Objective      To investigate the effect of dextran/gelatin composite hydrogels with different concentrations on constructing tissue-engineered nucleus pulposus in vitro and characterize the hydrogels so as to optimize the concentration of dextran/gelatin composite hydrogels for nucleus pulposus tissue engineering.       Methods      The dextran/gelatin composite hydrogels with different concentrations were fabricated by Schiff base crosslinking reaction

and the interior morphology

pore size

porosity

Young’s modulus and biocompatibility of the hydrogels were evaluated. Thereafter

porcine nucleus pulposus cells were isolated and cultured

and the first-passage cells were in situ seeded into the hydrogels

and then the cell viability as well as nucleus pulposus-related gene expression (aggrecan

type Ⅱ collagen) was tested by MTT assay and RT-PCR respectively.       Results      The interior morphology of the composite hydrogels under a scanning electron microscope was porous network structure. The pore size and porosity (38.182~82.675 μm and 95.5%~98.3%

respectively) were decreased along with increase of the hydrogel concentration

but Young’s modulus (0.135~11.366 kPa) was increased. The composite hydrogels showed favorable biocompatility

but the degradation of hydrogels with concentrations of 6% and 8% led to decline of cell proliferation and adhesion on the surface on day 4 and day 7. After 1 week

cell viability and expressions of aggrecan and type Ⅱ collagen in the nucleus pulposus cells seeded in the hydrogel with concentration of 10% were enhanced compared to other groups.       Conclusion      The dextran/gelatin composite hydrogel with concentration of 10% processes proper three-dimensional porous structure

stable mechanical structure and favorable biocompability

and is suitable for constructing tissue-engineered nucleus pulposus.