Fetal Cells Can Be Traced Back to the First Day of Embryonic Development

Amazing stuff! Wonders of life!

"... a new study ... demonstrates that when human embryos are composed of two cells, at just 1 day old, only one of these cells will create most of the fetal body cells in addition to placental cells, while the other will only create placental cells. The research changes the long-standing paradigm that the two cells at this stage both contribute equally to all parts of the developing embryo, suggesting that "specification"—the phenomenon of cells having specific individual roles—happens much earlier in development than previously believed. ..."

From the highlights and abstract:

"Highlights

• Lineage tracing of human embryos from the 2-cell to the blastocyst stage

• The majority of the epiblast is derived from only one 2-cell stage blastomere

• Early asymmetric divisions are a bottleneck controlling the embryo’s clonal composition

• First-dividing 2-cell blastomere generates more asymmetric divisions at 8-cell stage

Summary

Retrospective lineage reconstruction of humans predicts that dramatic clonal imbalances in the body can be traced to the 2-cell stage embryo. However, whether and how such clonal asymmetries arise in the embryo is unclear. Here, we performed prospective lineage tracing of human embryos using live imaging, non-invasive cell labeling, and computational predictions to determine the contribution of each 2-cell stage blastomere to the epiblast (body), hypoblast (yolk sac), and trophectoderm (placenta). We show that the majority of epiblast cells originate from only one blastomere of the 2-cell stage embryo. We observe that only one to three cells become internalized at the 8-to-16-cell stage transition. Moreover, these internalized cells are more frequently derived from the first cell to divide at the 2-cell stage. We propose that cell division dynamics and a cell internalization bottleneck in the early embryo establish asymmetry in the clonal composition of the future human body."

Fetal Cells Can Be Traced Back to the First Day of Embryonic Development - www.caltech.edu Though over 8 million babies have been born through in vitro fertilization (IVF), 70 percent of IVF implantations fail. As IVF is becoming a more common route to pregnancy in cases of infertility, there is a need for better understanding of embryonic development at this early stage.

The first two blastomeres contribute unequally to the human embryo (open access)

Graphical abstract

A human blastocyst expressing GFP in some cells (green), and stained to reveal the cell membranes (orange) and nuclei (blue). Labelling with GFP allowed the authors to reveal that only one 2-cell stage blastomere contributes most of the cells to the future body.