Rescue of human embryos by micromanipulation.
Review
Overview
abstract
This chapter evaluates clinical micromanipulation techniques aimed at rescuing abnormally developing zygotes and cleaved embryos. First, the possibility of reversing dispermic zygotes to a normal biparental diploid state was evaluated by extracting the distal pronucleus (that furthest from the polar body). The ratio of X:Y was determined in both groups of embryos by assessing a minimum of two blastomeres using duplex PCR or multiple colour FISH. The ratio of embryos containing only an X chromosome and those with X as well as Y chromosomes in the intact dispermic zygotes was 1.0:2.6, which is similar to the theoretical ratio of 1:3. This ratio was 1.0:1.5 in dispermic zygotes from which the distal pronuclei were removed. Although the ratio of X:Y was altered following removal of distal pronuclei, suggesting frequent targeting of male pronuclei, accidental removal of the female pronucleus could not be excluded. In a second set of mouse experiments, it was shown that the hatching process of embryos that develop with excessive amounts of degenerate material is adversely affected. It was shown that removal of such extracellular material by micromanipulation potentially reverted the hatching process. It was also indicated that immediate removal of the degenerate tissue was more beneficial than that following prolonged co-culture. Assisted hatching is probably the most frequently applied clinical embryo micromanipulation procedure. The outcome of assisted hatching is dependent largely on the mode by which the zona pellucida is breached, the size of the artificial gap and the thickness of the zona pellucida. Embryos with zonae thicker than 17 microns rarely implant. Zona drilling could be detrimental in embryos with thin zonae (< 12 microns). Superficial zona thinning has not enhanced implantation. These observations led to a routine procedure called selective assisted hatching, which involves measuring the zonae before zona drilling and replacement on day 3 of development. This appears to be most successful in older women and those with elevated basal FSH levels. Selective assisted hatching is routinely applied in consenting patients whose embryos have thick zonae, slow development or excessive fragmentation (> 20%). Zona drilling of all embryos, regardless of zona thickness, is being performed in patients aged over 40 years and in those with repeated failures or elevated basal FSH levels. Results in the first group of more than 900 patients indicate that nearly one-quarter of human embryos have the ability to implant.
