Cytoskeleton and chromatin reorganization in horse oocytes following intracytoplasmic sperm injection: patterns associated with normal and defective fertilization.

The research article investigates the reorganization of cytoskeleton and chromatin inside horse oocytes after undergoing intracytoplasmic sperm injection (ICSI). The study further explores the causes of low embryo development rates despite successful ICSI practices and presents insights about the changes that occur post-ICSI and their association with successful and faulty fertilization.

Objective and Methodology

• The aim of the study was to scrutinize the reorganization of the cell’s internal framework, the cytoskeleton, and chromatin (the genetic materials enclosed within the nucleus) in horse oocytes that were fertilized in-vitro using ICSI.
• Additionally, the researchers aimed to study which structures the sperm contributes to in the zygote, particularly the centrosome—a key cell structure involved in cell division.
• The researchers used sperm labeled with a mitochondrion-specific dye to facilitate this analysis.
• Fixed intervals after the sperm was injected, the oocytes were surveyed using confocal laser scanning microscopy.

Oocytes Pre and Post-ICSI

• Before the injection of sperm, microtubules were present only in specific regions: the metaphase-arrested second meiotic spindle, and the first polar body.
• Post-injection, a star-shaped array of microtubules was seen forming near the sperm head, enlarging with time and filling the entire cytoplasm at the time of pronuclear migration and apposition.

Reorganization of Microtubules

• During syngamy, the joining of gametes, the microtubule array reorganizes to form a mitotic spindle—an essential part of cell division—onto which the chromatin of both parents aligns.
• Once cell division is finished, the microtubules spread into the interphase daughter cells.

Incomplete Fertilization

• It was observed that 25% of sperm-injected oocytes failed to complete fertilization, mainly due to a lack of activation in the oocyte (65%), typically coinciding with a failure of sperm decondensation.

Conclusion

• The study concluded that the unification of parental genomes in horse zygotes is accompanied by a series of cytoskeleton-facilitated events. When these events fail, it results in developmental arrest, providing insight into reasons for low horse embryo development rates after ICSI.