Stem cells are incredible healers that can undergo self-renewal. However, getting them to do this in culture is often tedious and difficult. Stem cells requires their own specific set of surroundings to grow and self renew and recapitulating those surroundings takes a deep understanding of stem cell niches.
Fortunately there is a way to grow stem cells in a three-dimensional culture system that more closely resembles their native milieu, and this system utilizes biomaterials. One such biomaterial is a hydrogel; a water-loving material that consists of a polymer that is highly dissolvable in water. By varying the degree of cross-links between polymer chains, the properties of the hydrogel can vary and these diverse characteristics can adapt the hydrogel to particular stem cell cultures systems.
A research team and Case Western Reserve University in Cleveland, Ohio has created several different hydrogels. Some consist of molecules that compose a highly crosslinked three-dimensional checkerboard, while others are less highly crosslinked. The different degree of crosslinking changes the spaces within the hydrogel, and when stem cells are grown in these hydrogels, the spacing differences affect stem cell behaviors such as proliferation and differentiation.
Eben Alsberg, associate professor of biomedical engineering, said this about his research project; “We think that control over local biomaterial properties may allow us to guide the formation of complex tissues. With this system, we can regulate cell proliferation and cell-specific differentiation into, for example bone-like or cartilage-like cells.”
The degree of crosslinking between the soluble molecules of a hydrogel influences the rigidity of the hydrogel. Also the porosity of the hydrogel is decreased when the degree of crosslinking increases. Alsberg used a hydrogel of oxidized methacrylated alginate with an 8-arm (polyethylene glycol) amine . When the methacrylated alginate was reacted with the polyethylene glycol, it generated crosslinks that conveyed an organized internal structure to the hydrogel.
By playing with the mixture and the proportion of methacrylated alginate to polyethylene glycol, Alsberg and his coworkers made a second set of crosslinks that were light-activated. when they made these molecules in checkerboard masks, patterns of alternating single and double crosslinked spaces emerged that ranged, in size, from 25 to 200 micrometers across. These little molecule cubbyholes resulted from molecules that were evenly singly and doubly crosslinked.
When human stem cells isolated from fat were grown in the singly and doubly-crosslinked regions of the hydrogel, the cells grew better and differentiated better in the hydrogels with larger spaces. The larger the spaces, the better the growth and the clusters they formed, and the more efficiently the cells differentiated.
Alsberg commented on his results: “Potentially, what’s happening is the single-crosslinked regions allow better nutrient transport and provide more space for cells to interact and, because it’s less restrictive, there’s space for new cells and matrix production. CLuster formation, in turn, may influence proliferation and differentiation. Differences in mechanical properties between regions likely also regulate the cell behaviors.
Alsberg and his team would ultimately like to understand how micropatterning influences stem cell fate decisions. By using biomaterials to produce particular micropatterns, Alsberg and his colleagues hope to engineer multiple tissues composed of multiple cell types by using a single stem cell source whose differentiation is directed and control by the structures into which the cells grow and differentiation.