sperm head. These microtubules are necessary for pronuclear migration. Prior to mitosis the numerous cytasters in rodent zygotes come together to form the poles for the mitotic spindle which is centrally located. The sperm tail is not associated with this spindle. The sperm aster in nonrodent mammals splits to form two asters, which serve as the poles for the first mitotic spindle that may be eccentric within the cytoplasm. The sperm tail remains associated with one of the poles.

Imaging motility during fertilization.

Studying reproduction in domestic species is now possible at the cellular and molecular level due to advances in the production of large numbers of zygotes and embryos in these species. In this paper we review the microtubule patterns during fertilization in domestic species. These results indicate that domestic species accomplish fertilization in a similar fashion to one another but in a far different fashion from rodents. Recent results indicate that human fertilization is similar to that of domestic species. We discuss the significance this has on the use of domestic species as a model system for human studies and possible consequences for the alleviation of human infertility.