Intracytoplasmic sperm injection of bovine oocytes with stallion spermatozoa.

The research article discusses a series of 5 experiments designed to evaluate the fertilization & development of bovine, or cow, eggs using a process called intracytoplasmic sperm injection (ICSI) with horse sperm.

Experiment 1: Time for Pronuclear Formation

• The research started off by determining how long it takes for pronuclei (the nuclei of sperm and egg cell after fertilization) to form after ICSI.
• The scientists found that the decondensation (uncoiling) of horse sperm head, a critical step prior to forming the male pronuclei (MPN), starts 3 hours after ICSI.
• 42% of them were decondensed by 6 hours and MPN started forming from 12 hours.
• Interestingly, female pronuclei (FPN) formed as early as 6 hours after ICSI.

Experiment 2: Activation of ICSI Eggs

• The researchers then used various chemical treatments to try to ‘activate’ the eggs after ICSI.
• None of the eggs started to divide (cleaved) after treatment with a chemical called thimerosal.
• Treatment with ionomycin resulted in about 29 to 48% cleavage rates, indicating partial success.
• But when ionomycin was combined with another chemical called 6-dimethylaminopurine (DMAP), higher rates of cleavage, morula and blastocyst (early stages in embryonic development) were seen.

Experiment 3: Variability among Stallions and Chromosome Analysis

• The team then wanted to see if there was any difference in success rates between individual stallions, but the results were similar.
• Importantly, the genetic makeup of the resultant blastocysts was bovine, not a hybrid of bovine and horse, indicating the embryos were parthenogenetic (unfertilized).

Experiment 4: MPN and FPN Formation Synchronicity

• In their fourth experiment, the researchers attempted to manipulate the eggs to form MPN and FPN at the same time.
• Despite their best efforts, however, they were only partly successful, and the subsequent rates of embryonic development were low.

Experiment 5: Testing the Fusion of Capacitated Equine Sperm and Bovine Oolemma

• In the final experiment, they wanted to see if ‘capacitated’ (fully matured and ready to fertilize) horse sperm could fuse with the egg’s outer layer (oolemma).
• The horse sperm could indeed penetrate the egg’s outer layer and successfully instigate development up to the 8-cell stage, but then it stalled.

The researchers concluded that while horse sperm can certainly infiltrate bovine eggs and begin the fertilization process, the subsequent embryo appears to be exclusively bovine, not a hybrid. This indicates that although fertilization initiates, it doesn’t progress normally, leading to unfertilized (parthenogenetic) embryonic development.