Time 1. Sperm and Egg Fuse
Time 2. Cortical Granules Fuse with Plasma Membrane
Time 3. Fertilization Envelope Forms
H2O Molecule
Fertilization Envelope
Cortical Granule
Plasma Membrane
Perivitelline Space
Sperm Nucleus
Figure 1. Time course of events during fertilization of a sea urchin egg
6. Figure 1 shows the events during and immediately following fertilization of a sea urchin egg. Sexual reproduction relies on the successful fusion of only one sperm with one egg (time 1) during fertilization. In sea urchins, a diploid organism, several events prevent additional sperm from fertilizing the egg. One of the first events following fertilization is the fusion of secretory vesicles called cortical granules with the egg plasma membrane (time 2). The cortical granules release their contents, a mixture of polysaccharides and proteins, into the perivitelline space. Subsequent to this, water moves into the perivitelline space, causing it to expand and physically separate the fertilization envelope from the egg plasma membrane (time 3). The fertilization envelope prevents fusion of more than one sperm with the egg.
(a) Provide ONE reason that preventing fusion of multiple sperm with one egg in sea urchins has provided a survival advantage over evolutionary time.
(b) Describe how the release of the contents of the cortical granules into the perivitelline space results in the movement of water and causes the subsequent expansion of the perivitelline space.