Threedimensional Culture Systems

Animal cell lines have to be classified according to their ability to grow in suspension or be anchorage-dependent. For the large-scale production of biologicals, suspension cells are clearly the substrate of choice. With the development of microcarriers initiated by van Wezel 1967 (72), it is now possible to culture attached cells in bioreactors very similar to suspension cells. Growth of animal cells on microcarriers or immobilized systems facilitates production processes and allows high yield of cells, heterologous proteins, or viruses (73).

Microcarrier techniques are especially important for the handling of primary cell types or cell strains because these are, with only a few exceptions, mostly anchorage-dependent. The microcarrier technology offers several advantages: providing a high surface-to-volume ratio; allowing manipulation during culture without interrupting the fermentation process, and facilitating scale-up possibilities (74). A further advantage is the stabilization of cells in culture, which applies to both attached and suspension cells, thereby permitting prolonged culture periods. Ma-croporous microcarriers have proved to be suitable for the protection of cells from shear forces and have been applied successfully for the immobilization of adherent and suspension cells (75-77).

The ability of some cell lines to form aggregates is another feature used for increased productivity in perfusion bioreactors (16). However, aggregation has to be evaluated in a case-to-case study because it can result in inhomoge-neous expression of proteins and might impair monitoring (17).

Finally, three-dimensional culture conditions can simulate the natural environment of cells, including cell-cell interaction and differentiation processes that are not always possible to achieve in two-dimensional systems. An example is the establishment of a bioreactor that provides high, almost tissuelike cell densities and thereby simulates cell-cell interactions (78).

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