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.)

 

Convergent evolution towards big brains in humans and dolphins

From Not Exactly Rocket Science (via John Hawks’ Weblog):

Every whale and dolphin evolved from a deer-like animal with slender, hoofed legs,
which lived between 53 and 56 million years ago. Over time, these
ancestral creatures became more streamlined, and their tails widened
into flukes. They lost their hind limbs, and their front ones became
paddles. And they became smarter. Today, whales and dolphins –
collectively known as cetaceans – are among the most intelligent of
mammals, with smarts that rival our own primate relatives.

Now, Shixia Xu from Nanjing Normal University has found that a gene
called ASPM seems to have played an important role in the evolution of
cetacean brains. The gene shows clear signatures of adaptive change at
two points in history, when the brains of some cetaceans ballooned in
size. But ASPM has also been linked to the evolution of bigger brains in
another branch of the mammal family tree – ours. It went through similar bursts of accelerated evolution in the great apes, and especially in our own ancestors after they split away from chimpanzees.

It seems that both primates and cetaceans—the intellectual
heavyweights of the animal world—could owe our bulging brains to changes
in the same gene. “It’s a significant result,” says Michael McGowen,
who studies the genetic evolution of whales at Wayne State University.
“The work on ASPM shows clear evidence of adaptive evolution, and adds
to the growing evidence of convergence between primates and cetaceans
from a molecular perspective.”

… continue reading at Not Exactly Rocket Science. 

Note: it is one of the microcephalin genes, if you wondered.

 

Dolphin mtDNA phylogeny

Left to right: pig, dolphin and human brains

There is hardly any more iconic animal than the dolphin but there is also hardly any animal closer to us in a key identity element: intelligence. Lacking hands and living in water, dolphins have never developed some of the technological landmarks that we associate with human-like intelligence: fire management and tool creation, however their brains are, in comparison to body mass, very much our size, they demonstrate once and again to be very intelligent beings with some abilities (notably sonar perception and communication) rather beyond our comprehension. For instance only recently have we begun to understand that dolphin language is not framed in the mere two dimensions ours is but is actually tridimensional.

A few weeks ago, it came out in a discussion on “Neanderthals and us” whether dolphins, with their many different species, many of them (if not all) showing striking intelligence, could be a model to understand the relations between the various species of the genus Homo in the past. A problem, at least for me, is that we really do not know so much about dolphins either anyhow. Most documentaries are about the successful common bottlenose dolphin Tursiops truncatus or the also very successful and impressively bright orcas. But there are dozens of dolphin species.

In this sense it is very interesting to take a look at this new paper establishing a mitochondrial phylogeny on these sea mammals:

Julia T. Vilstrup et al. Mitogenomic phylogenetic analyses of the Delphinidae with an emphasis on the Globicephalinae. BMC Evolutionary Biology, 2011. Open access.

Particularly illustrative is the proposed phylogeny of figure 1:

It is notable that there are cases of hybridization (fertile hybrids!) in the wild as in captivity, of such different species as Turiops truncatus and Pseudorca crassidens, which would have diverged some 8.5 million years ago, roughly the (true) distance between chimpanzees (and bonobos) and us.

This really challenges the concept of species as defined in classical terms (absolute possibility of production of fertile hybrids) and reinforces the modern revised concept (normal reproduction in the wild).

The closest equivalent (always assuming all age estimates are correct) of “Neanderthal and us” (i.e. Homo ergaster and derived species by most accounts) in the tree above would be the relation between the long-beaked common dolphin (D. capensis) and the Indo-pacific bottlenose dolphin (T. aduncus). Just a little bit upstream and we get to the equivalent of all the Homo genus, while the overall dolphin relationships are in the range, as already mentioned of “Chimpanzees and us”. 

Probably this phylogeny will not be of much use for comparison, more so as the various intelligence levels and other cognitive, linguistic or social adaptions of dolphins are ill understood at the moment, but it is still better than nothing and hence I felt it was an interesting reference to have in mind.