Again upon fertilization, a portion of the chromosome will be expressed in triplicate. Any evidence of trisomy at all can be sufficient to cause the disorder In 1929, the average life span of an individual with Down s syndrome was 9 years. Morphologically, the brain of a Down s individual is smaller than a normal individual s with a reduction in the size of both cerebral and cerebellar components. Using methenamine silver staining techniques with a Nissl counterstain, facilitates the detection of pre-plaques or presumed early senile plaques. Large plaques are found to be in the localization of glial nuclei, capillaries and neuronal perikarya.
Smaller plaques tend to be localized around neuron cell bodies.
The gene encoding for this is thought to be on chromosome 21 producing at least three different forms of APP mRNA expressed in the brain, spleen, heart and kidney. It is still unknown whether or not a plasma precursor exists. In DS people over 40, features of AD including senile plaques and neurofibrillary tangles are seen as well as the development of dementia. In younger individuals they are not. Using the above mentioned staining technique, a large number of pre-plaques can be seen in the younger individuals suggesting that the deposition of amyloid precedes the degeneration of neurites and may originate from proximal dendrites of morphologically normal neurons.