Topographically and Chemically Modified Surfaces for Expansion or Differentiation of Embryonic Stem Cells

Abstract

Cells in the body respond to signals emanating from the interaction with neighbouring cells, from the surrounding extracellular matrix, and from soluble signalling molecules. These signals are perceived by the cell as topographical, mechanical, and chemical cues (Martinez et al., 2009). One example of a defined physical topography embedded in the extracellular matrix (ECM) is provided by the collagen structure. Collagen molecules are about 300 nm long and 1.5 nm wide and can form fibrils up to tens of microns in length and with a diameter of 260-410 nm (Bettinger et al., 2009). Furthermore, the spatial organisation and density of ECM is characteristic of individual tissue types (Martinez et al., 2009), and these natural structures serve to guide interacting cells in terms of cell morphology, migration, and function. Among mechanical cues, tissue elasticity has been found to vary from 0.1-0.3 kPa for embryonic stem (ES) cells and endoderm through increasing values for various differentiated cells to more than 30 kPa for demineralised bone (Reilly & Engler 2010). Such mechanical cues are also recognised in early development with cell-cortex tension being involved in germ-layer sorting (Krieg et al., 2008), indicating that mechanical signals have implications for the decision of stem cell fate. Chemical cues from the surroundings are provided from biochemical mixtures of soluble chemokines, cytokines, and growth factors, as well as insoluble receptor ligands and ECM molecules. Stem cells reside in threedimensional tissue-specific stem-cell niches were the cells are exposed to a controlled microenvironment including both chemical, mechanical, and topographical cues from the surrounding matrix and cells (Reilly & Engler 2010). For the development of cell-based therapies, where growth and differentiation of cells must be controlled in the laboratory or in the body, it is therefore a challenge to develop biomaterials that exploit these biological principles of guiding cells through specific interactions with their environment. Human embryonic stem (hES) cells are potentially valuable in cell-based therapies since they are able to differentiate into cells of all three germ layers as well as to selfrenew and being expanded without loss of pluripotency. However, one prerequisite for such clinical use is that expansion and differentiation protocols must fulfil defined quality standards including xenofree culture conditions (Unger et al., 2008), maintenance of pluripotency during expansion, and uniform differentiation into a specific cell type.