Fibrin in intervertebral disc tissue engineering

Fibrin is clinically employed as a versatile, safe, and clinically applicable sealant and cell carrier. It has been able to support disc cell survival, favor extracellular matrix production, and enhance the efficiency of cell transfer in the intervertebral disc (IVD). The aim of this review was to evaluate how fibrin has been used in vitro, in vivo, and in clinical trials for IVD tissue engineering. Within the in vitro studies, disc cells were cultured in fibrin alone or combined with other materials and a difference in the behavior of nucleus pulposus (NP) and annulus fibrosus (AF) cells was sometimes reported, but in general, the formation of fibrocartilaginous matrix was observed. Moreover, data concerning the fibrin long-term stability and its anti-inflammatory properties were found. Disc cells of human origin were never employed in combination with fibrin in vivo or in clinical trials. In vivo, disc degeneration models used to test the fibrin properties essentially involved NP injuries. The addition of cells, in particular if terminally differentiated, to the injected fibrin seemed to promote a more physiological matrix in comparison with fibrin alone. Important aspects should be further investigated in future studies such as the use of fibrin to treat AF lesions as well as the mechanical properties of the fibrin-based biomaterials and of the neoformed tissue. Finally, in vivo studies and clinical trials with in situ injection of fibrin and human disc cells should be performed.