ACAN and intervertebral disk degenerative disorder: Age-related spontaneous IDD animal models have been reported in a limited number of species including nonhuman primates,53 sheep,54 and mice.55 In our study, we first described mild signs of IDD in 3.5 year-old tree shrews, characterized by a replacement of notochordal cells by mature nucleopulpocytes associated with an early decrease in aggrecan content.