Biomechanical conditioning of chondrocyte-seeded constructs: Tuning for success Dan L. Bader The importance of mechanical stimulation in maintaining the integrity of healthy load-bearing soft tissue such as articular cartilage is widely accepted, both for metabolism and homeostasis. The manner in which the cartilage cells, termed chondrocytes, respond to different mechanical environments has attracted a wide interest, with studies investigating the role of various signalling pathways associated with mechanotransduction involving, for example, cytoskeletal elements and intracellular Ca 2+. Several groups are proposing in vitro mechanical conditioning strategies for chondrocyte-seeded scaffolds as an essential feature for the long term functionality of tissue engineered implants. This demands the development of suitable mechanical loading rigs for use in a controlled biological environment, such as a bioreactor, which provide a strain distribution appropriate to that experienced by the cells in vivo. Much of our work at QMUL has utilised a well-established model system, based on bovine articular chondrocytes seeded in agarose constructs subjected to compression. It has been shown that the degree of deformation of chondrocytes will depend on the magnitude and frequency of the gross applied compression, the organisation of its extracellular matrix and the nature of the scaffold material. In a well-described mechanical system, we have subjected constructs to a physiological compressive strain of 15% at a frequency of 1 Hz for varying periods. A selection of metabolic parameters, including cell proliferation, GAG synthesis and NO production, have been determined as output markers. Results have suggested that the metabolic response depends on: The sub-population of chondrocytes: surface versus deep, load bearing areas versus non-load bearing areas The temporal nature and the type of loading modality: intermittent versus continuous compression An association between mechanical and biochemical molecules The nature and type of the scaffold material The observations clearly suggest a complex interplay between stimuli and metabolic parameters, which appear to be uncoupled. Nonetheless provided suitable monitoring systems are available, there remains a possibility of fine tuning the mechanical stimulation to elicit a specific cellular response during the in vitro conditioning period.