In 1832 the young Charles Darwin embarked on one of the most epic journeys in the history of biology, if not of all science. As a naturalist on the H.M.S. Beagle, Darwin saw things that challenged the prevailing view of how life arose. On returning to England five years later, he began work on what he called “the species problem.”
“The Voyage of the Beagle” (not published until 1909), comprises the diaries Darwin kept on the journey.
One of his most celebrated observations concerns a group of 14 species of finches he encountered in the Galapagos Islands.
Darwin’s finches, as they have come to be known, have beaks of slightly different shapes and sizes, depending on where they live, what sort of seeds they eat and how they eat them.
Making a modest note about what would turn out to be a world-changing observation, Darwin wrote in “The Voyage”: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”
So the theory of evolution by natural selection was born, influenced in part by speculation about why birds had different shaped beaks. And why am I banging on about it now? Because although Darwin’s finches have been intensively studied, until recently no one knew how genes and cells influenced beak size and shape.
Recently, the journal Science published a paper about cells that had been transplanted from embryonic quails to ducks, and vice versa.
The duck embryos grew pointy little quail beaks and the quails grew distinctive flat, wide duck bills.
And why is this important? Because (aside from the fact that thinking about beak shape influenced Darwin) learning how this happens may advance our understanding of what causes facial birth defects in humans, such as cleft palate. These would also include defects caused by fetal alcohol syndrome, which develops in some babies when a mother drinks too much alcohol during pregnancy.
Jill Helms and Richard Schneider of the department of orthopedic surgery, University of California, San Francisco, knew that bird beaks have many functions, such as preening, eating, feeding chicks, fighting and courting.
And beaks come in an incredible variety of shapes and sizes, both within and between species. But all these shapes and sizes are determined by similar tissues in the embryos of each bird.
In particular, the cartilage and connective tissues of the beak are made from a group of stem cells in the embryo called neural crest cells.
To understand how beaks become so different when they derive from tissues that look the same, Helms and Schneider performed a series of technically elegant experiments that switched the neural crest cells of embryonic ducks and quails while they were still in their eggs.
“Neural crest cells fated to give rise to the beak were grafted from either quail to duck or duck to quail,” the authors write in the Science paper.
“The transplants require an exquisitely sensitive touch,” said Helms in an e-mail interview.
“[Schneider] transplanted a very, very small population of cells from one embryo to another, which had had the same population of cells carefully removed.”
Once the switch was made, the researchers let the eggs incubate until they were about 11 days old. By this stage the embryos are well formed and the shape of the beak is easily discerned and measured.
Helms and Schneider found that each bird had grown the other species’ beak.
“Our paper in Science demonstrates that a population of stem cells, known as the cranial neural crest, provides patterning information for facial morphology,” said Helms.
“Not only did neural crest cells direct their own morphogenesis, but they also patterned non-neural crest facial tissues in a manner characteristic of the donor species,” he added.
In other words, not only do the cells march to the sound of their own drum, creating the donor beak regardless of the host body they find themselves in, but they also call the tune for surrounding host cells, governing their maturation rate and gene expression. Neural crest cells are therefore integral to beak development and evolution.
The researchers also note that a regulatory, commanding role for certain tissues, such as neural crest cells, was anticipated by the Nobel Laureate and pioneering German embryologist Hans Spemann in the 19th century.
Imagining a conversation between a piece of tissue that has been grafted onto another species, just like in the duck/quail experiment, Spemann said: “You tell me to make a mouth; all right, I’ll do so, but I can’t make your kind of mouth; I can make my own and I’ll do that.”
Humans might have cells equivalent to the birds’ neural crest cells. If they do, it would shed light onto human craniofacial development.
“The implications from this work are that some types of craniofacial malformations might be treatable by using these cells to repair defects in utero,” Helms said.
“As fetal surgeons perfect their abilities to repair defects in utero, and imaging techniques improve by leaps and bounds, and as diagnoses of birth defects can be made earlier and earlier in gestation, it becomes more feasible to think about repairing some of these defects prior to birth — fetal surgeries produce no scar tissue; scar tissue typically impedes growth.”
So eventually, surgeons may be able to correct a cleft palate before birth, perhaps one caused by fetal alcohol syndrome, in a similar way to how Helms and Schneider changed beak development in their bird embryos.