Appropriate extracellular matrix content (high proteoglycan to collagen ratio, an abundance of aggrecan and collagen type II for NP; collagen types I and II for AF) and biomechanical properties (elastic modulus close to that of the native NP or AF) are further requirements for cellular aggregates designed for application in IVD repair [123,124,125]. This evidence concerns the gene ACAN and atrial fibrillation.