progenitor cells because they are associated with dorsal root ganglion and motor nerve development. emsc differentiation appears to be regulated by intrinsic genetic programs and extracellular signals in the surrounding environment. schwann cells are the source for both neurotropic and neurotrophic factors essential for regenerating nerves and a scaffold for guiding growth. nie, zhang et al. conducted a study investigating the benefits of culturing emscs within plga conduits. adding foskolin and bpe to an emsc culture caused the formation of elongated cell processes, which is common to schwann cells in vitro. thus, foskolin and bpf may induce differentiation into schwann cell-like phenotypes. bpe contains the cytokines gdnf, basic fibroblast growth factor and platelet-derived growth factor, which cause differentiation and proliferation of glial and schwann cells by activating map kinases. when implanted into the plga conduits, the emscs maintained long-term survival and promoted peripheral nerve regeneration across a 10 mm gap, which usually demonstrates little to no regeneration. myelinated axons were present within the grafts and basal laminae were formed within the myelin. these observations suggest that emscs may promote myelination of regenerated nerve fibers within the conduit. neural progenitor cells inserting neurons into a bioartificial nerve conduit seems like the most obvious method for replacing damaged nerves; however, neurons are unable to proliferate and they are often short-lived in culture. thus, neural progenitor cells are more promising candidates for replacing damaged and degenerated neurons because they are self-renewing, which allows for the in vitro production of many cells with minimal donor material. in order to confirm that the new neurons formed from neural progenitor cells are a part of a functional network, the presence of synapse formation is required. a study by ma, fitzgerald et al. is the first demonstration of murine neural stem and progenitor cell-derived functional synapse and neuronal network formation on a 3d collagen matrix. the neural progenitor cells expanded and spontaneously differentiated into excitable neurons and formed synapses; furthermore, they retained the ability to differentiate into the three neural tissue lineages. it was also demonstrated that not only active synaptic vesicle recycling occurred, but also that excitatory and inhibitory connections capable of generating action potentials spontaneously were formed. thus, neural progenitor cells are a viable and relatively unlimited source for creating functional neurons. neural stem cells neural stem cells (nscs) have the capability to self-renew and to differentiate into neuronal and glial lineages. many culture methods have been developed for directing nsc differentiation; however, the creation of biomaterials for directing nsc differentiation is seen as a more clinically relevant and usable technology. one approach to develop a biomaterial for directing nsc differentiation is to combine extracellular matrix (ecm) components and growth factors. a very recent study by nakajima, ishimuro et al. examined the effects of different molecular pairs consisting of a growth factor and an ecm component on the differentiation of nscs into astrocytes and neuronal cells. the ecm components investigated were laminin-1 and fibronectin, which are natural ecm components, and pronectin f plus (pro-f) and pronectin l (pro-l), which are artificial ecm components, and poly(ethyleneimine) (pei). the neurotrophic factors used were epidermal growth factor (egf), fibroblast growth factor-2 (fgf-2), nerve growth factor (ngf), neurotrophin-3 (nt-3), and ciliary neurotrophic factor (cntf). the pair combinations were immobilized onto matrix cell arrays, on which the nscs were cultured. after 2 days in culture, the cells were stained with antibodies against nestin, β-tubulin iii, and gfap, which are markers for nscs, neuronal cells, and astrocytes, respectively. the results provide valuable information on advantageous combinations of ecm components and growth factors as a practical method for developing a biomaterial for directing differentiation of nscs. neurotrophic factors currently, neurotrophic factors are being intensely studied for use in bioartificial nerve conduits because they are necessary in vivo for directing axon growth and regeneration. in studies, neurotrophic factors are normally used in conjunction with other techniques such as biological and physical cues created by the addition of cells and specific topographies. the neurotrophic factors may or may not be immobilized to the scaffold structure, though immobilization is preferred because it allows for the creation of permanent, controllable gradients. in some cases, such as neural drug delivery systems, they are loosely immobilized such that they can be selectively released at specified times and in specified amounts. drug delivery is the next step beyond the basic addition of growth factors to nerve guidance conduits. biomimetic materials many biomaterials used for nerve guidance conduits are biomimetic materials. biomimetic materials are materials that have been design such that they elicit specified cellular responses mediated by interactions with scaffold-tethered peptides from ecm proteins; essentially, the incorporation of cell-binding peptides into biomaterials via chemical or physical modification. synergism synergism often occurs when two elements are combined; it is an interaction between two elements that causes an effect greater than the combined effects of each element separately. synergism is evident in the combining of scaffold material and topography with cellular therapies, neurotrophic factors, and biomimetic materials. investigation of synergism is the next step after individual techniques have proven to be successful by themselves. the combinations of these different factors need to be carefully studied in order to optimize synergistic effects. optimizing neurotrophic factor combinations it was hypothesized that interactions between neurotrophic factors could alter the optimal concentrations of each factor. while cell survival and phenotype maintenance are important, the emphasis of evaluation was on neurite extension. a combination of ngf, glial cell-line derived neurotrophic factor (gdnf), and ciliary neurotrophic factor (cntf) was presented to dorsal root ganglion cultures in vitro. one factor from each neurotrophic family was used. it was determined that there is not a difference in individual optimal concentration and combinatorial optimal concentration; however, around day 5 or 6 the neurites ceased extension and began to degrade. this was hypothesized to be due to lack of a critical nutrient or of proper gradients; previous studies have shown that growth factors are able to optimize neurite extension best when presented in gradients. future studies on neurotrophic factor combinations will need to include gradients. combination of neural cell adhesion molecules and gfd-5 cell adhesion molecules (cams) and neurotrophic factors embedded together into biocompatible matrices is a relatively new concept being investigated. cams of the immunoglobulin superfamily (igsf), which includes l1/ngcam and neurofascin, are particularly promising, because they are expressed in the developing nervous system on neurons or schwann cells. they are known to serve as guidance cues and mediate neuronal differentiation. neurotrophic factors such as ngf and growth differentiation factor 5 (gdf-5), however, are well established as promoters of regeneration in vivo. a recent study by niere, brown et al. investigated the synergistic effects of combining l1 and neurofascin with ngf and gdf-5 on drg neurons in culture; this combination enhanced neurite outgrowth. further enhancement was demonstrated by combining l1 and neurofascin into an artificial fusion protein, which improves efficiency since factors are not delivered individually. not only can different cues be used, but they may even be fused into a single 'new' cue. topography in synergy with chemical and biological cues the effect of presenting multiple stimuli types such as chemical, physical, and biological cues on neural progenitor cell differentiation has not been explored. a study was conducted in which three different stimuli were presented to adult rat hippocampal progenitor cells (ahpcs): postnatal rat type-1 astrocytes (biological), laminin (chemical), and micropatterned substrate (physical). over 75% of the ahpcs aligned within 20° of the grooves compared to random growth on the non-patterned substrates. when ahpcs were grown on micropatterned substrates with astrocytes, outgrowth was influenced by the astrocytes that had aligned with the grooves; namely, the ahpcs extended processes along the astrocytic cytoskeletal filaments. however, the alignment was not as significant as that seen by the ahpcs in culture alone with the micropatterned substrate. in order to assess the different phenotypes expressed as a result of differentiation, the cells were stained with antibodies for class iii β-tubulin (tuji), receptor interacting protein (rip), and glial fibrillary acidic protein (gfap), which are markers for early neurons, oligodendrocytes, and astrocytes, respectively. the greatest amount of differentiation was seen with ahpcs cultured on patterned substrates with astrocytes.