Tracking cytoplasmic ca2+ distribution after a sperm enters the egg based on what they knew about the fertilization envelope (a protective layer that forms around the egg when a sperm fuses with it), steinhardt, shatten, and their colleagues hypothesized that changes in the distribution of ca2+ ions in sea urchin eggs are involved in the formation of the fertilization envelope. in sea urchin eggs (as in most eukaryotic cells) the concentration of ca2+ ions is much higher in the endoplasmic reticulum (er) than in the cytoplasm. to see how cytoplasmic ca2+ concentration changes in the egg during fertilization, the researchers injected a ca2+-specific fluorescent dye into the cytoplasm of unfertilized eggs. after adding sperm to the eggs, they observed the eggs with a fluorescence microscope. the following images show the changes in fluorescence that occurred after a single sperm entered the egg. the fluorescence of the dye increased with increasing cytoplasmic ca2+ concentration. the green color indicates the region of the cell with the highest fluorescence at that point in time.
sort the labeled regions of the fertilized egg above based on the status of the er ca2+ channels and ca2+ pumps in that region.