As we explored last week, the brain is a complex organ with highly specialized areas that control almost all of the body's functions (both conscious and unconscious). This week I hope to shed somelight on the differences between a 'typical' brain and one with Trisomy 21.
|Lateral surface of left cerebral hemisphere|
of the 'typical' brain
Through autopsy, MRI imaging and related studies, many anatomical differences have been noted in the brain with Trisomy 21. There is:
- Statistically a brain that is 18% smaller by volume
- A smaller than average occipital lobe and brain stem
- Alterations in the layers of the cortex (cortical lamination)
- A simplified appearance to the sulci (the furrows or wrinkles in the surface of the brain. The inside of the furrow is known as a sulcus, while the crest is known as a gyrus)
- A smaller cerebellum, which could account for the hypotonia, motor-coordination, articulation, fluency, syntactic, language and cognitive issues with Down syndrome.
- Smaller frontal lobes (although in proportion to the rest of the smaller sized brain) could account for cognitive deficits, executive dysfunction, inattention, tendency towards perseveration.
- Smaller temporal lobes than the average brain, although larger comparatively when size corrected for the smaller overall size of the brain with Down syndrome.
- Larger white matter volumes within the temporal lobe which could attribute to cognitive dysfunction
- Adults with DS have been found to have a larger parahippocampal gyrus (the fold of the cerebral cortex that lies over the hippocampus that is normally composed mainly of grey matter). One study found an inverse relationship between IQ and parahippocampal gyrus size.
- Smaller hippocampus volumes have been found in adults with DS which may contribute to memory and language deficits
- A comparatively smaller superior temporal gyrus which could significantly contribute to language deficits as it is the location of both the primary auditory cortex (region responsible for sound) and Wernicke's Area (region responsible for speech and language recognition)
- More grey matter in the parietal lobe. This could account for the strength of visuospacial processing and visuospacial short term memory.
|A brain with DS, noting the 'boxy' shape and shortened|
superior temporal gyrus (Image courtesy of
Virginia Commonwealth University's Department of Pathology)
There are also differences in the cells themselves in the Trisomy 21 brain:
- Neurons have a reduced number of dendrites, less synapses an are often clustered irregularly. Early in development, a infant with DS has a rapidly growing dendritic tree, which connects neurons together. Within the first year however, this growth slows.
- Oligodendrocytes, a type of glial cell, create the mylen sheaths which insulate the axons of a nerve cell. There is some dysfunction with these cells in Down syndrome which is seen as delayed mylenation in the frontal and temporal lobes
- There are more microglial cells found in Trisomy 21
- There can be a presence of nerve cell heterotopias in the white layers of the cerebellum (which could indicate some disturbance of cell migration in the embryo)
- A decreased amount of granular cells throughout the cerebral cortex
- A decreased amount of neurons in the occipital cortex and hypothalamus
- Larger amount of astrocytes in the temporal lobe
It is possible that over expression of the T21 gene affects apoptosis or programmed cell death. This could potentially account for lower numbers of neurons in specific areas of the brain and the prevalence of leukemia in the DS population. Also, compounds known as Reactive Oxidant Species could contribute to neurodegeneration by oxidation.
Other factors to consider:
Beta-amyloid expression in children with Down syndrome is no different than in normal children. However, it disappears after age two then reappears in adulthood.
The accumulation of beta amyloid deposits, senile plaques and neurofibrillary tangles starts at approximately age 40 which may represent or lead to an Alzheimer's like neurodegeneration
[Next week: Down syndrome, Alzheimer's and a Very Special Mouse]
Becker, L., T. Mito, S. Takashima, and K. Onodera. "Growth and Development of the Brain in Down Syndrome." Progress in Clinical and Biological Research, 373 (1991): 133-52. Web.
Lubec, G., and E. Engidawork. "The Brain in Down Syndrome (TRISOMY 21)." The Journal of Neurology 249.10 (2002): 1347-356. Web.
Pinter, Joseph D., Stephan Eliez, J. Eric Schmitt, George T. Capone, and Allan E. Reiss. "Neuroanatomy of Down’s Syndrome: A High-Resolution MRI Study." The American Journal of Psychiatry 158 (2001): 1659-665.