The differentiation behaviour of MDCK cells grown on matrix components and in collagen gels.

MDCK cells are grown on various substrates (Thermanox pure, extracellular matrix (ECM), dried or wet collagen type I or type III), on floating collagen and enclosed in collagen gels, and their differentiation behaviour is investigated electron microscopically. The cells grown on ECM or dried collagen (type I and type III) do not show any changes as compared with the controls (Thermanox). Differentiation processes can only be observed when the cells are grown on wet collagen (type I and type III), especially on floating collagen and enclosed in collagen gels. These differentiation processes comprise changes in the cell shape, an increase in the number of microvilli, an increase in the length of the lateral contact zone with the formation of gap junctions and desmosomes, and an increase in the number and size of the cell organelles. A basement membrane only develops in the form of short segments. Moreover, on floating collagen and in collagen gels three-dimensional, organoid structures develop: cell aggregates with central lumina and tubuli. They are formed by cuboid cells that also exhibit indications of differentiation. Basement membrane fragments occur more often and are longer. It can be concluded from these findings that the chemical structure of the substrate does not play the primary role in the described process. It is rather the physical properties, probably the plasticity, that are of significance. Due to this property the cells change their shape and the contact areas increase in size. The establishment of contacts might be the triggering factor for differentiation. Organoid structures with lumina develop when the apical surface comes into contact with other cells or collagen gels. The pronounced tendency towards polarization necessitates a re-arrangement of three-dimensionally growing cells to structures with lumina. The formation of the basement membrane is the result and not the cause of differentiation.