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Felicitous cleavages

This morning as I was doing my rounds at the marine lab I noticed a pile of eggs next to one of the bat stars (Patiria miniata) in a large table. Somebody, or more likely, multiple somebodies, had spawned overnight. I have absolutely zero time to deal with another ongoing project right now, but I have even less self-control when it comes to culturing invertebrate larvae. So I sucked up as many of the eggs as I could, along with a fair amount of scuzz from the bottom of the table, and took a look.

Assortment of bat star (Patiria miniata) embryos
Embryos of the bat star Patiria miniata, about 1 day old
2020-06-19
© Allison J. Gong

As I've come to expect with stars, the early embryonic stages are developing asynchronously. There were unfertilized eggs (obviously not going to develop at all), zygotes that hadn't divided yet, and other stages.

The coolest thing, though, will take some explaining. Animals begin life as a zygote, or fertilized egg. The zygote undergoes a number of what are called cleavage divisions, in which the cell divides but the embryo doesn't grow. A logical necessity of these two facts is that the cells get smaller and smaller as cleavage continues.

Now let's go back to the earliest cleavage divisions. One cell divides into two, each of those divides into two, and so on. The cell number starts with 1 and goes to 2, then 4, then 8, then 16, and so on. The process is more or less the same for all animals, but in only a few can these divisions be easily seen. Many echinoderms have nice distinct cleavage divisions and transparent-ish embryos, which is why the old-school embryologists in the early 1900s studied them.

Echinoderms are the major phylum in a group of animals called the deuterostomes. Incidentally, chordates (ahem--us) are also deuterostomes. The word "deuterostome" refers to the fact that during development in these animals the anus forms before the mouth does. That's right, folks, you had an anus before you had a mouth.

Another feature that is generally associated with the deuterostomes occurs in early cleavage. Picture this: A cell divides into two cells. Then each of those divides, resulting in four cells. Geometry dictates that the four cells form a plane. That makes sense, right? When the four cells divide again to make the 8-cell embryo, a second plane of cells is formed on top of the first. The second tier can either sit directly on top of the cells of the first tier (radial cleavage) or be twisted 45º so that the cells sit in the grooves between cells in the first tier (spiral cleavage).

Take a look at this embryo. Do you think it has undergone spiral cleavage or radial cleavage?

8-cell embryo of Patiria miniata
8-cell embryo of Patiria miniata
2020-06-19
© Allison J. Gong

This is a textbook example of radial cleavage. In all the sea urchin embryos I've watched over the years, I've never seen radial cleavage as clear and unambiguous as this. It was one of those moments when you actually get to see something that you've known (and taught) about forever.

So yes, echinoderms and other deuterostomes generally undergo radial cleavage. And I will hopefully have larvae to look after again! They will probably hatch over the weekend. On top of everything else that's going on now, additional mouths to feed are the last thing I need. But fate dropped them into my lap and who am I to argue with fate?

What do you think?

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