The slow block to polyspermy, as well as many other events that ensue after fertilization, are elicited by the release of calcium, first at the site of sperm penetration, and then sweeping across the egg in a wave. This "calcium wave" can be visualized using calcium indicators, dyes that change their fluorescence in response to the local concentration of calcium ions.

Pseudocolor image from a movie courtesy of Dr. Gerald Schatten (Univ. of Pittsburgh). High levels of calcium are shown as blue; low levels as red. See if you can tell where the sperm entered the egg!

Calcium dynamics can be quantitatively measured using indicators, and depicted as a "4-dimensional" movie. This image, courtesy of Michael Whitaker (Univ. of Newcastle) depicts elevated calcium levels using both color and a third axis, in which greater height also indicates higher calcium ion concentration. Again, see if you can tell where the sperm entered the egg!

4-dimensional depiction of calcium levels in a L. pictus zygote. From a movie courtesy of Michael Whitaker, Univ. of Newcastle.

Calcium dynamics can be imaged nicely in movies. Click on the corresponding thumnails to see movies corresponding to the two images shown above.

Calcium wave in a sea urchin zygote (0.4 Mb)
Calcium levels imaged in 4D in a sea urchin zygote (4.5 Mb).

Calcium elevation correlates with the site of sperm entry and the initial site of fertilization envelope elevation. Top: imaging of calcium (left) and fluorescent sperm (right). Note that the successful sperm enters near the top of the egg. The acrosomal processes of the sperm are also visible if one looks closely. Bottom: imaging of calcium (left) and fertilization envelope production via phase contrast (right). Calcium levels are highest near thefirst visible sign of a fertilization envelope. Images courtesy of Mark Terasaki, Univ. of Connecticut

Two additional movies (provided by Dr. c), show that calcium elevation iniates near the site of sperm entry, and that calcium elevation correlates with where fertilization envelope formation begins. In the first movie, fluorescently labeled sperm are used to fertilize an egg which has beeninjected with a caclium indicator. Notice that in this movie the calcium levels elevate in the same location as the first place where the fertilization envelope lifts off of the egg surface.

Calcium wave and sperm entry in a sea urchin zygote (0.? Mb)
Calcium elevation and fertilization envelope production (2.4 Mb)