To say that something is as "scarce as hen's teeth" is to imply that it is so rare as to call its very existence into question. The meaning of this phrase is based on the common knowledge that chickens are edentulous. The chicken has solved the problem of processing incoming food with the development of a hard, toothless beak, which is ideally suited to cracking open seed casings and insect exoskeletons. As with so many favorite folksy colloquialisms, the phrase "scarce as hen's teeth" originated in the southern United States in the 19th century, first appearing in print in 1858. It is usually uttered with some degree of exasperation mixed with amusement.
Here's the kicker: hens can grow teeth. They just don't live to tell the tale.
Junk DNA
Chickens, like many eukaryotes, only express a small fraction of their genome. Nature has a tendency to deprecate superseded genes, rather than removing them entirely. This is also the case in humans. The library of your genomic sequence is filled to the rafters with musty tomes from your evolutionary forbearers, most of which will never be transcribed or translated as your industrious cells go about their business of creating and maintaining you. These genetic "dead spots" are known as introns when they occur within the range of an active gene, and intergenic DNA or non-coding DNA when they occur outside of an active gene (or you can simply refer to it all as "junk DNA" if you're a writer for a lay publication who believes that your audience is too stupid to grasp the fundamental mechanics of life). Some of these non-coding sequences may still serve a purpose in providing marker regions for active genes, but, as a human being, around 98% of your nuclear genetic material will never experience the joys of protein synthesis. The process by which an active gene evolves into a non-coding sequence is poorly understood.
A chicken's genetic library has a somewhat different set of out-of-print texts than your own. More than 70 million years ago, the ancestors of Henny Penny and Foghorn Leghorn were running through a steamy Jurassic jungle, looking rather more fearsome than their modern-day descendents. The modern chicken evolved from a group of dinosaurs named Maniraptora, and is at least a close cousin (80 million times removed), if not a direct descendent of well-known beasties such as the velociraptor. The dead cells of the tasty creature you braised with wine and mushrooms last night may have carried the blueprint for razor-sharp teeth and a structure informally known as the "killer claw."
All of this genetic mumbo jumbo about birds and dinosaurs is interesting, but not exactly news to you. After all, you dutifully obsessed over dinosaurs for a brief period as a child. You survived the pop culture blitzkrieg that was Jurassic Park. You already know all about the evolutionary biology of dinosaurs and chickens, at least where it relates to the intersection between deadly teeth, slashing claws, and CGI effects, right? What you may not already know, however, is that someone is investing a ton of money to explore the dormant genetic secrets of the common chicken. And they have succeeded in unleashing Chicken Little's inner maniraptor. Sort of.
Of Chickens and Men
In order to understand how this all came about, you must first learn just a bit about embryogenesis. Because you occasionally managed to stay awake in high school biology class, much of this will sound vaguely familiar.
The embryos of birds and mammals look quite similar during the very early stages of development. What starts as a simple ball of cells quickly develops into three nested layers of tissue: the endoderm (literally, "inside skin"), the mesoderm ("middle skin"), and the ectoderm ("outside skin"). Shortly after this process occurs, the germ layers start rearranging themselves in new and interesting ways. Part of the ectoderm pinches off inwards and forms the neural tube, which will eventually sprout a brain at one end and become the organism's spinal cord. As the neural tube forms, it temporarily remains connected to the ectoderm by a band of tissue known as the neural crest. Although the neural crest is a transient structure, it is a developmental powerhouse, creating cells that will give rise to everything from nerves to bones to muscle. The neural crest is so important during early embryonic development that it is sometimes referred to as the fourth germ layer.
Both the neural crest and the mesoderm create a particular type of tissue called mesenchyme, which is unspecialized connective tissue with the potential to develop into cartilage, bones, and blood vessels, amongst other things. Almost every structure in your body was created with the aid of mesenchyme. In this early stage of development, mesenchyme is basically the goop that fills the spaces between everything else. Mesenchyme produced by the neural crest is sometimes called ectomesenchyme, and is responsible for much of the development of hard and soft tissues in the head and neck. There is still debate as to whether ectomesenchyme is substantially different from mesodermally produced mesenchyme, which forms much of the circulatory system and connective tissue in the rest of the body.
