Basics: Gastrulation, invertebrate style

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.

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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.

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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:

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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.

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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.