Knowledge Sharing

2013.02.07

FISH (FluorescenceInSituHybridization) and selective implantation of embryos

Researcher of Genetic Diagnosis Lab, CAO HUI-MEI

Do you think rebellion is restricted to teenagers? Do you think it's very unique to argue the toss? You can never imagine we scientists are against the Gods to change the fate he arranged for us.

Don't’ be ignorant, you have to know this! When everyone is talking about "FISH", don't doubt why fishing rods are not used ...

Do you believe God is a big naughty boy? It is said that humans have 23 pairs of chromosomes, but God will occasionally play tricks and make chromosomes abnormal, such as

  1. Numerical abnormalities:more or less chromosomes, the most common is that there is trisomy of chromosome 21, causing Down's syndrome
     
  2. Structural abnormalities:translocations, inversions, deletions, duplications, insertions, amplifications, ring chromosomes, isochromosomes, etc
     
FISH (fluorescence in situ hybridization) is a fast and precise analysisof chromosome copy or structure at metaphase or interphase. By labeling with florescent materials, specific nucleic acid sequences can hybridize with homologous segment on target chromosomes, and is a recombinant deoxyribonucleic acid (DNA) technology for chromosomal examination. Under fluorescence microscope, the specimen will be as colorful and charming as the neon signs.

The essential steps of FISH are as follows:
  1. Sample preparation and fixation on the slide(fix)
  2. Dehydration in graded series of alcohol(dehydrate)
  3. Denaturization of sample and probe DNA(dena ture)
  4. Hybridization of sample and probe DNA(hybridize)
  5. wash
  6. counterstain
  7. Observe the results under a florescence microscope
     
With the skilled scientists, sperms and oocytes of infertile couples can meet romantically in vitro andunion into fertilized eggs. However, depending on the developmental potential, there are several types of division resulted from fertilized egg to the embryo:
 
  1. Grade A- symmetrical equal size blastomeres/li>
  2. Grade B-symmetrical equal size blastomeres, but with some cytoplasmic fragments
  3. Grade C -different size blastomeres, <20%cytoplasmic fragments
  4. Grade C+ -different size blastomeres, >20%cytoplasmic fragments
  5. Grade D - The embryo is in a mess and can't figure out whether it is a cell or a fragment

FISH analysis revealed chromosome abnormalities in a high proportion of embryos below grade C, so obstetrician and gynaecologistprefer grade A embryos for transfer.Despite perfect morphology, some grade A embryos may contain genetic defects which could not be differentiated simply by morphology.

Recently developed embryo biopsy technology, i.e, preimplantation genetic diagnosis, allows scientists to extract 1 to 2 cells for genetic diagnosis of 6- to 8-cell stage embryos. The extended application of FISH on fixed nuclei of biopsied cells have take one step further to give birth to the healthiest baby.