Characterization of zebrafish nucleus pulposus in development and aging: A fish model for probing human intervertebral disc degeneration and regeneration

Intervertebral disc (IVD) degeneration (IDD) contributes to low back and neck pain. The nucleus pulposus (NP) of the IVD is usually degenerated first in age-related IDD. Addressing NP degeneration remains challenging due to insufficient understanding of its etiologies. Here, we characterize zebrafish IVD, particularly the NP, and note several characteristics that are common to or different from those of mammals. The adult zebrafish NP can be partitioned into three sections: the NP envelope with a monolayer of basal cells, the peripheral region with clustered vacuolated and non-vacuolated NP cells, and the central fibrotic region with low cellularity. With age, the percentage of vacuolated NP cells decreases, whereas that of non-vacuolated NP cells increases, as in mammals. However, a cluster of vacuolated cells persists in the NP periphery even in aged zebrafish. Notably, the ratio of the span of the fibrotic NP center to NP diameter remains largely constant at 50% regardless of age, suggesting that the zebrafish NP can better maintain its overall shape and structure, compared to mammalian NP. The fibrotic NP center and cell-populated NP periphery serve as internal controls to each other for studying NP maintenance and fibrosis. With external development, high fecundity, and great regenerative capabilities, zebrafish may be a unique NP model to complement mammalian models for studying how the NP degenerates and how the dormant capability of NP regeneration may be revitalized in humans.