Intracellular free Ca2+ changes during physiological polyspermy in amphibian eggs

Area of Science:

• Developmental Biology
• Cellular Physiology
• Reproductive Biology

Background:

• Polyspermic fertilization, where multiple sperm fertilize an egg, occurs in some species.
• Intracellular calcium ([Ca2+]i) signaling plays a crucial role in fertilization.
• Previous studies on calcium dynamics during fertilization have primarily focused on monospermic events.

Purpose of the Study:

• To investigate intracellular free calcium activity ([Ca2+]i) in urodele eggs during polyspermic fertilization.
• To determine if sperm number influences calcium signaling during polyspermy.
• To characterize the spatial and temporal dynamics of calcium changes triggered by sperm entry in polyspermic eggs.

Main Methods:

• Urodele amphibian eggs (Pleurodeles waltlii) were impaled with intracellular Ca(2+)-selective microelectrodes.
• Eggs were inseminated under varying sperm:egg ratios to induce different degrees of polyspermy.
• Local insemination techniques and dual-microelectrode recordings were employed to analyze calcium dynamics.

Main Results:

• A slow increase in [Ca2+]i (0.15 microM) was observed in 28 out of 45 eggs, starting approximately 15 minutes post-fertilization.
• The amplitude of the [Ca2+]i increase was independent of the number of sperm interacting with the egg.
• Calcium increases were not simultaneous across the egg cortex and could be localized to specific sites, suggesting non-propagating, localized signaling events triggered by individual sperm entries.

Conclusions:

• Sperm entry during polyspermic fertilization in urodele eggs can trigger localized, slow, and non-propagating calcium transients.
• These calcium changes, while not propagating throughout the egg cortex, can travel short distances from the site of sperm entry.
• The findings suggest a complex, localized calcium signaling mechanism during polyspermic fertilization in amphibians.