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This Week

8 August 2012

Chimeric birds could explain how brains get big

BODY of a quail, brain of a zebra finch. The most audacious brain transplant yet could help us understand how brains develop, perhaps opening the way to treating some brain conditions.

Since 2009, Chun-Chun Chen of Duke University in Durham, North Carolina, has performed over 100 brain transplants in bird embryos. In her latest study she transferred the cells destined to become the forebrain of zebra finches (Taeniopygia guttata) into Japanese quail (Coturnix japonica) embryos, after first removing the equivalent quail cells.

Chen incubated the chimeric eggs for up to 16 days, before opening some of them to find that the transplanted cells had integrated into their hosts, forming the normal neural pathways (PLoS One, DOI: 10.1371/journal.pone.0042477).

Of the eggs left to hatch, none did, perhaps because the chimeric embryos’ hybrid brains could not trigger breathing. Chen says she will try to crack the hatching problem by transplanting just half a zebra finch forebrain, leaving half the quail forebrain still in place.


Researchers have been attempting such transplants for decades. In 1988 Evan Balaban of McGill University in Montreal, Canada, transplanted chicken brain cells into a quail embryo, and hatched the resulting chimera. Some of the chicken’s behaviours, such as a propensity to crow, were transferred with the cells (Science, DOI: 10.1126/science.3413496).

Chickens and quails are developmentally similar, though, and quite unlike zebra finches.

“Zebra finches are like humans: they’re born naked and blind,” says Balaban, who also contributed to the latest study. The finch brain grows slowly but reaches a large size. By way of contrast, quails are active immediately after hatching and have a quick growing but ultimately small brain.

Chen is the first to transplant brain cells between two such different species. “This study was a technical tour de force,” says Georg Striedter of the University of California, Irvine.

The transplanted zebra finch brains grew faster in the quail than they would have done inside a zebra finch. That suggests large brains do not simply grow under their own steam. “We think that there are some signals coming from the quail cells that are influencing how much the finch brain cells can proliferate,” says Chen’s supervisor Erich Jarvis.

“The transplanted zebra finch brains grew faster in the quail than they would have done in a zebra finch”

The two species must use the same signals, otherwise the transplanted cells would not have integrated properly. But we don’t know what those signals are or where they come from, says Striedter. They might come from other parts of the brain, or elsewhere in the embryo.

Once the signals are known, they could be blocked or enhanced. “If we could identify that signal or set of signals, we might be able to use it to try to control overall brain size,” says Jarvis. Striedter says that might ultimately help us to understand and treat brain conditions like microcephaly, in which the circumference of the head is smaller than average.