Cortical softening elicits zygotic contractility during mouse preimplantation development

Area of Science:

• Developmental Biology
• Cell Biology
• Biophysics

Background:

• Actomyosin contractility drives early embryonic development, particularly preimplantation morphogenesis in mice.
• Periodic cortical waves of contraction (PeCoWaCo) are the first visible signs of contractility, appearing after the second cleavage.
• The precise mechanism initiating embryonic contractility remains largely unknown.

Purpose of the Study:

• To investigate the factors responsible for the activation of actomyosin contractility during mouse preimplantation development.
• To understand the role of cell size and cortical properties in the emergence of PeCoWaCo.

Main Methods:

• Utilized cell fusion and fragmentation to manipulate blastomere size (20–60 μm).
• Monitored the appearance and frequency of PeCoWaCo during cleavage stages.
• Assessed blastomere surface tension and formin Fmnl3 expression levels.

Main Results:

• PeCoWaCo emergence and frequency acceleration correlate with cleavage progression but are not explained by cell size reduction.
• Blastomere surface tension decreases progressively, and artificial softening enhances PeCoWaCo prematurely.
• Programmed down-regulation of formin Fmnl3 is identified as a critical event for cortical softening and PeCoWaCo manifestation.

Conclusions:

• Cortical softening, driven by Fmnl3 down-regulation, is the key mechanism awakening zygotic contractility.
• This process precedes preimplantation morphogenesis and is independent of cell size reduction during cleavage.
• Findings reveal a novel pathway for initiating embryonic contractility.