Meanwhile, the endoderm at the center of it all elongates into a tube that, together with tissue arising from the mesoderm, will eventually differentiate into most of the internal organs in the abdominal cavity. The ectoderm starts developing folds and ridges that will give rise to the skin and associated glands, the epithelium lining the mouth and nasal cavity, the lens and cornea of the eyes, and the beginnings of limb buds. At this point, the development of mammal and bird embryos begins to diverge as structures become more specialized. However, the similarities in early embryonic development make chick embryos an extremely popular subject for scientific study, as learning about early chicken development could also illuminate early stem cell differentiation during human embryonic development.
Jurassic Chicken Within Your (Grandchildren's) Lifetime
The story of hen's teeth started in the year 2000, with the publication of a study of chick embryos jointly funded by several American medical schools and a hospital in London. Earlier studies had noted that the oral epithelium in a chick embryo looks remarkably similar to the dental lamina in mammal embryos that eventually gives rise to teeth. The researchers hypothesized that the reason why chickens don't develop teeth is that they do not synthesize bone morphogenetic protein 4 (BMP4) in their jaws, which previous studies had shown is one of several proteins key to tooth development in mice.
When BMP4 was artificially introduced to developing chick embryos, it started a chain reaction of gene expression that resulted in the chicks developing rudimentary tooth buds. The researchers concluded that since a chick embryo's tissue is still responsive to biochemical pathways for forming dentition, it is possible that a chicken's DNA may still contain the (now inactive) genes necessary to initiate tooth growth. The embryos used in the study were not hatched.
After almost a year, the lay media picked up the study results and started reporting sensationalized stories about chickens and their toothy dinosaur ancestors. BBC News went so far as to suggest that this research was just a promising stepping stone on the path to engineering a mythical creature dubbed the "Jurassic Chicken," which they suggested could become a reality in the next 50-100 years (precisely how this timeline was calculated is not revealed). Of course, they did not elaborate on the reasons why the creation of a Jurassic Chicken might be desirable. This was in the summer of 2001. It was a slow news day.
Frankenchicken Cures Male Pattern Baldness, Cancer
In 2003, another study was published investigating the chicken's latent ability to grow chompers. This team of British and French researchers built on the previous study's discoveries, hypothesizing that the chick embryo's ectomesenchyme lacked the ability to signal for the production of BMP4 necessary to activate tooth growth. In order to prove this hypothesis, the researchers engaged in a seemingly gruesome exercise in vivisection, transplanting parts of neural tubes (along with the neural crest) harvested from the embryos of mice into very young chicken embryos. These chimera embryos were studied for 14 days following transplantation.
As the grafted tissue from the mouse donor started migrating throughout the chick's head, producing mesenchyme, tooth bud structures formed in the oral epithelium, confirming that chicken embryonic tissue retains the potential to develop teeth. However, the progress of the developing tooth buds more or less followed the development program of mammalian teeth, rather than the teeth of ancient birds or dinosaurs. Although the mouse and chick tissues were obviously capable of interacting with each other in order to form teeth, apparently the mouse's genes were running the show.
Again, the results of this esoteric study were reported in the mass media, with a bizarrely inaccurate twist. CNN chose to headline the story with "Baldness hope as birds get teeth." Mind you, the study never actually addressed the possibility of engineering hirsute chickens. The production of hair never even entered into consideration. Nevertheless, the editors at CNN felt that teeth and hair are, like, basically the same, y'know? And if those brainiacs down at the lab can get a damn chicken to grow some damn pearly whites, then they should totally be able to reverse your receding hairline, right? Fair is fair.
Problem Solved Without a Peep
In early 2006, researchers from the Universities of Wisconsin and Manchester published a study demonstrating that chickens sometimes develop a natural mutation that causes the formation of teeth. They were specifically studying chick embryos with the talpid mutation, first documented 50 years ago. The talpid mutation has been the subject of continuing scientific study because no one has identified the specific gene (or genes) involved, and it causes rather interesting deformities during the development of the limbs and head. The mutation is uniformly fatal: most talpid chicks die very early during embryonic development; none survive to hatch.
The study found that embryos with a particular talpid variant named talpid2 grow conical, "saber-like" teeth that are strikingly similar in appearance, position, and development progression to the teeth of alligator embryos. None of the previous studies had succeeded in initiating dental development in chicks without the use of exogenous proteins or tissue from other species. The presence of teeth in the talpid mutants suggests that chickens do indeed retain all of the genetic code necessary to grow teeth similar to their dinosaur ancestors. So why don't chickens grow teeth, except in the presence of a cataclysmic mutation?
