A week from today, my little girl turns 3 years old. As I’ve witnessed first-hand, kids grow up way too quickly. So, in that light, I’m already thinking through how I’ll give her “the talk.” We’ll start out easy, talking about birds and bees, then I’ll watch her squirm with discomfort while we talk about the real deal. Lucky for her, our sex talk will be peppered with phrases like zona pellucida and gamete recognition, thrown in for technical completeness. Today’s image is from a paper describing the proteins that allow sperm to attach to an egg.
Fertilization in mammals begins when sperm attaches to an egg’s zona pellucida, a glycoprotein membrane around the egg. Gamete recognition prevents fertilization between different species—human sperm cannot attach to a mouse egg, for example. In a recent paper, Baibakov and colleagues replaced mouse zona pellucida proteins with human zona pellucida proteins (ZP1-4). The transgenic mouse eggs containing human ZP2 protein allowed the attachment of human sperm. Human sperm were able to penetrate the zona pellucidae, but were not able to fertilize the mouse eggs. In the images above, human sperm are attached to a mouse egg with human ZP2 in the zona pellucida. Sperm heads are in blue, sperm tails are red.
Baibakov B, Boggs NA, Yauger B, Baibakov G, & Dean J (2012). Human sperm bind to the N-terminal domain of ZP2 in humanized zonae pellucidae in transgenic mice. originally published in the Journal of Cell Biology, 197 (7), 897-905 PMID: 22734000 
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