The article about gastrulation from the other day was dreadfully vertebrate-centric, so let me correct that with a little addendum that mentions a few invertebrate patterns of gastrulation—and you'll see that the story hasn't changed.
Remember, this is the definition of gastrulation that I explained with some vertebrate examples:
The process in animal embryos in which endoderm and mesoderm move from the outer surface of the embryo to the inside, where they give rise to internal organs.
I described frogs and birds and mammals the other day, so lets take a look at sea urchins and fruit flies.
Echinoderms are one of the best systems for studying gastrulation. They're well characterized, and best of all, the embryos are transparent: just focus on the interior of the animal, and you can watch all the cells move and divide and change shape to generate the changes in tissue organization. In addition, the blastula divides in a predictable and stereotyped way, and you can trace the cell lineages late into development, seeing what each piece of the puzzle does. The diagram below is a simplified diagram of the process; the top 6 images show the early cleavages that lead to the formation of a hollow ball.
The bottom 6 images show the course of gastrulation. The yellow sheet of cells will become ectoderm; the red cells at the vegetal pole will introgress into the fluid-filled space, forming the primary mesenchyme. (Mesenchyme, by the way, is a term for loosely associated, often migratory mesodermal cells). The primary mesenchyme crawls about in the interior, and will form the internal mineral skeleton of the pluteus larva. The endoderm (in blue) invaginates and forms a hollow tube, the animal's gut. Some of its cells also peel out of the sheet to form the secondary mesenchyme, and the ascending end of the endoderm fuses with the ectoderm to create a mouth.
You can find out much more at this page on sea urchin gastrulation that is loaded with lovely photographs of it all.
Flies also engage in a similar series of gastrulation movements—compare the photographs below to the 7th-9th images in the urchin diagram.
The Drosophila embryo is more cigar-shaped than spherical, and gastrulation occurs along a ventral seam; these are cross-sections through the tube of the embryo's body. What you can see is that a plate of cells buckles inward and the cells roll inward as a sheet. The sheet then collapses into a mesenchymal mass that will contribute to the mesoderm of the embryo. Note that one difference here is that you aren't seeing endoderm; the tube of the gut arises from invaginations at the anterior and posterior ends, which aren't seen in this section.
One of the strengths of the Drosophila system is that we can look at gene expression around the circumference of this ring of cells. Prior to gastrulation, it looks like this:
Those green cells are expressing the mesodermal markers twist and snail; they're the ones that will move inward. The orange cells just lateral to them will shift towards the ventral midline, and will form the neurectoderm (insect nervous systems form along the ventral midline, unlike ours that form along the dorsal midline). The dorsal ectoderm expresses a gene call decapentaplegic (dpp for short) that is homologous to a gene called Bmp in vertebrates; Bmp induces ventral fates in vertebrates, while its homolog is a dorsal gene in flies. Similarly, sog is going to be expressed on the ventral side of the fly, while its vertebrate homolog, chordin, will be active in the dorsal organizer. This is part of the evidence that vertebrates and invertebrates are upside-down versions of one another.
One other important historical fact I have to mention about the gastrula. The morphological features were first identified in the early 19th century by Rusconi and Dutrochet, and later by Karl Ernst von Baer (that's a name that turns up extremely often in the history of embryology), but was actually first named in 1872 by the gentleman to the right, whose name comes up just as often.
That's Ernst Haeckel.
Nowadays, it seems that the only time anyone brings up that name is in the context of his failed theory of the biogenetic law, but that's hardly fair. He was an energetic and influential figure in the history of developmental biology, and he was responsible for synthesizing many of the scattered observations of experimentalists and natural historians into a more coherent and universal set of explanations for animal development. He seems to have named a lot of the developmental phenomena we take for granted now, along with a British scientist who is also less well appreciated than he deserves, Ray Lankester (Lankester, for instance, is responsible for the germ layer concept that named ectoderm, endoderm, and mesoderm, and made the distinction between triploblastic phyla that have all three layers, and the diploblasts, that have only two).
Haeckel generalized the idea of the gastrula to a wider domain than just amphibians, and argued that it was a common phenomenon in all triploblastic animals. He also gave an evolutionary explanation, suggesting that it arose as a feeding adaptation in the urmetazoa, in which the inward migration was part of a process to establish a gut cavity (which is how the name was derived—'gastrula' and 'gastric' have the same root, referring to a 'stomach'). His gastraea hypothesis is illustrated below, with a hypothetical placozoan like organism that evolved to form an internal chamber used for holding food for processing, a process that internalized an epithelial layer and opened up the potential for novel cell interactions and greater complexity.
There are real problems with the gastraea hypothesis, but like all of Haeckel's work, it was an early, flawed explanation based on a valid and universal observation—in this case, the ubiquity of gastrulation in many animal embryos.
Aha, I remember that Lankester name. He was one of the nine people at Karl Marx's funeral.
what, no cephalopod gastrulation?
That was great, Im a big fan of your site-- long time reader, first time commenter.
I would love to read your take on some of these old theories i.e. the Gastrea and planula about the origin of triploblasty and how they came and went.
I studied sea urchin development for my dissertation and loved it so much that I essentially have the first figure tattooed on my arm.
Thanks a lot for posting this PZ. So very interesting. I love the diagrams and photos you use to demonstrate your points.
What a great post again. Dev Bio is so much more interesting than creationism (yeah, I know that battle has to be fought). I am also glad you're doing something for Haeckel's reputation. Presumably (and hopefully) the majority of visitors here have read The Ancestor's Tale where Dawkins also pays tribute to Haeckel, including the magnificent drawings that he made. See: http://en.wikipedia.org/wiki/Kunstformen_der_Natur .
(And, after all, Mendel also cheated with the data, only he guessed right and Haeckel was wrong.)
Very nice! It would be great to get your take on recent claims that even the lowly sponge embryos undergo gastrulation.
I do not believe that Haeckel's reputation ought to be rehabilitated. While he was certainly brilliant, Haeckel was wrong - often grievously so - about far more than the Biogenetic Law. Namely, his ostensibly "scientific" writings on race, eugenics, euthanasia, society, and the weird quasi-religion of Monism, which inspired totalitarians both right and left. Although Daniel Gasman probably overstates the case, Haeckel's impact was both immense and mainly noxious. (At least the claim that he was a member of Thule Gesellschaft turns out to be incorrect - see Robert Richards.)
He is a towering figure in biology - where would modern biology be without the terms he coined? - but his legacy is even worse than that of Francis Galton. He cast a dark shadow over evolutionary biology for decades. It is no coincidence that Haeckel was left out of the Modern Synthesis. Stephen Jay Gould had good reason for his negative assessment of Haeckel.
Haeckel is a gift to the creationists that keeps on giving. Yes, we should acknowledge Haeckel's specific contributions - including his wonderful artwork illustrating various metazoan taxa - while condemning his overall legacy. Haeckel is a tragic figure; he ought to be an acknowledged ancestor, but not an honored one.
American Historical Review on Gasman's recent book
Historian Robert Richards has a more positive evaluation of Haeckel. Some of Richards' writings are posted on his website.
Hmm. I'm still curious about where Echinodermata really fits. Seems odd to me that radial symmetry would show up again, but perhaps then again it might not be surprising. Their larvae are bilateral after all. But here's the simplicity. All one has to do to get bilateral symmetry out of a radially symmetric animal is flatten part of the radial circle. Not much of a jump here. But to go from bilateral to radial seems unlikely. The there is mandible symmetry in insects. There is virtually no literature on this, and it is certainly neither random nor bilaterally symmetrical.