The fate of the mosaic embryo: chromosomal constitution and development of Day 4, 5 and 8 human embryos.
BACKGROUND
Post-zygotic chromosome segregation errors are very common in human embryos after in vitro fertilization, resulting in mosaic embryos. However, the significance of mosaicism for the developmental potential of early embryos is unknown. We assessed chromosomal constitution and development of embryos from compaction to the peri-implantation stage.


METHODS
From 112 cryopreserved Day 4 human embryos donated for research, 21 were immediately fixed and all cells were analysed by fluorescent in situ hybridization (FISH) for chromosomes 1, 7, 13, 15, 16, 18, 21, 22, X and Y. The remaining 91 embryos were thawed, with 54 embryos undergoing biopsy of one or two cells which were fixed and analysed by FISH. Biopsied embryos were kept in standard culture conditions for 24 h. Embryos arrested before cavitation (n = 24) were fixed whereas developing Day 5 blastocysts (n = 24) were co-cultured for a further 72 h on an endometrial monolayer followed by fixation. Cell numbers were counted and all nuclei were analysed by FISH. Data from a previous FISH analysis on cryopreserved good-quality Day 5 blastocysts (n = 36) were also included in the present study.


RESULTS
FISH analysis was successful for 18 Day 4 fixed embryos and, according to our definition, 83% were mosaic and 11% showed a chaotic chromosomal constitution. FISH analysis of two blastomeres from Day 4 developing embryos showed that 54% were mosaic, 40% were normal and 6% were abnormal. Analysis of Day 4, 5 and 8 whole embryos showed a decrease in incidence of mosaicism over time, from 83% on Day 4 to 42% on Day 8. A significant positive correlation was observed between the total cell number and the percentage of normal cells in developing Day 5 and Day 8 embryos but not in developing Day 4 or embryos arrested before cavitation.


CONCLUSIONS
These data suggest that both the developmental arrest of a significant proportion of mosaic embryos on Day 4, and the cell death or reduced proliferation of aneuploid cells within an embryo may be responsible for the observed decrease of aneuploid blastomeres from compaction to the peri-implantation stage.
Santos
Margarida Avo
MA
Department of Reproductive Medicine and Gynaecology, University Medical Centre, Utrecht, The Netherlands. mavoribe@umcutrecht.nl
Teklenburg
Gijs
G
Macklon
Nick S
NS
Van Opstal
Diane
D
Schuring-Blom
G Heleen
GH
Krijtenburg
Pieter-Jaap
PJ
de Vreeden-Elbertse
Johanna
J
Fauser
Bart C
BC
Baart
Esther B
EB
D001755:Blastocyst / Q000648:ultrastructure; D002877:Chromosomes, Human / Q000737:chemistry* / Q000648:ultrastructure; D018920:Coculture Techniques; D046149:Embryo Culture Techniques; D047108:Embryonic Development / Q000235:genetics*; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D009030:Mosaicism / Q000196:embryology*