Articular cartilage is a connective tissue with cellular and matrix properties that normally vary with depth from the surface, providing a low-friction load-bearing surface for efficient joint articulation. When injured, cartilage does not heal, and frequently leads to degenerative and debilitating conditions including osteoarthritis. Several options for treatment exist, but generally result in some loss of function. Recently, investigators have attempted to use cell-based therapies to produce a tissue-engineered cartilage replacement. While there has been limited success, the development of an engineered articular cartilage with the depth-varying properties of normal articular cartilage has not been achieved. This dissertation aims to develop and model the growth of cartilaginous tissue with depth-dependent properties akin to those of native cartilage, including secretion of lubricating molecules at the surface, and increasing compressive modulus with increasing depth.

Chondrocytes from the superficial and middle/deep zones of bovine articular cartilage were layered after pre-culturing in alginate, and formed cohesive tissue with production of a lubricant molecule, superficial zone protein, localized to the superficial surface of the tissue. By culturing the superficial cells in monolayer prior to seeding the constructs, the superficial phenotype was enhanced, and the resulting tissues secreted more of the lubricant, in a manner that was dependent on the number of superficial cells seeded. The mechanical properties of the constructs were also depth-dependent, but exhibited soft regions at both surfaces, unlike the monotonically increasing compressive stiffness of both fetal and immature bovine cartilage. A continuum model was proposed to explain the properties of these constructs, with particular emphasis on the effects of membrane pore-size and bioreactor perfusion on accumulation and loss of matrix macromolecules.

Specific points in the fabrication of tissue-engineered cartilage can be used to modulate the depth-dependent properties, and may lead to an articular cartilage replacement that is engineered specifically to exhibit the functional depth-dependent properties of normal articular cartilage. Such tissues could provide the advantages of better host-implant integration and lubrication to provide an advanced biological repair option for the growing population of patients with limitations of joint function.