O-186 A novel micro-insemination method, assisted sperm fusion insemination, increases the embryo utilization rate compared to conventional intracytoplasmic sperm injection
Authors: Hatakeyama S , Koizumi K , Kuramoto G , Horiuchi Y , Ohgi S , Yanaihara A 
Published: July 2024

Abstract

Study question
Does a novel micro-insemination method, assisted sperm fusion insemination (ASFI), improve fertilization and embryonic development compared to conventional intracytoplasmic sperm injection (C-ICSI)?

Summary answer
ASFI showed comparable fertilization and blastocyst formation rates to C-ICSI, while achieving significantly higher embryo utilization rates. Furthermore, there were no degenerated oocytes after ASFI.

What is known already
We have reported that ASFI yielded fertilization and blastocyst rates similar to those obtained with rescue ICSI performed at 6 hours after in vitro fertilization (IVF). In ASFI, fertilization is achieved by pressing a zona pellucida (ZP)-bound sperm onto the oocyte membrane without breaking the oocyte membrane. However, there was a risk of artificial polyspermy because the sperm were pressed against the unfertilized oocytes after IVF. Moreover, it is uncertain whether fertilization occurred with sperm that penetrated into the oocyte via IVF or that were pressed against the oocyte membrane. Clarifying this requires performing ASFI of the oocyte without IVF.

Study design, size, duration
We prospectively evaluated the effectiveness of ASFI in a sibling oocyte study including 14 subjects from which at least two metaphase II (MII) oocytes and one immature or degenerated oocyte were retrieved. Between December 2022 and October 2023, 119 MII oocytes were obtained in 22 oocyte retrieval cycles and used in the study. Sibling MII oocytes were randomly assigned to a C-ICSI or ASFI group. The present study was approved by the local ethics committee.

Participants/materials, setting, methods
In the ASFI group, 10,000 motile sperm were incubated with at least one immature or degenerated oocyte for 3 hours in 100 μL of medium per patient. After incubation, the motile sperm bound to the ZP were aspirated directly using an injection pipette. The heads of these ZP-bound sperm were pressed onto the membrane of MII oocytes for 10 seconds using the injection pipette. C-ICSI was conducted using the conventional method.

Main results and the role of chance
The mean and standard deviation of age of the women and number of MII oocytes per cycle were 39.9 ± 2.2 years and 5.5 ± 1.8, respectively. All sperm collected from the ZP adhered to the MII oocyte membrane after ASFI. Furthermore, time-lapse imaging revealed that all sperm adhering to the MII oocyte membrane were subsequently incorporated into the oocyte. Comparison of the ASFI and C-ICSI groups did not reveal any significant differences in 2 pronuclear embryo rate (ASFI group: 85.4% vs. C-ICSI group: 71.8%), degeneration rate (0% vs. 5.6%), good-quality embryo rate at day 3 (63.4% vs. 52.9%), blastocyst formation rate (67.6% vs. 51.9%), or good-quality blastocyst (Grade 3BB and above by the Gardner criteria) formation rate (29.4% vs. 15.4%). On the contrary, the embryo utilization rate (defined as the total number of embryos transferred and cryopreserved divided by the number of MII oocytes) was significantly higher in the ASFI group compared to the C-ICSI group (47.9% vs. 25.4%, P &lt; 0.05).

Limitations, reasons for caution
This study was limited by using the ZP for sperm selection. To expand its applicability, a method to identify sperm capable of fusing with an oocyte without using the ZP must be established. Additionally, since our sample size was small, larger studies are required to confirm our findings.

Wider implications of the findings
ASFI is expected to enhance the survival rate of oocytes and to increase the number of embryos available for implantation, not only compared to rescue ICSI but also C-ICSI. ASFI could aid in understanding fertilization mechanisms by enabling direct observation of oocyte-sperm fusion during the process.

Trial registration number
UMIN000050184