ACAN and atrial fibrillation: Human disc NP tissues have a higher content of glycosaminoglycans than AF in donors aged 25 years or younger but thereafter lose them; further, NP specimens have a higher level of newly synthesized aggrecan than AF in donors five years or younger however lose it markedly by 5 to 15 years [5], corresponding well with the disappearance of notochordal cells [22].