Glycosaminoglycans (GAGs) govern the osmotic environment of cartilaginous tissues and hence determine their ability to resist the large compressive forces encountered during normal activity. In degeneration GAGs are lost and there is now much interest in biological repair processes where cells from cartilaginous tissues synthesise replacement GAGs and other matrix components in situ. In addition, cells can be grown in tissue engineered constructs. Unfortunately, GAG synthesis is slow.

The aim of this study was to determine whether GAG accumulation could be hastened by increasing cell density in a construct using articular cartilage and intervertebral disc cells cultured in alginate beads.

Bovine chondrocytes and intervertebral disc cells were placed in alginate bead suspension at varying cell densities. GAG synthesis rates, total GAG accumulation and lactate production rates were determined by standard methods. The cell viability profile across intact beads was determined using fluorescent probes.

Increasing cell density causes a reduction in lactate production and sulphate incorporation per million live cells. At greater than 20 million cells per ml, cell death is increased compared with lower densities. GAG produced per bead is not increased in proportion to increasing cell density.

These results show that there is a limit to the rate at which matrix per volume of tissue can be produced and accumulated. At high cell densities cellular activity is limited by toxicity arising from low pH and hypoxia.