Chapter category: Reproductive Biology
Cytogenetics of Human Preimplantation Embryos
Cytogenetics in Reproductive Medicine
Edited by: Dagan WellsISBN: 1-58706-205-4
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Chapter authors:
J.D.A. Delhanty* and D. Wells
Chromosomal abnormalities are common at all stages of human development and are the main cause of lethality between conception and birth. In natural conceptions the fecundity rate is about 25% while each embryo derived from in vitro fertilisation has on average a 1 in 5 chance of survival. These anomalies may arise during gamete formation, at fertilisation or post-zygotically during embryogenesis. Molecular cytogenetic studies show that 1–2% of sperm are aneuploid; in comparison there is at least a tenfold higher rate in mature oocytes. The addition of the 2% of errors arising at fertilisation (triploidy and possibly X monosomy) combined with data from spontaneous abortions leads to an estimate of 25% of conceptions with chromosome abnormalities. The post-zygotic errors that lead to chromosomal mosaicism in over 50% of 3-day old embryos must be added to those arising at or before conception to give an idea of the overall burden. The chromosomes most likely to be involved in aneuploidy are 15, 16, 21 and 22, while those least likely to be affected are 6, 14, X and Y. The survival potential of fully and mosaic aneuploid embryos can be predicted from data obtained from chromosome analysis of blastocyst stage embryos and of material from spontaneous abortions. Apart from trisomies of chromosomes 1 and 17, most other trisomies are compatible with survival to the stage of recognised pregnancy. Monosomies are far more lethal; only those affecting chromosome 21 and the X allow survival to the late blastocyst stage. The development of mosaic embryos where over 50% of cells are abnormal is affected to such an extent that successful implantation is not possible. Mosaic embryos with low levels of abnormal cells may have the potential to implant if the embryo proper is derived from the normal cells but such conceptions will be at risk of confined placental mosaicism. The incidence and type of post zygotic errors leading to the mosaicism that has been consistently observed in human cleavage stage embryos is totally unlike any observed in cultured somatic cells, suggesting that the mechanisms operating are peculiar to this stage of development and that the normal cell cycle checkpoints are not operating normally during early cleavage. We have hypothesized that prior to activation of the embryonic genome mammalian embryos follow a relatively rigid program of cellular growth and division, governed by a reservoir of maternal mRNA and protein derived from the oocyte. This rigidity may result in cell cycle checkpoints that are more permissive or inactive, leading to predisposition to a range of problems including chromosome malsegregation.
Additional chapters from this book:
Cytogenetics of Human Preimplantation Embryos
J.D.A. Delhanty* and D. Wells
Chromosomal abnormalities are common at all stages of human development and are the main cause of lethality between conception and birth. In natural conceptions the fecundity rate is about 25% w...
Uterine Leiomyomata: Aspects in Reproduction, Susceptibility, and Tumor Cytogenetics
Jennelle C. Hodge and Cynthia C. Morton
Uterine leiomyomata (UL), commonly referred to as fibroids, are the most frequent neoplasm of the female genital tract with such high morbidity as to account annually for one-third of hysterecto...
Preimplantation Genetic Diagnosis (PGD): Screening for Aneuploidy in Human Oocytes and Polar Bodies
J. Navarro,* C. Gutierrez-Mateo, A. Pujol, M. Durban, J.F. Sanchez-Garcia, J. Egozcue and J. Benet
Since it has been reported that in humans 90% of embryos are aneuploid as a result of malsegregation mechanisms in maternal meiosis I,1 the detection of abnormal oocytes in IVF treatments has become...
Cytogenetics of Human Sperm
Renee H. Martin
Constitutional chromosome abnormalities are seen frequently in infertile men. The consequences of these abnormalities in sperm are outlined. Infertile men with a normal chromosome karyotype also hav...
Rapid Prenatal Diagnosis of Common Chromosome Aneuploidies by Quantitative Fluorescent PCR
Vincenzo Cirigliano and Matteo Adinolfi
Prenatal diagnoses of chromosome abnormalities are performed by cytogenetic analyses of metaphase chromosomes obtained from fetal cells after several days of in vitro culture. The main disadvan...
Chromosome Abnormalities in Human Oocytes: Their Role in Reproductive Failure
Michelle Plachot
Chromosome number abnormalities are remarkably common in human reproduction, as a consequence of both whole chromosome nondisjunction and premature chromatid separation during oocyte meiosis I. ...
Prenatal Detection of Chromosome Abnormalities
Brynn Levy and Nataline Kardon
Chromosome abnormalities have been diagnosed from prenatal specimens for almost 40 years. In the United States, the current standard of care in obstetrical practice is to offer either chorionic ...
Chromosome Abnormalities in Spontaneous Abortions
Brynn Levy, Kurt Hirschhorn and Nataline Kardon
Most chromosome abnormalities lead to fetal loss, with the majority of losses occurring very early in gestation. The greater the size of the chromosomal aberration (imbalance), the earlier the f...
