I’ve been tinkering with a lovely software tool, the 3D Virtual Embryo, which you can down download from ANISEED (Ascidian Network of In Situ Expression and Embryological Data). Yes, you: it’s free, it runs under Java, and you can get the source and versions compiled for Windows, Linux, and Mac OS X. It contains a set of data on ascidian development—cell shapes, gene expression, proteins, etc., all rendered in 3 dimensions and color, and with the user able to interact with the data, spinning it around and highlighting and annotating. It’s beautiful!
Unfortunately, as I was experimenting with it, it locked up on me several times, so be prepared for some rough edges. I’m putting it on my list of optional labs for developmental biology—3-D visualization of morphological and molecular data is one of those tools that are going to be part of the future of embryology, after all—but it isn’t quite reliable enough for general student work. At least not in my hands, anyway. If one of my students were to work through the glitches and figure out how to avoid them, though, it could be a useful adjunct to instruction in chordate development.
If you want to play with it, I’ll give you a quick overview of what’s going on in the dataset. A paper by Munro et al. has used these kinds of data to summarize key events in the transformation of a spherical ball of cells into an elongate, swimming tadpole larva.
Ciona is a tunicate, a marine invertebrate chordate, that in its adult form is a kind of sessile barrel-shaped blob called a sea squirt that attaches to a substrate and spends its life filter-feeding. As an embryo, though, it rapidly forms a tadpole shaped larva that displays its chordate affinities: it has a notochord, and gill slits, and a long post-anal tail, and a dorsal nerve cord. It uses these features to swim, as a dispersal apparatus, before settling down and metamorphosing into the adult form.
Their development is different from ours in some significant ways. They exhibit an invariant, stereotyped pattern of cleavages, with a mosaic pattern of development in which each newly divided cell is assigned to a specific and relatively inflexible fate. Each cell has a specific and predictable lineage, which has been a real boon to embryologists since Conklin described them in 1905—you can recognize each cell in the embryo, give it a name, follow what it does, experiment on it, and ask what tissues it will generate. The cells also have different distributions of pigment, so even at the turn of the last century it was relatively easy to follow their development; it was as if they came with a colored label.
The diagram below shows some of the stages. As you can see, the divisions can be asymmetric, and shape and size are also indicators of cell identity.
One other quirk of ascidians is that there is almost no cell migration, and a cell generally hangs on to its nearest neighbors throughout embryonic development. In vertebrates, we’re used to seeing cells uncouple from each other extensively and stream to new locations in the embryo. One classic example is gastrulation: in the two-layered early embryo, a subset of cells move into the interior of the animal to establish a third germ layer, the mesoderm. Ciona also gastrulates, but cells don’t dissociate from each other. Instead, the embryo forms a flattened disc, and then curls in on itself to move cells into an interior space.
Finally, a round ball of cells has to reshape itself into that elongate tadpole form you can see at the top of this page. In this process, we get into more familar ground for vertebrate embryologists: elongation of a cluster of cells is accomplished by intercalation. Cells push medially, into the interior of the tissue. When a cell moves inward, it pushes neighboring cells to either side, at right angles to the direction of movement. All of the cells jostling to squeeze in between each other, to intercalate, generates a force for elongation. Imagine a pile of coins loose on your desk. If you put your hands on the edges of the pile and push all the coins inward, they’ll stack on one another and pile higher while reducing their overall footprint on the desk. The same principle operates here; cells are more obliging than rigid metal coins, though, and will eventually get into a perfect stack that maximizes the length of the column. (You’ll often see the notochord described with the term “stack of coins” in the literature—it actually looks a bit like that.)
That should get you started in figuring out what’s going on. Now go play with the free software and watch all these events happen! The paper by Tassy et al. listed below also describes how the software is used to examine cell shape changes and interactions in ascidian development.
Munro E, Robin F, Lemaire P. (2006) Cellular morphogenesis in ascidians: how to shape a simple tadpole. Curr Opin Genet Dev. 16(4):399-405.
Tassy O, Daian F, Hudson C, Bertrand V, Lemaire P (2006) A quantitative approach to the study of cell shapes and interactions during early chordateembryogenesis. Curr Biol 16:345-358.