Abnormal distribution of chromosomes in the first division of nuclear transferred mouse embryos

Area of Science:

• Reproductive Biology
• Developmental Biology
• Cell Biology

Background:

• Somatic cell nuclear transfer (SCNT) is a technique used in assisted reproduction, but SCNT embryos often fail to develop to term.
• Previous research has focused on early nuclear remodeling and cytoskeletal changes in SCNT embryos.
• The developmental status of SCNT embryos beyond the first cell division, specifically at the 2-cell stage, remains poorly understood.

Purpose of the Study:

• To investigate and compare the morphological status of chromosomal division in somatic cell nuclear transferred (SCNT) embryos and intracytoplasmic sperm injection (ICSI) embryos from the 1-cell to the 2-cell stage.
• To identify potential causes for the high failure rate of SCNT embryos during early development.

Main Methods:

• Comparison of chromosomal division and spindle formation in SCNT and ICSI embryos during the transition from the 1-cell to the 2-cell stage.
• Assessment of nuclear morphology, including fragmentation, in both SCNT and ICSI embryos.
• Utilization of TdT-mediated dUTP-biotin Nick End Labeling (TUNEL) assays to detect apoptotic DNA fragmentation.

Main Results:

• While the initial mitotic spindle formation was normal in both SCNT and ICSI embryos, microtubules and cytoplasmic asters disappeared at the pronuclear stage.
• Nuclear fragmentation was observed in a significant percentage of 2-cell SCNT embryos (30%) compared to ICSI embryos (12%).
• Chromosome distribution was disturbed during the 1-cell to 2-cell stage division in both SCNT and ICSI embryos, with no evidence of apoptosis.

Conclusions:

• Abnormal chromosome distribution during the first mitotic division (1-cell to 2-cell stage) is a key feature of SCNT embryos.
• The observed nuclear fragmentation and disturbed chromosome segregation at the 2-cell stage likely contribute to the high mortality rates of SCNT embryos before or after implantation.