Unlike many tissues, cartilage does not have an internal capillary network. Instead cells in this tissue acquire their nutrients and oxygen by diffusion from the surface. As a consequence implanted cartilage generally survives well in the patient, as there is no problem with connecting it to the patient’s vascular system. This makes cartilage a useful ‘halfway-house’ in the development of a clinically effective tissue.
Cartilage is also a clinically important tissue; as it does not recover well from injury, and its deterioration is associated with debilitating diseases of old age, such as arthritis. It is also a very versatile structural tissue, different forms being used as lubricating pads in joints, as flexible support tissues in the nose and ears, and as a substrate/precursor to bone formation. The development of clinically useful cartilage through tissue engineering is therefore a useful goal in its own right.
Above right: Fluorescence micrograph showing type II collagen as a green haze about chondrocytes in engineered cartilage. This defines the extent of the chondron, the functional sub-unit of cartilage.
In nature cartilage is laid down by chondrocytes, specialised cells which are encapsulated in a chondron. The chondron, composed largely of type VI collagen, is important for protecting the chondrocyte from mechanical shock within the tissue. The complete chondron is further embedded in a matrix high in collagen type II that provides the tissue with its desirable mechanical properties.
We are developing culture techniques that give rise to a cartilage with composition and structure as similar to that seen in nature. To do this a wide range of culture conditions have been investigated on a number of different scaffolds – including synthetic spiders’ silk.
Clinical evidence suggests that mechanical stimulation is important for correct development of both cartilage and bone. Consequently the effect of mechanical stimulation on the growing tissue is now also being investigated.
Rright: Cartilage plugs grown in vitro ready for analysis and implant.