Human somatic cells are diploid, they normally have 23 pairs of chromosomes (46 chromosomes), in which all the genes that make up the genome are distributed. Sex cells (eggs and sperm) are haploid and only contain half as many chromosomes.
A cell that contains a normal chromosome load is known as euploid and when it has an abnormal number of chromosomes due to gains or losses in one or more chromosomes, it is called aneuploid. When the abnormal number results in a number of chromosomes less than 46 it is known as monosomy, while when there is a gain of chromosomes it is known as trisomy (more than 46).
Most chromosome duplication and deletion errors originate during gametogenesis, mainly in oogenesis (during meiosis I), while another considerable percentage of errors arise during the first mitosis after fertilization.
Around 50% of all embryos produced through assisted reproduction techniques are aneuploid, the rate of aneuploidy increases as maternal age increases. From the age of 37, a higher percentage of aneuploid embryos is produced.
The trisomies most frequently found in IVF embryos are 16, 19 and 21, while monosomy 22 and partial loss of the Y chromosome are the most common monosomies. All monosomies (except X chromosome monosomy or Turner syndrome) are incompatible with life; However, all trisomies can give rise to a baby with serious health problems. The type of involvement, phenotypic characteristics and life expectancy of the affected individual will depend on the type of trisomy. Trisomy 21, for example, a condition that gives rise to Down syndrome and one of the most frequent trisomies (1 case in 700 occurs), is characterized by a phenotype prone to congenital heart disease, mental retardation of varying degrees, and a long life expectancy. average of 50 years.
Around 35% of all implantation failures that occur in assisted reproduction treatments are due to the transfer of aneuploid embryos. Additionally, in a study carried out at our Institute, it was determined that a mostly aneuploid embryonic cohort negatively affects the implantation result.
Recommending and carrying out Preimplantation Genetic Diagnosis (PGD Ingenes) for all embryos achieved through IVF to know their chromosomal profile will avoid the transfer of aneuploid embryos. This behavior will increase the chances of implantation and pregnancy. PGT should be performed on all embryos during In Vitro Fertilization, especially if the woman's age is over 37, or if the couple has a family history of aneuploidies.
Sources
Hassold, T., and Hunt, P. (2001). To err (meiotically) is human: the genesis of human aneuploidy. Nature reviews Genetics 2, 280-291.
Ramia, M., Musharrafieh, U., Khaddage, W., and Sabri, A. (2014). Revisiting Down syndrome from the ENT perspective: review of literature and recommendations. European archives of oto-rhino-laryngology : official journal of the European Federation of Oto-Rhino-Laryngological Societies 271, 863-869.
Rubio, C., Bellver, J., Rodrigo, L., Castillon, G., Guillen, A., Vidal, C., Giles, J., Ferrando, M., Cabanillas, S., Remohi, J., et al. (2017). In vitro fertilization with preimplantation genetic diagnosis for aneuploidies in advanced maternal age: a randomized, controlled study. Fertility and sterility 107, 1122-1129.
