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#fast block

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Regeneration -ll

The fast block to polyspermy lasts approximately:

Fast block is transient electrical depolarization preventing extra sperm fusion until slow block completes. Sea urchin resting potential -70 mV shifts to +20 mV within seconds after first sperm entry. This positive potential makes membrane refractory because sperm-egg fusion voltage-dependent and favored only at negative potentials. Depolarization lasts about a minute until cortical granule exocytosis elevates fertilization envelope. Voltage clamp experiments show positive-held eggs reject sperm, return to negative restores receptivity. One to two minutes ensures overlap with slow block; shorter seconds insufficient, longer many minutes would impair subsequent ionic homeostasis required for development.

Ref: Jaffe LA, Fast block to polyspermy in sea urchin eggs - membrane potential duration about one minute, CSIRO Reproduction.

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Sea urchin -l

The fast block to polyspermy lasts approximately:

Fast electrical block depolarizes plasma membrane within one to three seconds after sperm fusion, creating immediate short barrier preventing additional fusions. By holding membrane positive, further sperm cannot undergo voltage-sensitive merger. This transient depolarization provides critical window lasting roughly sixty to one hundred twenty seconds before permanent slow block assembles. During this calcium wave triggers cortical exocytosis and progressive elevation and hardening of fertilization envelope. Voltage clamp experiments showing artificially negative holding permits supernumerary fusions confirm mechanism. Duration of fast block generally quoted as one to two minutes covering overlapping transition between blocks.

Ref: Jaffe, Development 1976, Duration of fast block; Gilbert Chapter 7: Timescale of electrical and slow blocks.

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