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Worm sperm have new tricks

A new mechanism for sperm to activate an egg.

Apr 25, 2016

Calcium Wave

The ovum is a non-active cell. It does not synthesize materials or grow, but when fertilized with sperm, it begins actively synthesizing and leads to cell division. This is referred to as "egg activation." The trigger of this transformation is a phenomenon called a calcium wave that propagates calcium concentration changes in the egg.

In a new study published in Cell Reports [1], Jun Takayama, a post-doctoral researcher with Shuichi Onami, the team leader of the QBiC Laboratory for Developmental Dynamics analyzed the generation of the calcium wave in the nematode worm, C. elegans, by high-speed imaging and image processing as pictured above. The researchers leveraged two distinct advantages of this model organism, namely its body is transparent and it is amenable to genetic manipulation.

Another advantage of C. elegans is that it is good at reproduction. Takayama explains, “From an evolutionary point of view, humans are not good at reproduction; human sperm can fertilize the egg at a 0.000001% probability, whereas a C. elegans sperm can fertilize the egg with close to 100% probability.”

C. elegans has a sperm-specific Ca2+-permeable channel, TRP-3, which humans lack. With a combination of genetic experiments and simulations, the researchers revealed that this channel causes a rapid increase in calcium concentration in the vicinity of the sperm entry point immediately after fertilization, this triggers the generation of calcium waves that propagate throughout the egg.

Furthermore, the sperm membrane including TRP-3 and ovum membrane were confirmed to be fused during fertilization. Therefore, it is thought that TRP-3 is working as a "conduit" for calcium ions flowing into the egg from the outside.

Despite the differences with human sperm, Takayama hopes that studying fertilization in C. elegans will shed light on human reproduction where the significance of calcium influx from the extracellular space especially in the context of triggering calcium waves is not well understood. “C. elegans egg activation might become a good model to investigate the physiological importance of the extracellular calcium on human egg activation.”

In the future they’d like to delve deeper into the details, “The first thing we must do is identify the molecules involved, especially those involved in the global calcium waves, because the molecular underpinnings for the calcium waves in C. elegans are largely unknown.”

  1. Takayama, J. & Onami, S. “The Sperm TRP-3 Channel Mediates the Onset of a Ca2+ Wave in the Fertilized C. elegans Oocyte” Cell Reports doi:10.1016/j.celrep.2016.03.040