|Development begins with the zygote dividing into two daughter cells. At a water temperature of -1.5 C here, developmental rates are very slow. First division occurs 8 hrs after fertilization, and about every 6 hrs after that. Frames were shot at 10 min. intervals.|
|If you want to know where this thing fits in the picture, then read on.|
The question is how does a single egg become a sea urchin. This in itself is very complex, but when you throw in an environmental variable like freezing water temperatures, it seems impossible that such a delicate process as embryo formation can work in a harsh polar environment.
|Fertilized Eggs||Antarctic Sea Urchin|
We have been working with the sea urchin Sterechinus neumayeri in McMurdo Sound. The left photo is of fertilized eggs that have been mixed with a blue dye. There is a thick 'jelly-coat' around each of the eggs which is visualized as the 'clear' space where the dye has been excluded. Around each zygote now is a heavy protein shell called the vitellin layer. This layer protects the zygote from direct contact with the environment. Our research goal: measure how much energy it takes for an egg to develop into a sea urchin in this extreme environment.
|An egg is fertilized by a single sperm cell. Immediately after fertilization, a protective membrane is produced.|
|The first cell divisions are easy to follow as the egg now divides into two equal cells......|
|...and then those 2 cells divide into four....|
|...and then those 4 cells divide into eight (the microscope is focused on the middle 4 cells such that there are two cells in front of them and another two cells behind them that are out of focus)...|
|....but now the tricky stuff starts when those 8 cells divide into 16 cells. What happens here is that the top layer of 4 cells divides evenly along their sides to produce a circle of 8 cells on top (mesomeres). But, the lower 4 cells divide unequally along their bottom to form 4 large cells (macromeres) on top of 4 very small cells (micromeres). At this 16 cell stage, the first level of differentiation begins.|
|Cell division continues but it is very difficult to follow individual cells now. As cell number increases a hollow ball of cells is formed (morula).|
|Soon cell the cell number reaches about 2,500 and there are now two distinct layers surrounding the empty space in the middle. This stage is called a blastula.|
|Now that all the cells are in place, the first real part of the future animal begins to form. Here a portion of the cells in the wall of the blastula begin to migrate in to the center space to produce a large invagination. This invagination is just like what happens when you poke your finger into a balloon. What you see forming right now will eventually become the digestive tract of the sea urchin. Do you recognize the small cells at the tip of the invagination (archenteron)? Those are derived from the micromeres that were produced at the 16 cell stage and are called primary mesenchyme cells.|
|This is an intermediate 'prism' stage that is hard to describe other than saying it looks like a hat with little arms beginning to bud off of the brim. You can see more internal structures forming.|
|The arms no grow out to form a pluteus larva. The arms are covered by tracts of cilia that allow the animal to swim and capture algal cells. The larva is really upside down with its mouth located in the circular area between the arms. This is a 4-arm stage. When the larva is ready to form a juvenile it will have grown another 4 arms for a total of eight. For this sea urchin species, it takes about 150 days for an egg to fully develop into a small juvenile.|