One of the all-around coolest things I do with my students is spawn sea urchins to show them fertilization. We can actually watch fertilization occur under the microscope. And since the early stages of development are the same in sea urchins and humans the students get to see how their own lives started–not in dishes of seawater and probably not on a microscope slide, but you get the drift. I’ve probably spawned and fertilized sea urchins dozens of times, and I never get tired of it.
Part of the reason we can spawn urchins on demand (sort of) is that they are broadcast spawners. In nature, urchins of both sexes shed their gametes to the outside and fertilization and all ensuing development occur in the water column. This is convenient for us because it means we can culture the larvae and observe them at various stages of development.
Gametogenesis is seasonal in urchins, with the local purple urchin (Strongylocentrotus purpuratus) generally ripe from December through March-ish. In the lab we can manipulate the timing of gametogenesis by subjecting the urchins to artificial photoperiod, tricking them into “thinking” that they are experiencing winter when the calendar says otherwise.
Fertilization is a complex series of events, some of which happen very quickly and some of which are a bit slower. Here’s a brief rundown:
- Sperm fuses with the outer layer (called the vitelline layer) of the egg.
- Sperm nucleus begins to enter the cytoplasm of the egg. This causes the egg membrane to become impenetrable to other sperm and is called the fast block to polyspermy. The egg is impenetrable about 1 second after the sperm and egg membranes begin to fuse.
- Once an egg has been penetrated by a sperm, vesicles in the outer layer of the egg fuse with the egg’s plasma membrane and release cortical granules into the space between the plasma membrane and the vitelline layer.
- The granules trigger a cortical reaction that results in the lifting of the vitelline layer off the egg surface. The vitelline layer hardens and is now called a fertilization envelope. The hardened envelope keeps other sperm from penetrating the egg and is referred to as the slow block to polyspermy.
Why are there two blocks to polyspermy? Everyone knows that it takes only one sperm to fertilize an egg. It turns out that if multiple sperm enter an egg at the same time, development goes awry. I’ve had cultures that I fertilized with too high a concentration of sperm that get through the early stages fine but crash soon afterwards. So polyspermy is bad and it definitely makes sense that natural selection would come up with redundant ways to prevent it.
All this is to set up the following video clip. These eggs were spawned at the end of February 2012 for my zoology class, and after all these years I was finally to record fertilization as it occurred in real time. Actually, I can’t take credit for the recording; Sid and Moriah were the ones who figured out how to make the camera play nice with the microscope and actually record video to a computer.
What you will see at the beginning is several large dark eggs on a yellow background, with lots of little sperms wiggling around. There are way too many sperm for this particular set of eggs NOT to be dealing with polyspermy, by the way. A few seconds into the video you will see what looks like a bubble forming around some of the eggs. The bubble is the fertilization envelope rising off the egg surface. There’s one egg that seems to be holding out, but by the end of the 1-minute-long video all the eggs will be fertilized.
Pretty cool, eh?