The diagram above shows the early cleavages of the embryo of the scaphopod mollusc, Dentalium. You may notice a few peculiarities: the first cleavage is asymmetric, producing a cell called AB and a larger sister cell, CD. Before the second division, CD makes a large bulge, called a polar lobe, and it almost looks like it’s a three-cell stage—this is called a trefoil embryo, and can look a bit like Mickey Mouse. The second division produces an A, a B, a C, and a D cell, and there’s that polar lobe, about as large as the regular cells, so that it now resembles a 5-cell embryo. What’s going on in these animals?
Here’s another example, another mollusc with the beautiful name Ilyanassa (seriously, my kids almost got named after some invertebrates. Ilyanassa, Ciona, Dentalia…but no, not Crepidula):
Again, we see the peculiar movement of the cytoplasm prior to division, a process called ooplasmic segregation. The contents of the cell stream to one side (A), and when division occurs, one of the sister cells is left holding the polar lobe (B), forming a trefoil embryo. The lobe contents flow back into the cell, so that cell is much, much larger than its sister (C). The D cell and its descendants may do this for the next few divisions, making it obviously different from the other cells in the embryo, even as the animal begins the pattern of molluscan spiral cleavage.
What we’re seeing is a simple and overt mechanism for setting up early asymmetries in an animal. Molecular determinants that define certain fates in the embryo are segregated and localized to a single cell in the dividing blastula. We only see it now in certain spiralian embryos, as other animals seem to have evolved more subtle mechanisms for doing the same thing.
We know the polar lobe is part of an essential mechanism for development because, as you might guess, when embryologists see such a discrete blob of cytoplasm, they have an irresistible urge to snip it off. It’s hanging there by a tiny stalk of membrane, so it’s fairly easy to do…and the embryos survive and continue to divide. Unfortunately, they’ve lost some crucial information. Their anterior-posterior axis is undefined, and among the structures that fail to form are the velum (a ciliated feeding structure in the larva), foot, shell, heart, intestine, and eyes. Other bits do form, but it’s basically a doomed partial embryo.
When you see a polar lobe in an embryo, what you are seeing is an early morphological for an animal with polarity and specialization—it means you’re looking at an organism that is definitely going to grow into something more complex than a ball of cells. Since the processes that preserve fossilized embryos only seem effective at very early stages of development, this is good to know…especially when you find a collection of 580 million year old cleavage stage embryos and are looking for clues about what they are.
Now that you’ve had a little context, isn’t it obvious what we’re looking at? These are phosphatized specimens from the famous Doushantuo Formation of China, a layer of rock dated to the Precambrian, about 580 million years ago. The odd number of blastomeres would be suspicious if you didn’t know that polar lobe formation was a primitive mode of symmetry breaking in developing embryos. What this suggests is that these are bilaterian embryos that, if they hadn’t been suffocated in layers of fine silt over half a billion years ago, would have grown up to be some interesting and relatively sophisticated animal.
We don’t know exactly what they would have grown up into, but one interesting observation is that in older embryos from the same deposit, there is no indication that any developed by spiral cleavage. While lobe formation is restricted to spiralians among extant animals, what we see here are probably non-spiralians that are using this method of establishing early asymmetries. That suggests that polar lobe formation may be a very primitive mode of segregating cytoplasmic determinants in the metazoa.
Chen J-Y, Bottjer DJ, Davidson EH, Dornbos SQ, Gao X, Yang Y-H, Li C-W, Li G, Wang X-Q, Xian D-C, Wu H-J, Hwu Y-K, Tafforeau P (2006) Phosphatized Polar Lobe-Forming Embryos from the Precambrian of Southwest China. Science 312(5780):1644-1646.