What happens to abnormally fertilized embryos? A scoping review
Reproductive BioMedicine Online29 May 2023
Embryos derived from oocytes lacking the typical two pronuclei (2PN), a sign of normal fertilization, show different development potentials, according to a comprehensive review of the literature by James Kemper, Yanhe Liu, Masoud Afnan, Ben Mol & Dean Morbeck. The analysis focused on embryos resulting from oocytes with no pronuclei (0PN), a single pronucleus (1PN), or three pronuclei (3PN), collectively termed non-2PN oocytes. The study revealed a significant gap in scientific knowledge about these non-standard fertilization outcomes.
In a standard In Vitro Fertilization (IVF) procedure, the presence of two pronuclei and two polar bodies in an oocyte signifies a normally fertilized egg. However, a scarcity of research exists on embryos derived from oocytes that do not adhere to this norm. Despite their rarity, these non-2PN oocytes could potentially increase the pool of embryos eligible for transfer, providing hope for couples who lack high-quality embryos.
The literature review, which considered 33 eligible articles, found that while non-2PN oocytes occur less frequently, a significant proportion of them stop developing between day 1 and day 6. As a result, they show a reduced chromosome integrity and diminished clinical utility compared to their 2PN counterparts. The review also identified that blastocyst rates were lower in 1PN oocytes (68.3% versus 32.2%), and that larger 1PN oocytes have a better developmental potential compared to smaller ones. Blastocysts from 1PN oocytes appeared to have a slightly reduced implantation potential and a reduced ongoing pregnancy rate when compared to 2PN blastocysts (33.3% versus 35.9%). While 13 of the included studies reported live birth rates, the results varied significantly, with rates ranging from 0 to 66.7%, pointing to a notable heterogeneity in practices.
The study underlines the pressing need for additional research to improve understanding and interpretation of non-2PN oocytes. The authors suggest that advancements in technology such as time-lapse imaging and artificial intelligence algorithms could provide valuable insights into the development and potential viability of embryos from non-2PN oocytes.
The study's findings challenge the conventional guidelines that only zygotes with two pronuclei are considered diploid, signifying a significant change in perspective and underscoring the complexity and dynamism of human reproduction. Further studies will be required to confirm these findings, but the potential for new advances in this area holds great promise.
[Note: This study has not been peer reviewed by IVF.net]