Astrocytes: a key "wiring" element behind human intelligence

Recent experiments with mice have shown that those with transplanted human glial cells known as astrocytes perform much better in learning and memory tests afterwards.

Xiaoning Han et al., Forebrain Engraftment by Human Glial Progenitor Cells Enhances Synaptic Plasticity and Learning in Adult Mice. Stem Cell 2013. Pay per view → LINK [doi:10.1016/j.stem.2012.12.015]

See the news article at Science Daily for details. 

One wonders if they gave mice some sort of humanity and the many ethical questions behind this experiment, of course. But what got me wondering after that is do chimpanzees have the same kind of astrocytes as we do?

And the answer seems to be yes but no.

Nancy A. Oberheim et al., Uniquely Hominid Features of Adult Human Astrocytes. The Journal of Neuroscience 2009. Freely accessible → LINK [doi:10.1523/​JNEUROSCI.4707-08.2009]

Chimpanzees and humans share a type of astrocytes not found in our monkey or rodent relatives but the density and complexity of these particular glial cells in humans is much greater than in chimpanzees.

Fig 2 (legend)

One of the most striking features distinguishing humans and chimpanzee from other lower primate and rodent astrocytes was the presence of a previously undescribed pool of morphologically distinct GFAP⁺ cells residing in layers 5–6, characterized by long fibers with prominent varicosities (Fig. 2A). (...) In our analysis of primate tissue, we were able to locate a small number of varicose projection astrocytes within layers 5 or 6 of the chimpanzee cortex (Fig. 2A, inset). These cells differed from those seen in human in that they were smaller and less complex, with fewer main GFAP⁺ processes.

This is not the only difference, another subgroup, the interlaminar astrocytes also shows differences:

In addition to being more numerous than their chimpanzee counterparts, the morphology of interlaminar astrocytes is subtly different in humans. Human interlaminar astrocytes have small spheroid cell bodies and several short processes that contribute to the pial glial limitains, creating a thick network of GFAP fibers not seen in the primate.

A third category, the protoplasmatic astrocytes, is also different:

... the average diameter of protoplasmic cortical astrocytes in the chimpanzee brain was 81.7 ± 1.9 μm (n = 36), which is significantly smaller than human astrocytes, but significantly larger than protoplasmic astrocytes in mouse brain...

So what about cetaceans, which include some of the non-human animals most famed for their intellectual capabilities? The brain structure seems different, so maybe not as easy to compare as with our closest relatives, also cetaceans do not seem so well researched. But we know (source) that at least that the proportion of glial cells in bottlenose dolphin forebrains is almost double than that of humans:

Glial cells outnumber neurons by at least 6 to 1 but the ratio differs in different parts of the nervous system. The ratio can be 100 glials to 1 neuron along nerves in the white matter tracts in the brain; in the frontal cortex the ratio is 4 to 1. Interestingly, whales and dolphins have 7 glials for every neuron in their gigantic forebrains. (Fields, R. Douglas, PhD. The Other Brain. P P 24. NY:Simon & Schuster, 2009.)