Figure 5.

A model of disc growth. (A) At birth, intervertebral disc grows through proliferation and differentiation of the cell population (thin arrows) in the nucleus pulposus (NP) and annulus fibrosus (AF). (B) Under mechanical loading (direction indicated by the thick black arrows), NP cell clusters continue to expand in the lateral direction. Remodeling of extracellular matrix (ECM) begins in AF. (C) At a later stage, ECM deposition increases in both the NP and AF, leading to a considerable reduction of cell density in the disc. The more proteoglycan-rich matrix increases the swelling pressure in the NP (hollow arrows) against the mechanical loading (black arrows), with the NP cell population regressing in the matrix. Matrix remodeling continues in AF. (D) As the disc matures, the cartilaginous endplates are formed after completion of secondary ossification in the epiphysis of vertebra. The colors in the panels depict the FAST staining patterns observed at different stages of growth in the mouse. NP, nucleus pulposus; IA, inner annulus; OA, outer annulus; SA, superficial annulus; EP, cartilaginous endplate; VB, vertebral body.