- Fertilization triggers 2 stereotyped series of calcium waves in ascidians.
These calcium signals are necessary for the completion of the the meiotic cell cycle, as manifested by the emission of polar bodies.
The repetitive calcium waves are generated by 2 cortical wave pacemakers (PM1 and PM2)
- First, a large calcium wave is intiated at the site of sperm entry by pacemaker PM1. The wave crosses the egg in 15 seconds and triggers an actomyosin-driven contraction within 30 seconds of sperm fusion .This is the first major phase of cortical and cytoplasmic reorganization. The contraction moves the sperm nucleus and centrosome vegetally. Pacemaker PM1 moves with the sperm nucleus and becomes located in a cortical accumulation of ER forming around the sperm aster. PM1 stops 5 minutes after fertilization as the second polar body is emitted.
- Second; a serie of 6 to 12 calcium waves are generated by a stable cortical pacemaker (PM2) situated in the contraction pole .This calcium wave pacemaker PM2 is situated in an ER-rich cortical domain that forms as a result of the cortical contraction. Calcium waves from this pacemaker propagate preferentially in the egg cortex and maintain a higher calcium level in the vegetal hemisphere.
The calcium waves stop 20-25 minutes after fertilization, just before meiosis completion.
- A third artificial calcium waves pacemake (PM3) can be generated by uncaging injected IP3. All waves originate from the animal pole cortex which surprisingly is the region of the egg most sensitive to IP3.
- The role of these calcium wave pacemakers in development is still a mystery. Note that calcium wave pacemakers located in the vegetal pole cortex, originally discovered in ascidians, have now been identified in fertilized mouse and annelid eggs in the process of completing their meiotic cell cycle.

- NEW :Journal of Cell Science (2001) 114,2471-248 (Please use IE 5)
Dumollard R., Sardet C.: 3 different calcium wave pacemakers in ascidian eggs.
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- Full references on calcium signals in ascidians
- Methods