An ultrastructural review of collagen gels, a model system for cell-matrix, cell-basement membrane and cell-cell interactions.

Collagen gels, prepared from rat tail tendon, have been shown to be a useful system for the provision of a more physiologically relevant culture milieu than the rigid inert substratum of tissue culture plastic. The gel is penetrable by cells, but this depends on their origin, either mesenchymal or epithelial, normal or tumour derived. Furthermore, endothelial cells form a basement membrane on the gel surface from which they can be removed with detergent, providing a cell secreted substratum for testing the invasive ability of tumour derived cell lines. Various interactions between the cell and gel occur depending on cell type. Normal fibroblasts plated within the gel will stress and contract the matrix if the gel is freed and allowed to float in the culture medium. The fibroblasts also migrate to the surface and encapsulate the entire gel. Normal lymphocytes rapidly migrate into the gel and continue to move through the gel in a random manner, demonstrating gross changes in surface morphology. Studies are in progress to monitor the behaviour of leukaemic lymphocytes within the gels. Collagen gels will also support in vitro haemopoiesis, providing a useful environment for the positive selection of stromal cells. As an in vitro model for invasion and metastasis the plating of melanoma cells onto a monolayer of endothelium on the gels allows investigation of the interactions between the normal cells which form a barrier to circulating tumour cells and the tumour cells themselves.