Trophectoderm specification: compaction, polarisation and inner and outer cells occur simultaneously in the human preimplantation embryo

Research output: Unpublished contribution to conferenceUnpublished abstract

Abstract

Study question:
How is the trophectoderm lineage specified in the human preimplantation embryo?
Summary answer:
Outer cells at the fully compacted embryo stage display characteristics of trophectoderm cells such as
apical polar distribution of p-ERM and nuclear GATA3/YAP1/TEAD4 protein expression.
What is known already:
In mice, the first indication of inner cell mass (ICM) and trophectoderm (TE) segregation occurs three
days post fertilisation (dpf3). After compaction, asymmetric divisions give rise to inner and outer
blastomeres that ubiquitously express transcription factor TEAD4 and sequester polarity markers (p-
ERM), and TE lineage specifiers (YAP1, GATA3) to outer blastomeres. This is considered the onset of TE
lineage segregation in the mammalian embryo. Studies regarding this event in the human embryo are
absent and extrapolations from the mouse model cannot be a surrogate for studies in the human
embryo.
Study design, size, duration:
Human embryos used in this study, were donated to research as a surplus of IVF-ICSI treatment at our
IVF-center after the cryo-storage expiration date of 5 years with informed consent. The project was
approved by the institutional ethical committee and the federal committee for research on human
embryos.
Participants/materials, setting, methods:
Cryopreserved human 8-cell stage embryos, donated to research, or zygotes created for research after
informed consent were warmed (Vit Kit -Thaw, Irvine Scientific, USA) and cultured until dpf5 according to
standard laboratory procedures of the associated IVF-clinic. Time course immunofluorescence was
performed on fixed embryos at predefined stages for TEAD4, YAP1, GATA3, p-ERM as well as F-actin and
Hoechst for imaging (LSM800, ZEISS) and the manual estimation of individual blastomere counts per
embryo (ImageJ).
Main results and the role of chance:
The compaction process of the human embryo is interrupted by cleavage divisions this results in a fully
compacted embryo at dpf4.These fully compacted embryos are comprised of a total of 16±3 cells, n=21,
of which 76%, n=16/21 contain 1±1 inner blastomere in their core excluded from the exterior. The nuclei of
the outer blastomeres are positive for TE-specifier YAP1 (69±22%, n=8) and begin to co-express TEtranscription
factor GATA3 (15±22%, n=8). The apical membranes of the outer blastomeres appear
polarised by p-ERM staining (100%, n=8) and TEAD4 is expressed ubiquitously (100%, n=8). While inner
cells also express TEAD4 they remain apolar and rarely express YAP1. This inner/outer pattern is
maintained in the blastocyst ICM and TE. All nuclei of TE cells of dpf5 blastocysts are positive for
YAP1/GATA3 and TEAD4 and apical membranes of TE cells show polar distribution of p-ERM. Whereas
the ICM remains apolar, TEAD4 positive, GATA3 negative, and very rarely contains YAP1 positive cells.
Limitations, reasons for caution:
In spite of available functional data on the interaction between YAP1 and TEAD4 and their effect on
GATA3 expression and embryo development in the mouse and the bovine model, functional experiments
in the human embryo still need to confirm the link between polarity, nuclear TEAD4/YAP1/GATA3, TE
specification and blastocyst formation.
Wider implications of the findings:
The same molecular determinants are involved in the TE lineage segregation of mice and humans, but
their expression differs according to the morphological development of the respective species. In
contrast to the stepwise model o
Original languageEnglish
Number of pages1
Publication statusPublished - 2020
EventESHRE 36th Annual meeting : ESHRE 2020 - Bella Center, Copenhague, Denmark
Duration: 5 Jul 20208 Jul 2020
Conference number: 36th

Conference

ConferenceESHRE 36th Annual meeting
CountryDenmark
CityCopenhague
Period5/07/208/07/20

Keywords

  • Embryology
  • DIFFERENTIATION

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