MicroRNAs are also proposed to be involved.The dementia that occurs in Down syndrome is due to an excess of amyloid beta peptide produced in the brain and is similar to Alzheimers disease, which also involves amyloid beta build-up. Amyloid beta is processed from amyloid precursor protein, the gene for which is located on chromosome 21. Senile plaques and neurofibrillary tangles are present in nearly all by 35 years of age, though dementia may not be present. Those with Down syndrome also lack a normal number of lymphocytes and produce less antibodies which contributes to their increased risk of infection. Epigenetics
Down syndrome is associated with an increased risk of many chronic diseases that are typically associated with older age such as Alzheimers disease. The accelerated aging suggest that trisomy 21 increases the biological age of tissues, but molecular evidence for this hypothesis is sparse. According to a biomarker of tissue age known as epigenetic clock, trisomy 21 increases the age of blood and brain tissue (on average by 6.6 years). Diagnosis
Before birth
When screening tests predict a high possibility of Down syndrome, a more invasive diagnostic test (amniocentesis or chorionic villus sampling) is needed to confirm the diagnosis. The false-positive rate with screening is about 2–5% (see section Screening below). Amniocentesis and chorionic villus sampling are more reliable tests, but they increase the risk of miscarriage by between 0.5 and 1%. The risk of limb problems may be increased in the offspring if chorionic villus sampling is performed before 10 weeks.
If reproduced in humans, this could potentially enable infertile men to father children with their own sperm.Researchers from Montana State University developed precursors of sperm from skin cells of infertile men.Sharpe et al. comment on the success of intracytoplasmic sperm injection (ICSI) in women saying, "[t]hus, the woman carries the treatment burden for male infertility, a fairly unique scenario in medical practice. Ironically, ICSIs success has effectively diverted attention from identifying what causes male infertility and focused research onto the female, to optimize the provision of eggs and a receptive endometrium, on which ICSIs success depends." Prevalence
Currently, there are no solid numbers on how many couples worldwide experience infertility, but the World Health Organization estimates between 60 and 80 million couples are affected. The population in different regions have varying amounts of infertility. Starting in the late 20th century, scientists have expressed concerns about the declining semen quality in men. A study was done in 1992 with men who had never experienced infertility showed that the amount of sperm in semen had declined by 1% per year since 1938. Further research a few years later also confirmed the decline in sperm count and also seminal volume. Various studies in Finland, Southern Tunisia, and Argentina also showed a decline in sperm count, motility, morphology, and seminal volume. Males from India had a 30.3% decline in sperm count, 22.9% decline in sperm motility, and a 51% decrease in morphology over a span of a decade.