Sea urchin egg undergoing mitosis with fluorescent-tagged/stained DNA (blue), microtubules (green).
Cells divide. One single piece of life tugs itself apart and splits in two. It sounds like a purely destructive process, reminiscent of medieval woodcuts where the hands and feet of some unfortunate thief are tied to horses heading in opposite directions. But that's the macro world. On the micro scale, to split is to live. A dividing cell doesn't just rip itself to pieces. Instead, the cell first makes a copy of its genetic information. When the cell splits, what it's really doing is making a new home for that copy to live in. Make enough copies—and enough copies of the copies—and you eventually end up with a living creature.
Back in May, I took part in the Marine Biological Laboratory Science Journalism Fellowship, a 10-day program that gives journalists hands-on experience in what it means to be a scientist. The program is split into two tracks. As part of the environmental track, I went to the Harvard Forest, where nature is one giant laboratory. But, at the same time, other journalists were busy in a different sort of lab.
Steven Ashley is a contributing editor at Scientific American and writes for a host of other publications. He took part in the fellowship's biomedical track. Ashley and the other journalists fertilized the eggs of sea urchins and other small ocean creatures, and then used specialized biomedical microscopes and cell imaging software to create brilliant photos and mesmerizing movies of cell division and growing animals.
Ashley was kind enough to send me some of those images and movies. In them, you can see the tiny structures and every day processes that form the basis of life.
That's Steven Ashley working the pipettes in the image above. Pipettes are just tools that scientists use to measure out small volumes of liquid and transport that liquid. You know how you can stick a straw into a glass of water and suspend some of the liquid in the straw by crimping the top, and creating a little vacuum seal? Pipettes work a lot like that.
Here's what Ashley had to say about his lab experience:
I worked with fellows Catherine de Lange, Alaina Levine, Euna Lhee, Sue Nelson and Maria Stenzel. Under the direction of Professors David Burgess and Brad Shuster, we took some sea floor creatures and processed them—their eggs and embryos—in the lab for viewing on the microscale.
There was lots of pipetting and waiting for cellular development to happen, followed by the incredible opportunity to operate $100,000 state-of-art Zeiss microscopes and create some pretty amazing images.
What you see in the slideshow are the results of only a couple of days of working on our stained and incubated specimens with Zeiss Axio Observer regular and inverted microscope systems(as well as other microscopes). We managed to produce some 'virtual 3D' contrast views and brilliant fluorescent-tagged images (and movies) of fragile live cells, embryos and other critters.
It starts at the spines. Sea urchins are spiny creatures. These spines are, in fact, probably their most distinguishing characteristic, from the human perspective. Sea urchins do have two separate sexes, but it's not easy to tell which is which. Luckily for the urchins, they don't really need to spend much time worrying about it. In nature, sea urchins breed by releasing eggs and sperm into the ocean and letting the sex cells find each other. In the lab, an injection of potassium chloride prompts the urchins to release eggs and sperm. Ashley and the other fellows had to "milk" the urchins to collect these cells.
Get the sperm and egg together, and you're on the road to cell division.
In this image you can see multiple fertilized sea urchin eggs at different stages of mitosis. Mitosis is an important part of cell division. During this process, the chromosomes (in blue) are separated out into two identical sets and those sets shift into position so that the cell can split, creating two cells that carry all the information necessary for life. Microtubules, protein bands that help maintain cell structure, (shown in green) make sure the chromosomes get sorted accurately and line up where they need to be.
Seven cell divisions later, when you have a 128 cells, what you've got is a blastula. Blastula are little hollow balls of cells. See how one side of the wall of the ball is thicker, though? That's important. That thick part will eventually become the sea urchin's digestive tract.
A week after fertilization, you get to the pluteus—a larval stage that now includes a basic skeletal structure. These little arrow shaped creatures move through the water, eating whatever they can. But they don't move in the direction that their arrow points. That's because the mouth of the pluteus in between its arms, and larval sea urchins deal with the world mouth-first. In fact, those arms probably help direct food towards the mouth.
A month later, this little larva will go through a stage of metamorphosis and become, officially, a baby sea urchin. Ashley and the other fellows weren't around long enough to see that happen, but if you want to know more about sea urchin development (and see more photos) I recommend checking out these links:
• The Early Development of Sea Urchins — from the book Developmental Biology, by SF Gilbert
• Echinoderms, an introduction — from Dr. Jeff Hardin at the University of Wisconsin-Madison
• The sea urchin: A stinging, but amazing, animal — by Jean-Marie Cavanihac in the July 2000 issue of Micscape Magazine