Sea Urchin Embryology Back to Contents page






The idea came in a dream, as do most of my creative ideas. How could I present fertilization in a way that students would want to learn? One person I talked to even quipped "You donít have to interest teenagers in SEX." The problem is of course that there is a lot more to fertilization than sex. How do you teach all the dull dry boring details? Ok, the stereotypes portrayed in the James Bond movies are not "Politically Correct", but a "comic book" character is a fun way to learn. The students, who have seen James Sperm so far, have enjoyed the show. I have kept the non-science as low key as possible [no car chases, exploding buildings or bikini clad sea urchins], but tried to stick with the typical Bond movie plot and sense of FUN.

FRAME 1 - {Title Screen}

James "Sperm" instead of Bond, Halpoid {sperm are haploid genetically} instead of Agent, and GoldenEgg to represent the yellow color of the typical S. purpuratus or L. pictus egg, but also to play with the world GOLD apparent in several James Bond movies. The target, if you look closely is an egg being fertilized.

FRAME 2 - {Miss Money Penny}

Some people have trouble with the perspective on this image. James Sperm is MUCH closer to the camera than Miss Money Penny. How can you tell the Miss Money Penny is now Mrs? [by the "ring" made by the fertilization membrane]

FRAME 3 - {"M"}

M is the reputed head of operations, not actually seen here. The schematic shows a typical sea urchin egg with the size on the left, cortical granules around the inside edge and the nucleus off center in the egg.

FRAME 4 - {"FatherLand"}

What species of sea urchin is this? [Looks like a S. purpuratus?]


This is a typical scene in a Bond film. When visiting Q, James invariably plays with something and sets it off. In this case an acrosome reaction has occurred.

FRAME 6 - {Sperm Schematics}

This image is straight out of the CORE LAB. I have added some explanation as to why sea urchin sperm have one large mitochondria instead of the 6 small ones seen in humans. This has not been proved or disproved as far as I know. The "live span" difference is true.

FRAME 7 - {Sea Urchin Beds}

This sets the scene of where all this is taking place. In the Bond films, he is usually on a plane or train going somewhere. Of course the typical sea urchin sperm probably does not cover that much distance, being subject to the currents in the sea.

FRAME 8 - {FatherLand}

View of a S. purpuratus sea urchin spawning in a shallow tide pool with other sea urchins nearby. Lends a sense of urgency to the story.

FRAME 9 - {Sperm Cockpit}

One of the least realistic scenes. Sperm of course do not have cockpits with people in them. Interestingly though, it was thought at one time that the sperm in fact did have a miniature little person inside "homunculus".

FRAME 10 - {Spawned}

Sodium ions are pumped into the mitochondria, changing the internal pH and activating the "sperm motor", the tail, which is done here when the student clicks on HIT.

FRAME 11 - {Tail Motion}

Review how the sperm moves, by rotating its tail, animations 2D & 3D. Seen from the point of view of the sperm head, the world is spinning.

FRAME 12 - {Jelly Coat}

The jelly coat is an attractor for sperm and a barrier to too many reaching the egg at the same time. Also somewhat species specific, though not as specific as the receptors we wil see later.

FRAME 13 - {Agglutination}

Sperm can often be seen sticking together. This is especially true if there are too many sperm and/or if the jelly has partially dissolved or broken off. [Remember sperm are attracted to the jelly.]

FRAME 14 - {Borrowing in}

The egg jelly, as mentioned earlier, is a significant barrier that slows down the sperm reaching the egg.

FRAME 15 - {Acrosome Reaction}

See the Acrosome animation. Basically, an egg jelly molecule is picked up on a jelly receptor on the sperm. This activates the acrosome reaction causing the actin molecules inside to polymerize and elongate. This pushes the head of the sperm outward, exposing the egg receptors and releasing proteases that help get through the egg jelly.

FRAME 16 - {FatherLand sperm "lands" on GoldenEgg}

Activation is not usually instantaneous. A few seconds are needed to match up with the sperm receptors on the egg surface. In this case there is another problem, as we shall see.

FRAME 17 - {James sperm lands}

James Sperm lands and locks into the egg receptors. Since he is the correct species, ie, the same as GoldenEgg, the match is good and activation starts. The FatherLand sperm, being the wrong species cannot activate the egg. Related species can sometimes activate each other, as in the case of S. purpuratus and S. francicanus.

FRAME 18 - {Electrical De-Polarization}

The electrical potential goes from about -60 mVolts to about +20 mVolts at activation. Any other sperm that may have attached itself to the egg is effectively "fried" and can not now fuse with this or any other egg.

FRAME 19 - {Sperm Egg Fusion}

Close-up view of the sperm of one of the egg's microvilli. There is a lot of controversy about what exactly happens here, but somehow the calcium wave is started. This calcium wave starts a lot of the biochemical machinery in the now embryo, including protein and DNA synthesis. First though, is the release of the cortical granules.

FRAME 20 - {Cortical Reaction}

Cortical granules fuse with the plasma membrane [see fusion animation]. These add new membrane and new membrane proteins to the surface of the egg. Also, the fertilization membrane is raised to act as a deterrent for further sperm entry and a protective barrier to bacterial attack and mechanical damage.

FRAME 21 - {Inside the Egg}

The outer layers of the sperm do not enter the egg, only the nucleus and the centrosome (centriole in some texts). The cortical granule(s) directly beneath the sperm entry point are unable to fuse with the plasma membrane of the egg, probably because the sperm is in the way. The egg is full of mitochondria and yolk bodies (power plants and fuel respectively). Microtubules, actin and myosin fibers attach to the sperm nucleus and start to bring it and the female nucleus to meet in the center of the egg.

FRAME 22 - {Near the Nucleus}

The nucleus is seen full of tiny holes to let small molecules in and out, but keep DNA in. Fusion of the two nuclei is the goal of the fertilization process.

FRAME 23 - {Dramatic Pause}

FRAME 24 - {Punch Line}

WE ARE ONE, WE ARE EMBRYO. The haploid sperm and egg are now one and diploid again. Fully capable now of becoming an adult sea urchin with luck and time.