FIGURE 4.

Study type utilized to investigate cellular regeneration of the NP. Studies investigating potential cellular therapy to regenerate the NP expanding over 26 years (from January 1994 to December 2020) were classified according to the type of study performed; the number (A), percentage (B) of study type and the percentage of study type relative to the year (C) from the literature search that used the following model systems: in vitro (including 2D and 3D culture), tissue explants, organ culture, in vivo (subcutaneous or injected into a healthy or degenerate intervertebral disc), canine clinical trials and human clinical trials