Q from U: Embryo Fragmentation

This week, a reader contacted me regarding fragmentation in her embryos. She was told by her RE that things were looking “good” but then he also mentioned that some of the embryos on day 3 were showing some signs of fragmentation. Her question was how concerning was this observation that her embryos had some fragmentation?

Research findings:

The article , “Human embryo fragmentation in vitro and its implications for pregnancy and implantation”, published by Richard Scott MD, Jacques Cohen PhD and colleagues looked at the question of how much and what kind of embryo fragmentation was significant in terms of hampering further development. They found that very large fragments were more problematic than more smaller fragments. Jacques Cohen is one of the  pioneers in micromanipulation techniques and his team promoted the practice of fragment removal on day 3 of development. Their experience described in this paper suggested that embryos with a wide range of fragmentation (as low as 6% to as much as 35%) could experience equivalent implantation rates after fragment removal.

Mina Alikani, PhD, a member of the Scott/Cohen clinical research team published a comprehensive review of the implications of embryo fragmentation, entitled  “Pathogenesis, developmental consequences, and clinical correlations of human embryo fragmentation”, Fertility and Sterility Volume 95, Issue 4, Pages 1197-1204, 15 March 2011.

Summary of the main points of her paper:

Fragmentation is a cellular process that generates cell fragments that do not contain an nucleus during early embryo development. Fragmentation is inversely correlated with implantation; meaning that the more fragmentation you have, the worse the prognosis for implantation. Because of its potential significance for implantation, the percent of embryo fragmentation is a standard component of various embryo grading systems. Although it can be correlated with poor developmental potential, it is important to remember that fragmentation is commonly observed for a majority of human embryos in culture.

Based on time-lapse photography studies, fragmentation occurs during the process of cell division and fragment reincorporation into other cells has infrequently been observed, suggesting that in some cases, it may even be a normal part of cell division. However, it is also well established that moderate to extensive fragmentation is associated with chromosomal abnormalities and mosaicism. Chromosomal abnormalities in this context usually means the condition of an incorrect number of chromosomes after cell division, aka. aneuploidy.

Mosaicism refers to the situation in which different numbers of chromosomes are present in different cell lineages in the embryo. For instance, if one cell of eight on day 3 finds itself with an incorrect number of chromosomes, this abnormality may be passed on to all the future daughter cells it gives rise to with successive divisions, creating an embryo with a mix of normal and abnormal cell lines. Mosaicism has been an issue for determining whether an embryo is normal based on genetic testing of a single cell, because when mosaicism is present, the single cell tested may not be representative of the rest of the embryo, leading to a misdiagnosis.

Fragmentation reduces the total functional cell numbers in either the trophectoderm or inner cell mass of the embryo. Lower  numbers of cells in these vital compartments may reduce the embryo’s implantation potential. In addition, if fragments lay between the embryo and the zona pellucida, they have the potential to interfere with normal hatching of the embryo.

The following types of fragmentation patterns have been described in human embryos:

  • Type 1: few small fragments typically associated with only one blastomere;
  • Type 2: many small and localized fragments associated with one or more cells; may result in fewer cells overall
  • Type 3: small and scattered fragments associated with multiple cells;
  • Type 4: large and scattered fragments associated with several unevenly sized cells; may result in fewer cells overall

A localized pattern of fragmentation is more conducive to fragment removal by a technician trained in this micromanipulation technique. Fragments scattered among and in between other cells are impossible to remove and one paper suggests this pattern of scattered fragmentation may be associated with a higher incidence of chromosomal abnormalities.

There is a mix of inconclusive studies in the literature regarding what effect, if any, fragmentation has on clinical outcomes. Some published studies found an association with increased fragmentation and miscarriage, compelling the authors to recommend that patients who only have severely fragmented embryos (50% or greater) be counseled regarding their increased risk for miscarriage.

Some researchers have suggested that poor egg quality in regards to the egg’s nuclear and cytoplasmic competence for development may be a cause of increased fragmentation. Poor egg quality could result from inherent deficiencies in mitochondrial DNA content or suboptimal follicular environment at the level of the granulosa cells which are vital for guiding egg maturation. Some studies suggest that stimulation protocols may be a contributing factor to poor egg quality resulting in increased fragmentation but these studies are still inconclusive.

Cellular mechanisms for fragmentation are also poorly understood and several possible pathways have been suggested including programmed cell death (apoptosis), defects in the microtubule and cytoskelatal framework of the cell that orchestrates the mechanical movements that happen during cell division, reactive oxygen species involvement and faulty cholesterol metabolism. More research is needed.

The graphic summary  below (copied from Alikani et al, March 2011) summarizes the correlations between embryo fragmentation, developmental milestones and clinical outcomes. Which of these correlations are causative requires further investigation but the underlying message is clear: A little localized fragmentation is well tolerated by the embryo and may even be normal. A great deal of fragmentation can have severe developmental consequences and if it can be minimized by improved stimulation protocols or technical defragmentation, that is probably beneficial.

 

© 2012, Carole. All rights reserved.

©2012 Fertility Lab Insider. All Rights Reserved.

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