Re-building a whole new body

Imagine how handy it would be if you could rebuild your whole body from a tiny fragment. Some animals can do this, but most such animals (e.g. sponges, hydra, and planarians) have elegantly simple internal structure. But it turns out that many botryllid ascidians, among our closest living invertebrate relatives, are also able to do this [1-3]. As mentioned in a previous post, these marine animals have the same basic organization that we do: each individual has a heart, central nervous system, and through-gut, as well as a number of other features shared with vertebrates.

Adult botryllid ascidians can reproduce sexually, with eggs that develop in a precisely choreographed manner to form a swimming larva. The larva metamorphoses into the founding member of a colony of interconnected individuals that live attached to surface. Each individual reproduces asexually by forming buds as pouches off the tissues surrounding its heart. But under certain conditions (which vary by species [1-3]) the colony can form a whole new individual from aggregates of blood cells. Circulating cells clump up on the wall of one of the blood vessels that reach to the outer edge of the colony [1]. Then the clump hollows out, and starts to mold itself into a new body from scratch*. Because these organisms are fairly closely related to humans, this process likely holds insights for regenerative medicine [3].

However, what I find most exciting is that it shows that development can tolerate remarkable variation. Developmental biology tends to focus on the complicated interplay of interactions required in "normal" development from the egg. We usually think of development as fragile, easily thrown off by a mutation here, or a sip of wine there. But this example dramatically shows that – at least in a few organisms surprisingly closely related to ourselves – cells can organize themselves into the same body from lots of different starting points. Botryllids can form complete new individuals in at least three very different ways: a precise series of events from egg to larva to juvenile; from pouches of a specific epithelium; or from disorganized aggregates of blood cells. Understanding how they do this could reshape how we think about animal development.

*The outer epithelium apparently comes from the cells lining the outside of the blood vessel. Their blood vessels do not have the inner lining of cells that our vessels have.

1) The clearest images and diagrams of this process come from the first description, which is freely available online: Oka, H., and Watanabe, H. 1957. Vascular budding, a new type of budding in Botryllus. Biological Bulletin 112:225–240.
2) This has some neat videos, and some interesting results about the importance of maintaining blood flow for regeneration. Voskoboynik A, Simon-Blecher N, Soen Y, Rinkevich B, De Tomaso AW, Ishizuka KJ, Weissman IL. 2007. Striving for normality: whole body regeneration through a series of abnormal generations. The FASEB Journal 21:1335–1344.
3) This one also has neat videos and discusses the some of the possible molecular pathways involved. Brown, F.D., Keeling, E.L., Le, A.D., and Swalla, B.J. 2009. Whole Body Regeneration in a Colonial Ascidian, Botrylloides violaceus. Journal of Experimental Zoology (Mol. Dev. Evol.) 312B:885-900