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Infertility article review and discussion

Title: Treatment-related chromosomal abnormalities in human embryos

Authors: S. Munne, C. Magli, A. Adler, G. Wright, K. de Boer, D. Mortimer, M. Tucker, J. Cohen and L. Gianaroli

Source: Human Reproduction April, 1997 (Vol. 12) Pages 780-784.

Definitions:

Aneuploidy is a condition where cells have abnormal number of chromosomes. An example of aneuploidy is trisomy 21 in which there are 46 normal chromosomes and one extra chromosome number 21 for a total of 47.

Polyploidy is a condition where cells have an extra set of chromosomes. For example, triploid embryos have 3 sets of 23 for a total of 69 instead of the normal 2 sets of 23 for a total of 46.

Mosaicism is a condition where some cells in a tissue have a different chromosomal complement as compared to other cells in the same tissue or organism. For example, a 4-cell embryo can have 2 cells that have 46 normal chromosomes and 2 cells that have 46 normal chromosomes and an extra chromosome number 18 for a total of 47.

Background

It is known that in vitro cultured human embryos have a high incidence of abnormal chromosomal content.

The proportion of chromosomally abnormal embryos (and pregnancies) is increased as the age of the female increases.

Polyploidy and mosaicism is increased in embryos that have stopped dividing (arrested) and those that look abnormal under the microscope (morphologically abnormal).

This study tried to determine whether embryos from patients that had undergone ovarian stimulation with different drug regimens, from different IVF centers, handled and cultured under different conditions, etc., have different rates of chromosomal mosaicism.

How the study was done

This study was performed on 138 donated spare human embryos from 4 different IVF centers.

Each cell in the embryos was analyzed using a technique called FISH to determine its karyotype (chromosomal content).

From the relative proportions of cells in the individual embryo that were normal or mosaic, it was determined at which cell division the abnormality arose.

Results

34 of 138 embryos (24.6%) were found to be mosaic and 12 of 138 (8.7%) were aneuploid.

There were significant differences in the rates of mosaicism from the 4 different centers. These figures ranged from 11% to 52%.

There were also differences in the rates of mosaicism based on the type of ovarian stimulation used. The highest rates were seen with clomiphene citrate regimen and lower rates were found with down-regulated gonadotropin regimens.

Comments about the study

This is a fascinating study of chromosomal anomalies in human in vitro cultured embryos. Studies like this are quite difficult to perform because of the labor-intensive and costly work that is required, and because of the rarity of human embryos that have been donated for research.

The authors point out that even though the rates of mosaicism varied significantly between the different IVF centers, the pregnancy rates from the centers did not correlate with mosaicism rates. There are many other important factors involved in establishing ongoing pregnancies, which makes it very difficult to single-out individual aspects and determine how much that factor contributes to the final outcome.

The authors also speculated that the high rate of mosaicism from the first cellular division from one center may have been related to the lack of a heated stage on the microscope at that center during the study period. This is an interesting possibility, although impossible to prove.

More and larger studies of this nature are needed.