Although no one knows precisely what causes the talpid mutation, the researchers believe they know why talpid is associated with the atavistic growth of dinosaur-like teeth. They hypothesized that the deactivation of the genes for initiating tooth development has as much to do with anatomy as biochemistry. The gross craniofacial deformities of the talpid mutants mean that the oral/aboral junction (literally, the boundary between that which is mouth and that which is not mouth) is in a different position during early embryonic development compared to a normal chicken. The location of the oral/aboral junction in talpid chicks results in a malformed jaw and beak, but also brings the oral epidermis into close contact with the chick ectomesenchyme. It appears that the proximity of these various tissues is what induces the expression of the dormant genes for tooth formation.
Despite the potential to cover the story under attention-grabbing headlines like "Mutant chickens grow dinosaur teeth," this study received almost no coverage in the mainstream press. A short summary in Scientific American, a one-sentence blurb in Harper's, a lengthy piece in the Milwaukee Journal Sentinel, and coverage in lesser known "science-oriented" publications - that's it. The only national news organization to even mention the study was, surprisingly, FOX News, which picked up a syndicated story on the study results and published it on their website.
In Conclusion
In terms of evolutionary biology, these studies increase our understanding of the mechanics of evolution, but they also raise more questions than they answer. Mechanics do not always illuminate motivation. Which came first, the beak or the loss of teeth? What selective pressures induced this change in avian anatomy?
In terms of gene therapy and stem cell research, these studies and others contribute to a growing body of knowledge, while simultaneously proving that we don't really know what we're doing. Early efforts in these areas have concentrated on mapping particular genes to the particular proteins they encode, to the particular effect of those proteins. However, the key to the eradication of diseases like cystic fibrosis, multiple sclerosis, and Alzheimer's may not lie in the proteins most closely associated with these conditions, but somewhere upstream in the astonishingly complex chain reaction that is life.
In terms of journalism, always greet any story announcing the results of a scientific study with skepticism, especially if it occurs somewhere under the heading of "Odd News." Accuracy in scientific reporting in the lay media is as scarce as hen's teeth.
Clarification: La petite mort points out "Don't forget hens are born with a tooth to crack open the eggs, so they are not that rare." It is true that many egg-bearing species hatch with the help of a structure known as an egg tooth. In some reptiles, this is a true tooth that is later lost or resorbed as the individual matures. In chickens and other birds, the egg tooth is not really a tooth at all (in that it has no pulp, dentin, enamel, or any of the other characteristics associated with teeth), but instead is a small, horny protuberence at the end of the beak that falls off within a couple weeks after hatching. However, the egg tooth is probably sufficiently similar to a true tooth to further invalidate the saying.
Sources:
- "Baldness hope as birds get teeth." June 4, 2003. CNN.com. http://edition.cnn.com/2003/TECH/science/06/04/teeth.birds/. Accessed: July 21, 2007.
- Chen, YiPing, et al. "Conservation of early odontogenic signaling pathways in Aves." Proceedings of the National Academy of Sciences. August 22, 2000. http://www.pnas.org/cgi/content/full/97/18/10044. Accessed: July 21, 2007.
- Gilbert, Scott F. Developmental Biology, 6th ed. Sunderland, Massachusetts: Sinauer Associates. 2000. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio. Accessed: July 24, 2007.
- Harris, Matthew P., et al. "The Development of Archosaurian First-Generation Teeth in a Chicken Mutant." Current Biology. February 21, 2006. http://dx.doi.org/10.1016/j.cub.2005.12.047. Accessed: July 21, 2007.
- "Hen." Oxford English Dictionary. http://dictionary.oed.com/cgi/entry/50104762. Accessed: July 19, 2007.
- "Jurassic chicken '50-100 years off'". BBC News. July 19, 2001. http://news.bbc.co.uk/1/hi/sci/tech/1446706.stm. Accessed: July 21, 2007.
- "Maniraptora." Wikipedia. http://en.wikipedia.org/wiki/Maniraptora. Accessed: July 20, 2007.
- Mitsiadis, Thimios, et al. "Development of teeth in chick embryos after mouse neural crest transplantations." Proceeedings of the National Academy of Sciences. May 9, 2003. http://www.pnas.org/cgi/content/full/100/11/6541. Accessed: July 21, 2007.
- Rust, Susanne. "Teeth discovered in mutant chickens: Genetic trait returns after millions of years." JS Online: Milwaukee Journal Sentinel. February 21, 2006. http://www.jsonline.com/story/index.aspx?id=403228. Accessed: July 29, 2007.