Sonic hedgehog-expressing cells in the developing limb measure time by an intrinsic cell cycle clock

Area of Science:

• Developmental biology
• Cell cycle regulation
• Embryogenesis

Background:

• Classical models proposed intrinsic cell cycle clocks in development, but their existence is debated.
• Sonic hedgehog (Shh) signaling patterns the chick limb bud's anteroposterior axis, determining digit identity.

Purpose of the Study:

• To investigate the role of intrinsic cell cycle clocks in timing developmental signals.
• To determine if cell cycle clocks regulate Sonic hedgehog (Shh) expression duration in limb development.
• To explore the implications for embryonic self-renewal and digit regeneration.

Main Methods:

• Utilized chick limb bud explants.
• Monitored Sonic hedgehog (Shh) expression.
• Investigated the influence of retinoic acid signaling.
• Assessed the reset potential of cell cycle timing and positional values.

Main Results:

• Demonstrated an intrinsic cell cycle clock in polarizing region cells that times Shh expression duration.
• Identified the initiation of this clock upon cells exiting retinoic acid signaling range.
• Showed that Shh transcription timing can be reset, indicating embryonic self-renewal.
• Found that anteroposterior positional values are not reset, suggesting a constraint on digit regeneration.

Conclusions:

• Provided the first evidence for an intrinsic cell cycle timer controlling a major embryonic signaling center.
• Established a link between cell cycle dynamics, morphogen signaling duration, and embryonic patterning.
• Highlighted potential mechanisms underlying regenerative limitations in limb development.