Back pain is a leading cause of global disability and is commonly associated with intervertebral disc (IVD) pathologies. Injuries to the ligamentous annulus fibrosus (AF) of the IVD are often implicated in painful spinal problems and are difficult to repair, since they lack regenerative capacity and exist in a challenging mechanical and biological environment. There is a critical need for the development of repair strategies that can restore native AF structure and function and promote biological regeneration, and for an improved understanding of fundamental AF cell (annulocyte) biology. This thesis generated knowledge that advanced these fundamental needs by defining cellular subpopulations of the AF, establishing biomechanical tests to precisely assess IVD function, creating an in vivo functional model of IVD regeneration, and elucidating cell sources and mechanisms of IVD regeneration. Of the several cell populations that exist within the AF, Scleraxis (Scx) expressing cells are an interesting and understudied subpopulation. This thesis defines Scx as a novel AF phenotypic marker that can be useful for identifying AF regeneration. Extensive development of methods to repeatedly and reliably create neonatal injuries and to detect functional changes using ex vivo biomechanical testing were performed, allowing for characterization of neonatal injury responses during early, mid, and latestage healing. Low force testing conditions that generate less AF fiber tension were best at identifying small defects, which were identified in multiple biomechanical properties. Neonatal IVD regeneration was characterized by restored IVD height and biomechanical function, and annulocyte differentiation. Neonates deposited dense and organized extracellular matrix (ECM) at the injury site despite lack of complete structural regeneration. Genetic lineage tracing and immunostaining of multiple cell types determined that annulocytes were the predominant cells involved with regenerative neonatal healing, involving dedifferentiation, proliferation, and redifferentiation. In contrast to neonatal functional and cellular regeneration, adult IVDs heal poorly with limited cell recruitment or ECM deposition. The neonatal model of functional and cellular IVD regeneration can be used as a template to identify mechanisms of AF differentiation and regeneration, which will inform future repair strategies to treat low back pain.