Human Stem Cell Research

New tools are becoming available to enable controlled studies of stem cells under the conditions that mimic some aspects of the developmental milieu. Specialized biomaterial scaffolds are of particular relevance to studies of human stem cells. We found that human embryonic stem cells (hESCs) have active binding sites and receptors for hyaluronic acid (HA) that are involved in standard cultures of these cells on feeder layers, and that hESCs are able to internalize and process HA. We then designed a completely synthetic HA hydrogel matrix, polymerizable by light, that supports long-term self-renewal and directed differentiation of hESCs.

In addition to their developmentally relevant chemical composition, HA hydrogels have the advantage that they can be tailored with respect to architecture, stiffness and degradation. We showed that the encapsulated hESCs maintain their undifferentiated state, preserve normal karyotype, and can be induced to differentiate by simply altering soluble factors (Gerecht et al. 2007). Hydrogels thus enable a simple “switch” between the self-renewal and differentiation of hESCs.

To further enhance control of the vascular differentiation of hESCs, we developed hydrogels containing combinations of regulatory factors, such as a tethered RGD peptide and microencapsulated VEGF165 (Fereira 2007). By controlling the growth factor delivery, it was possible to regulate the fractions of cells expressing specific receptors (such as the VEGF receptor KDR/Flk-1) and differentiation markers (such as ectodermal marker including nestin or endodermal marker-fetoprotein). In a related study, we showed that hESCs contain a population of CD34+ vascular progenitor cells that can be selectively differentiated into endothelial and smooth muscle cells, and form vascular networks that were integrated with the host vasculature (Ferreira 2007).