There is a treasure trove in China: the well-preserved phosphatized embryos of the Doushantuo formation, a sampling of the developmental events in ancient metazoans between 551 and 635 million years ago. These are splendid specimens that give us a peek at some awesomely fragile organisms, and modern technology helps by giving us new tools, like x-ray computed tomography (CT), scanning electron microscopy (SEM), thin-section petrography, synchrotron X-ray tomographic microscopy (SRXTM), and computer-aided visualization, that allow us to dig into the fine detail inside these delicate specimens and display and manipulate the data. A new paper in Science describes a survey of a large collection of these embryos, probed with these new techniques, and rendered for our viewing pleasure…that is, we’ve got pretty pictures!

One of the things that can be done is that the embryos can be virtually dissected into their component cells, and even the sub-cellular organelles examined. In the figure below, one animal was scanned and all the boundaries between cells identified, allowing the computer to pull it apart and present each cell separately. This is a 16-cell embryo; about 3/4 of the specimens have 2n cells, an indicator that these are members of a synchronously dividing population, and the roughly equal volumes of the cells also support the idea that these are cells of an early blastula. The remaining quarter of the specimens show deviations that indicate either asynchronous cell divisions or naturally occurring asymmetry.

The techniques allow the interior of the cells to be examined, and in many cases large organelles (Vesicles? Nuclei? Their identity is ambiguous) are preserved, shown as the darker green blobs in the images.

(A) Scanning electron micrograph of a 16-cell embryo. (B) Iso-surface model of the exterior of a 16-cell embryo (specimen DOU-10). (C) Schematic drawing with cells labeled to correspond to their models [(E) to (T)]. (D) An x-ray section showing very faint cell boundaries and subcellular spheroidal structures (arrowheads). Grayscale values in image have been inverted and the image placed on a black background to match (A). (E to T) Models of all 16 cells. Cells with intracellular structures (shaded dark green) are rendered transparent and those without are opaque. Corrugation on cell faces is an isocontouring artifact. (M) shows the internal cell. Scale bar in (A), 200 Ám; in (E), 85 Ám.

The images below illustrate the level of subcellular detail that can be achieved. The embryos aren’t completely preserved—no one is going to pull molecules of a signal transduction pathway out of there—and are subject to all kinds of taphonomic and diagenetic artifacts, some of which I’ve discussed before. The actual chemical contents have been replaced, so there’s no way to tell if, for instance, a large kidney shaped vesicle in the interior once contained massive amounts of DNA (it’s a nucleus!) or lysozyme (it’s a vesicle!). There are suggestive elements, such as the fact that some of the organelles are consistently aligned relative to the cleavage planes, suggesting that they might be nuclei, which do the same thing in modern embryos. Numerous smaller blobs suggest lipid granules, another common feature of developing cells. In particular, compare the “A” figures to “B”; “A” is a two celled embryo from Doushantuo, while “B” is a two-cell sea urchin embryo. The similarities are striking.

(click for larger image)

(A) A two-cell embryo (specimen MPKxiv). (A1) Surface rendering of the embryo based on tomographic scans, showing the position of the orthoslice (A2), which reveals internal preservation of spheroidal subcellular structures analagous to modern lipid vesicles or yolk granules. (A3) High magnification of an orthoslice at the boundary between the two cells. (B1 and B2) Two-cell embryo of the sea urchin Heliocidaris erythrogamma, including an enlarged view of the lipid vesicles. Such vesicles are spheroidal and can be even larger in large-egged embryos of other modern invertebrates. (C) Orthosliced volume rendering of a possible embryo (specimen SB0604Clea), illustrating intact, deflated, and collapsed spheroidal and ovoidal interior structures, coated by several generations of cement. The left hemisphere is dominated by a clotted fabric. Representative deflated (white arrowhead) and collapsed (black arrow) spheres are indicated. Slices in (A) and (C) are negatives: Darker grayscale values represent lower x-ray attenuation. (D and F) Photomicrographs of four-cell embryos with subcellular structures (arrowheads). (F) Extracted, embedded, and thin-sectioned specimen. (E1) X-ray section through a four-cell embryo (specimen DOU-8), showing three of the cells with paired subcellular structures (one pair is indicated by arrowheads), each with slightly greater attenuation (dark regions in image). (E2 to E4) Isosurface models of the four tetrahedrally arranged cells, with two of the cells extracted. All cells have paired subcellular structures, but only the upper two cells are rendered transparent to show intracellular structures (shaded green). (E4) Embryo from (E2), rotated horizontally 180░;. (G) TEM images of reniform (G1) to spheroidal (G2) structures like those in (D) to (F). Although the exact orientation of the embryo in (G1) is poorly constrained because of difficulties inherent in sample embedding and microtoming, it is hypothesized that this represents a tangential section through a reniform subcellular structure, so that each kidney-shaped structure is represented by two subcircular, more electron-dense areas (dark regions in image). (H) Transmitted-light photomicrograph of much smaller algal or cyanobacterial cells, each with one or more intracellular structures that occupy much of each cell. Relative scale bar size is as follows: for (A) and (B), 170 Ám; for (C), 270 Ám; for (D), 150 Ám; for (E), 150 Ám; for (F), 190 Ám; for (G1), 8 Ám; for (G2), 5 Ám; for (H), 20 Ám.

The feature that jumps out at us from these fossils are their similarities to modern organisms at early stages of development. However, the authors also not a significant absence, a difference from modern embryos: even in the later stage fossils of a thousand cells or more, there is no sign of epithelialization. This is a major lack. One of the hallmarks of the development of modern metazoans, from sponges on up, is the early formation of sheets of cells—even the terms diploblast and triploblast refer to the number of tissue layers formed, and in derived organisms like the zebrafish what we watch in early development is the formation of ectodermal and mesodermal sheets of tissue, and their movements and interactions. These embryos don’t exhibit any sign of doing that! The absence of such a key feature suggests that these animals are very primitive, and that what we’re seeing in this collection are the preserved remains of the embryos of stem-group metazoans.

Hagadorn JW, Xiao S, Donoghue PC, Bengtson S, Gostling NJ, Pawlowska M, Raff EC, Raff RA, Turner FR, Chongyu Y, Zhou C, Yuan X, McFeely MB, Stampanoni M, Nealson KH (2006) Cellular and subcellular structure of neoproterozoic animal embryos. Science 314(5797):291-4.