Intrinsic chiral properties of the Xenopus egg cortex: an early indicator of left-right asymmetry?

Area of Science:

• Developmental biology
• Cell biology
• Embryology

Background:

• Vertebrate embryos establish anteroposterior, dorsoventral, and left-right (LR) axes for proper development.
• While microtubule-dependent mechanisms drive dorsoventral axis determination in Xenopus, early LR axis determination remains unclear.
• No prior studies identified cytoskeletal polarization for maternal component displacement in early embryonic LR axis determination.

Purpose of the Study:

• To investigate the earliest events in left-right (LR) axis determination during the first cell cycle in Xenopus embryos.
• To explore the role of cortical cytoskeletal organization in establishing embryonic asymmetry.

Main Methods:

• Treatment of cleaving Xenopus embryos and parthenogenetically activated eggs with 2,3-butanedione monoxime (BDM) to induce cortical torsion.
• Analysis of actin fiber formation and organization within the cortical shear zone.
• Disruption of cortical actin during the first cell cycle to assess effects on LR axis orientation.

Main Results:

• BDM treatment induced a consistent counterclockwise cortical shear in the animal hemisphere, accompanied by the development of long actin fibers.
• Actin organization was independent of microtubules and involved reorganization of existing f-actin, not de novo polymerization.
• Disruption of cortical actin during the first cell cycle led to randomized LR orientation of tadpole heart and gut.

Conclusions:

• The invariant chirality of the BDM-induced response suggests a maternally inherited, microfilament-dependent cortical organization.
• This cortical organization likely plays a crucial role in establishing the LR axis during the first cell cycle.
• Cortical actin dynamics are critical for the proper establishment of visceral asymmetry in early embryonic development.