Interaction ofCartilage andCeramic Matrix - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 300

for the physical and chemical properties of the biomaterial's surface [ 13 , 32 ].
Biological aspects of cell reactions including signal mechanisms inside the cells
will not be discussed here.
Thus, research aims to control cell reactions and tissue development with the help
of an adequate material. Because of the various interactive factors influencing tissue
formation—e.g. geometry, composition, porosity and surface structure including
macroscopic topography, roughness and grain size—the appraisal of results is dif-
ficult and the effects of modified substrate structure are therefore not yet fully
understood [ 18 ]. Furthermore, cell phenotypes and cells at different maturation states
react differently to material characteristics, and culture conditions and different
methods of analysis also influence the results [ 13 , 18 , 22 , 25 ]. Literature review
shows that little is known of how chondrocytes and cartilage tissue respond to
different ceramic substrate properties [ 19 ]. Therefore, the following sections high-
light the impact of a carrier on cartilage quality compared to unsupported tissue over
a long cultivation period, important surface characteristics of biomaterials for car-
tilage tissue engineering, and the influence of carrier surface structure modifications.
3 Impact of a Hydroxyapatite Carrier
on Cartilage Formation
The results presented in the following are based on a cultivation principle
developed by Nagel-Heyer et al. [ 33 ]. Here, the osteochondral implant consists
of a ceramic carrier which acts as bone equivalent. On top of this carrier,
in vitro generated cartilage is cultivated without any scaffold. The principle
combines the advantages of AOT and ACT (autologous chondrocyte transplan-
tation), as autologous chondrocytes and synthetic scaffolds are used for the
formation of the three-dimensional cartilage-carrier constructs. This concept
allows controllable proliferation, differentiation and matrix production phases
[ 33 - 36 ]. As shown in Fig. 1 , tissue is explanted from the defective articular carti-
lage. After an enzymatic digestion, chondrocytes are expanded within three sub-
cultivation steps in monolayer culture, as the initial cell number is limited by the
small size of the biopsy (step a). However, proliferation is accompanied by dedif-
ferentiation of cells. A portion of the dedifferentiated cells is cultivated on top of a
ceramic carrier to form a cell layer (step b). In previous studies, it was found that this
initial cell layer is necessary to improve bonding between in-vitro cartilage and the
ceramic carrier [ 37 ]. The other cells are re-differentiated in an alginate gel supported
by the addition of specific growth factors (step c). In step d, cells together with their
cell-associated matrix are recovered from the gel, seeded on top of the cell-coated
carrier and cultivated in a high-density cell culture. Afterwards, cartilage-carrier
constructs can be implanted into the defective cartilage-bone site.
Chondrocytes from 4-to 6-month-old pigs were used in experiments. The
concept was successfully applied in mini-pigs. After 1 year, the cartilage defect
was completely closed and the recently formed cartilage showed no difference