Defective sperm head decondensation undermines the success of ICSI in the bovine

The efficiency of intracytoplasmic sperm injection (ICSI) in the bovine is low compared to other species. It is unknown whether defective oocyte activation and/or sperm head decondensation limit the success of this technique in this species. To elucidate where the main obstacle lies, we used homologous and heterologous ICSI and parthenogenetic activation procedures. We also evaluated whether in vitro maturation negatively impacted the early stages of activation after ICSI. Here we showed that injected bovine sperm are resistant to nuclear decondensation by bovine oocytes and this is only partly overcome by exogenous activation. Remarkably, when we used heterologous ICSI, in vivo-matured mouse eggs were capable of mounting calcium oscillations and displaying normal PN formation following injection of bovine sperm, although in vitro-matured mouse oocytes were unable to do so. Together, our data demonstrate that bovine sperm are especially resistant to nuclear decondensation by in vitro-matured oocytes and this deficiency cannot be simply overcome by exogenous activation protocols, even by inducing physiological calcium oscillations. Therefore, the inability of a suboptimal ooplasmic environment to induce sperm head decondensation limits the success of ICSI in the bovine. Studies aimed to improve the cytoplasmic milieu of in vitro-matured oocytes and to replicate the molecular changes associated with in vivo capacitation and acrosome reaction will deepen our understanding of the mechanism of fertilization and improve the success of ICSI in this species.

Sperm capacitation pretreatment positively impacts bovine intracytoplasmic sperm injection

The efficiency of intracytoplasmic sperm injection (ICSI) in bovines is low compared to other species due in part to inadequate egg activation and sperm nucleus decondensation after injection. We hypothesized that this low efficiency is due to the lack of complete sperm capacitation, so we evaluated the effects of isobutylmethylxanthine (IBMX) and methyl-β-cyclodextrin (MβCD) on bovine sperm capacitation and on the preimplantation developmental potential of bovine embryos generated by ICSI. Treatment with IBMX and MβCD decreased sperm viability (between 13-30%); nevertheless, 0.4 mM IBMX and 1 mM MβCD increased (p < 0.05) capacitation metrics-that is, acrosome exocytosis, intracellular calcium level, plasma membrane fluidity, and tyrosine phosphorylation-compared to the control. After ICSI, embryos injected with IBMX- and MβCD-treated sperm showed similar cleavage to the untreated group (range 82-88%). Pronucleus formation rate was higher with MβCD-pretreatment (54%) compared to the control group (25%), and blastocyst rate was significantly improved with MβCD-pretreatment (24%) compared to the IBMX (18%) and control (17%) groups. Importantly, embryo quality-as assessed by the total number of cells, cell allocation, and apoptotic cell index-was not affected by the sperm treatments. In conclusion, MβCD pretreatment of sperm improved the efficiency of blastocyst production in bovine ICSI.