Into brains of newborn mice, researchers implanted human "progenitor cells." These mature into a type of brain cell called astrocytes (see below). They grew into human astrocytes, crowding out mouse astrocytes. The mouse brains became chimeras of human and mouse, with the workhorse mouse brain cells - neurons - nurtured by billions of human astrocytes.
Neuroscience is only beginning to discover what astrocytes do in brains. One job that is known is that they help neurons build connections (synapses) with other neurons. (Firing neurotransmitter molecules across synapses is how neurons communicate.) Human astrocytes are larger and more complex than those of other mammals. Humans' unique brain capabilities may depend on this complexity.
Human astrocytes certainly inspired the mice. Their neurons did indeed build stronger synapses. (Perhaps this was because human astrocytes signal three times faster than mouse astrocytes do.) Mouse learning sharpened, too. On the first try, for instance, altered mice perceived the connection between a noise and an electric shock (a standard learning test in mouse research). Normal mice need a few repetitions to get the idea. Memories of the doctored mice were better too: they remembered mazes, object locations, and the shock lessons longer.
[caption id="attachment_183" align="alignright" width="255" caption="Astrocytes (red) embedded among neurons (green)"][/caption]
The reciprocal pulsing of billions of human and mouse brain cells inside a mouse skull is a little creepy. Imagine one of these hybrid mice exploring your living room. Would you feel like a Stone Age tribesman observing a toy robot? Does the thing think?
Neuroscience has no idea - none - of how a mind rises like a genie from the fleshy human brain. It supposes, however, that the magic trick's spoiler will turn out to reside in physics and chemistry of brain cells. That is the discipline's fundamental assumption. Nowhere else can the mystery be hiding.
But we have no idea what's happening as billions of human astrocytes animate rodent awareness inside the tiny skulls. And "awareness" is one quality of "mind." Do billions of human cells have no effect on mouse awareness? That seems unlikely.
S. H. Saey, Mice get brain boost from transplanted human tissue. Science News, Vol. 183 #7, April 6, 2013
Xiaoning Han, et al., Forebrain Engraftment by Human Glial Progenitor Cells Enhances Synaptic Plasticity and Learning in Adult Mice, Cell Stem Cell, vol. 12, No.3, March 7, 2013.