Myelination of the corpus callosum in male and female rats following complex environment housing during adulthood

Inspired by yesterday's post on this review article I read this paper on myelination of the corpus callosum in male and female rats following complex environment housing during adulthood.  The authors were looking to see if putting adult rats (4 months) in an enriched environment would increase myelination in the corpus callosum.  Previous studies have shown that rats in an enriched environment grow more synapses and also increase the size number of astrocytes in the brain.  Myelination has been seen in rats raised in enriched environments but hadn't been well studied in adult rats before this study.

Just to jump to the end, the authors did not see and increase in myelination.  But they did see an increase in volume of the splenium of the corpus callosum which was driven by an increase in unmyelinated axons and glial cell processes.  They conclude that its unlikely that oligodendrocytes are increasing in size (since the amount of myelin stays the same) and so deduce that there must be more astrocytes in the splenium.  They tested the genu of the corpus callosum (at the front) and didn't see any changes.

I think this is wonderfully interesting: if rats don't increase the amount of myelin but do increase the number of supporting cells, then maybe we shouldn't be hypothesizing an increase in myelin with experience in humans.  Obviously I understand that rats and humans are very different, but this may be a situation in which the power of glial cells is being overlooked.  Maybe increasing the amount of myelin isn't what's necessary, rather an increase in the support for the electrical signal transmission.  Quality rather than quantity.  Having said that, I don't know what the unmyelinated axons are doing.....any suggestions?

Stereological point counting technique. Length of the corpus callosum is shown on a mid-sagittal section through a corpus callosum stained with osmium tetraoxide (each tic mark on the scale at the top equal to 0.5 mm) and the boundaries for the regions sampled (genu on the right, splenium on the left) are indicated by black lines (A). Toluidine blue stained section through the corpus callosum (B). The arrows (white) denote myelinated axons and the * indicate the location of two glial cell bodies. The area occupied by other material (unmyelinated axons and glial processes) is the amorphous, interstitial space surrounding the myelinated axons. Scale bar = 10 μm.

Myelination of the corpus callosum in male and female rats following complex environment housing during adulthood.

Markham JA, Herting MM, Luszpak AE, Juraska JM, Greenough WT.

Brain Res. 2009 Sep 8;1288:9-17. Epub 2009 Jul 9. 
PMID: 19596280