Reprogramming of cells during embryonic transfating: overcoming a reprogramming block.

Area of Science:

• Developmental biology
• Cellular reprogramming
• Molecular mechanisms of embryogenesis

Background:

• Regulative development allows embryos to replace missing cells, but its molecular basis remains unclear.
• Sea urchin micromere removal disrupts mesoderm formation and triggers endoderm cell fate changes (transfating).
• Pigment cells fail to regenerate after micromere removal, unlike other mesodermal cells.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying regulative development and cell fate plasticity.
• To investigate the sequential gene regulatory state transitions during transfating in sea urchin embryos.
• To identify the conditions necessary for the successful regeneration of pigment cells.

Main Methods:

• Removal of sea urchin micromeres at the 16-cell stage.
• Single-cell RNA sequencing to track cell fate changes over time.
• Manipulation of signaling pathways (Delta and Nodal) to assess pigment cell rescue.

Main Results:

• Single-cell RNA sequencing revealed a stepwise progression of endoderm cells through endomesoderm and mesoderm states.
• Skeletogenic and blastocoelar cell fates were successfully reprogrammed, but pigment cell fate was not.
• Pigment cell regeneration was rescued by timing signal expression: Delta expression before Nodal restored pigment cells.

Conclusions:

• Transfating involves a series of gene regulatory state transitions enabling cell fate reprogramming.
• The timing of signaling pathway activation is critical for successful cell fate reprogramming.
• Failure in pigment cell regeneration is linked to the sequence of endogenous negative and positive signals during reprogramming.