An embryo developed from an ”abnormally” fertilized embryo was tested chromosomally normal. Embryo transfer resulted in a healthy live birth.
For a couple undergoing a fertility treatment at Ovumia, only one of the oocytes appeared to have fertilized normally. The transfer of the embryo developing from this did not lead to pregnancy. Each of the remaining four embryos had developed from an oocyte that had either three (3PN) or four (4PN) pronuclei instead of the normal two pronuclei (2PN). Hence it was assumed that embryos developing from these might have one or two extra copies of each chromosome, i.e. a total of 69 or 92 chromosomes instead of the normal 46. However, the progress of fertilization and embryo development looked normal in the video monitoring, so it was possible that one or more embryos could have developed from an egg with a normal number of chromosomes. The decision was to do a chromosome test for the abnormally fertilized embryos. The test result was in line with our assumption: the embryo developed from the oocyte with four pronuclei was chromosomally completely normal. The healthy baby boy born as a result of embryo transfer of this embryo is now a brisk 5-year-old young man.
This is the first report of a healthy offspring following transfer of an embryo developed from an oocyte with four pronuclei. The case report has been published in Human Reproduction and there is a link to the article at the end of this page.
The fertilization of oocytes is checked microscopically the morning after fertilization in the embryo laboratory. The presence of two pronuclei in the oocyte is considered a prerequisite for normal fertilization. In the presence of two pronuclei it can be assumed that each of the 23 chromosomes appears in pairs in the oocyte (so-called diploid), half of which originate from the sperm and half from the oocyte. However, in about five percent of fertilized oocytes, an abnormal number of pronuclei is seen, which indicates a deviation from the normal number of two sets of chromosomes, i.e. 46 chromosomes. Often three pronuclei are seen, in which case the assumption is that the embryo developing from the oovyte has 69 chromosomes. Transferring such a triploid embryo can lead to pregnancy, but with fatal consequences. In order to avoid risks, embryos developed from abnormally fertilized eggs have as a rule been rejected until these days.
Oocytes the morning after fertilization. Two pronuclei (2PN) can be seen in the middle of the oocyte on the left. The assumption is that the chromosome number of the embryo is normal 46, half of spermatozoa origin and half of oocyte origin. Three pronuclei (3PN) can be seen in the middle oocyte, so it can be suspected that it has an extra set of chromosomes, i.e. a total of 69 chromosomes. The oocyte on the right has only one pronucleus (1PN), in which case the oocyte is assumed to have only 23 chromosomes.
In recent years embryos developing from abnormally fertilized eggs have been studied in more detail. It has been discovered that many of these are actually chromosomally normal. The proportion of normal embryos varies depending on the fertilization method and the number of pronuclei. For example, 40-50 % of the embryos developed from oocytes fertilized by microinsemination (ICSI) are normal. However, with conventional in vitro fertilization (IVF), the proportion of normals is clearly lower. Children are known to have been born from embryos that were abnormally fertilized, but whose chromosomes were tested as normal in the embryonic stage.
Chromosomally normal embryos have thus been rejected based on microscopic findings. However, the proportion of abnormally fertilized but normal embryos is small, and rejecting embryos with abnormal number of pronuclei does not often affect the outcome of the treatment. The importance can still be crucial in treatments where the desired result is not achieved with other embryos, and in treatments where, based on microscopic examination, only abnormally fertilized eggs are found.
In treatments where only one or more abnormally fertilized eggs are available, chromosome testing of the embryos can be considered. Due to the test costs, it would be good to get an estimate of whether the tests are is worth doing. The probability of finding a normal embryo depends not only on the number of pronuclei and the fertilization method, but also on the quality and growth rate of the embryo. It has been observed that if embryos developing from 3PN oocytes are diploid (chromosomally normal), they develop on average faster than those embryos that were triploid. Video monitoring of the fertilization event can also give indications that an abnormal-looking fertilization may indeed be normal. For example, the differences in the size of the embryos may give an indication of how many chromosomes there are in the oocytes. Indeed, the observed pronuclear size differences was the most significant reason why we decided to test the embryos described in the case report.
Case report oocyte 19 hours after fertilization. The pronucleus indicated by the black arrow contains sperm-derived chromosomes and the pronucleus indicated by the white arrow contains oocyte-derived chromosomal material. The smaller size of the oocyte-derived pronuclei raised the assumption that there are no extra sets of chromosomes, but that the oocyte-derived 23 chromosomes were divided into these three small pronuclei.
A more detailed investigation of the possibilities of embryos developing from abnormally fertilized eggs would improve treatment results in some situations. Our recently published case report will hopefully encourage further research on abnormally fertilized oocytes.
Peter Bredbacka, Antonio Capalbo, Kirsi Kananen, Ludovica Picchetta, Candido Tomás (2023): Healthy live birth following embryo transfer of a blastocyst of tetrapronuclear (4PN) origin: a case report, Human Reproduction, Volume 38, Issue 9, September 2023, Pages 1700–1704. https://doi.org/10.1093/humrep/dead151
Preimplantation chromosome testing (PGT-A, niPGT-A)
Pre-implantation genetic diagnosis (PGT‑M, PGT‑SR)