Related Experiment Videos
Fate of microinjected spermatid mitochondria in the mouse oocyte and embryo
J M Cummins1, T Wakayama, R Yanagimachi
1School of Veterinary Studies, Murdoch University, Western Australia. cummins@central.murdoch.edu.au
Abstract:
Mouse round spermatids labelled with MitoTracker were microinjected into Sr(2+)-activated mouse oocytes. The labelled mitochondria were tracked up to the morula/blastocyst stage using fluorescence microscopy. The overall incidence of embryos with labelled mitochondria fell from 80% in the 1-cell zygote to 25% in 2-cell, 9% in 4-cell and approximately 1% in 8-cell or later stages. Thus it appears that almost all round spermatid mitochondria finally disappear from embryos during the 4-cell to 8-cell transition, as happens for mature spermatozoa (Cummins et al. Zygote 1997, 5:301-8). The spermatid mitochondria remained tightly bound together during this process. In contrast, labelled primary spermatocyte and cumulus mitochondria dispersed rapidly throughout the oocyte cytoplasm within 3 h. We hypothesise that spermatid mitochondria may be bound together by cytoskeletal elements produced in the early haploid spermatid. These elements, together with terminal differentiation of the sperm mitochondria, may be central to the processes by which the embryo 'recognises' the sperm mitochondria and inhibits inheritance of paternal mitochondrial DNA. These results suggest that round spermatid injection for infertile men will not pose a significant risk to offspring by transmitting abnormal mitochondrial genomes.
Insights
Round spermatid mitochondria injected into oocytes largely disappear by the 8-cell stage, similar to mature sperm. This suggests round spermatid injection is unlikely to transmit abnormal paternal mitochondrial DNA to offspring.
Area of Science:
- Reproductive Biology
- Mitochondrial Genetics
- Developmental Biology
Background:
- Mitochondrial inheritance in mammals is primarily maternal.
- The fate of paternal mitochondria after fertilization is crucial for understanding inheritance patterns.
- Previous studies indicate mature sperm mitochondria are eliminated from the zygote.
Purpose of the Study:
- To investigate the fate of mitochondria from round spermatids after microinjection into oocytes.
- To determine if round spermatid mitochondria are eliminated during early embryonic development.
- To assess the potential risk of transmitting abnormal paternal mitochondrial DNA via round spermatid injection.
Main Methods:
- Fluorescence microscopy was used to track MitoTracker-labelled round spermatid mitochondria.
- Mitochondria were microinjected into Sr(2+)-activated mouse oocytes.
- Embryos were analyzed at various stages, from 1-cell zygote to morula/blastocyst.
Main Results:
- The incidence of labelled mitochondria decreased significantly from 80% at the 1-cell stage to ~1% by the 8-cell stage.
- Round spermatid mitochondria remained aggregated during elimination.
- In contrast, primary spermatocyte and cumulus mitochondria dispersed rapidly.
Conclusions:
- Round spermatid mitochondria are largely eliminated during the 4-cell to 8-cell transition, similar to mature sperm mitochondria.
- Hypothesized cytoskeletal elements and terminal differentiation contribute to this elimination process.
- Round spermatid injection poses a low risk for transmitting abnormal paternal mitochondrial genomes.