also, the ha hydrogels modified with laminin were able to promote neurite extension. these results support ha gels as a promising biomaterial for a nerve guidance conduit. cellular therapies in addition to scaffold material and physical cues, biological cues can also be incorporated into a bioartificial nerve conduit in the form of cells. in the nervous system there are many different cell types that help support the growth and maintenance of neurons. these cells are collectively termed glial cells. glial cells have been investigated in an attempt to understand the mechanisms behind their abilities to promote axon regeneration. three types of glial cells are discussed: schwann cells, astrocytes, and olfactory ensheathing cells. in addition to glial cells, stem cells also have potential benefit for repair and regeneration because many are able to differentiate into neurons or glial cells. this article briefly discusses the use of adult, transdifferentiated mesenchymal, ectomesenchymal, neural and neural progenitor stem cells. glial cells glial cells are necessary for supporting the growth and maintenance of neurons in the peripheral and central nervous system. most glial cells are specific to either the peripheral or central nervous system. schwann cells are located in the peripheral nervous system where they myelinate the axons of neurons. astrocytes are specific to the central nervous system; they provide nutrients, physical support, and insulation for neurons. they also form the blood brain barrier. olfactory ensheathing cells, however, cross the cns-pns boundary, because they guide olfactory receptor neurons from the pns to the cns. schwann cells schwann cells (sc) are crucial to peripheral nerve regeneration; they play both structural and functional roles. schwann cells are responsible for taking part in both wallerian degeneration and bands of bungner. when a peripheral nerve is damaged, schwann cells alter their morphology, behavior and proliferation to become involved in wallerian degeneration and bungner bands. in wallerian degeneration, schwann cells grow in ordered columns along the endoneurial tube, creating a band of bungner (bob) that protects and preserves the endoneurial channel. additionally, they release neurotrophic factors that enhance regrowth in conjunction with macrophages. there are some disadvantages to using schwann cells in neural tissue engineering; for example, it is difficult to selectively isolate schwann cells and they show poor proliferation once isolated. one way to overcome this difficulty is to artificially induce other cells such as stem cells into sc-like phenotypes. eguchi et al. (2003) have investigated the use of magnetic fields in order to align schwann cells. they used a horizontal type superconducting magnet, which produces an 8 t field at its center. within 60 hours of exposure, schwann cells aligned parallel to the field; during the same interval, schwann cells not exposed oriented in a random fashion. it is hypothesized that differences in magnetic field susceptibility of membrane components and cytoskeletal elements may cause the magnetic orientation. collagen fibers were also exposed to the magnetic field, and within 2 hours, they aligned perpendicular to the magnetic field, while collagen fibers formed a random meshwork pattern without magnetic field exposure. when cultured on the collagen fibers, schwann cells aligned along the magnetically oriented collagen after two hours of 8-t magnetic field exposure. in contrast, the schwann cells randomly oriented on the collagen fibers without magnetic field exposure. thus, culture on collagen fibers allowed schwann cells to be oriented perpendicular to the magnetic field and oriented much quicker. these findings may be useful for aligning schwann cells in a nervous system injury to promote the formation of bands of bungner, which are crucial for maintaining the endoneurial tube that guides the regrowing axons back to their targets. it is nearly impossible to align schwann cells by external physical techniques; thus, the discovery of an alternative technique for alignment is significant. however, the technique developed still has its disadvantages, namely that it takes a considerable amount of energy to sustain the magnetic field for extended periods. studies have been conducted in attempts to enhance the migratory ability of schwann cells. schwann cell migration is regulated by integrins with ecm molecules such as fibronectin and laminin. in addition, neural cell adhesion molecule (ncam) is known to enhance schwann cell motility in vitro. ncam is a glycoprotein that is expressed on axonal and schwann cell membranes. polysialic acid (psa) is synthesized on ncam by polysialyltransferase (pst) and sialyltransferase x (stx). during the development of the cns, psa expression on ncam is upregulated until postnatal stages. however, in the adult brain psa is found only in regions with high plasticity. psa expression does not occur on schwann cells. lavdas et al. (2006) investigated whether sustained expression of psa on schwann cells enhances their migration. schwann cells were tranduced with a retroviral vector encoding stx in order to induce psa expression. psa-expressing schwann cells did obtain enhanced motility as demonstrated in a gap bridging assay and after grafting in postnatal forebrain slice cultures. psa expression did not alter molecular and morphological differentiation. the psa-expressing schwann cells were able to myelinate cns axons in cerebellar slices, which is not normally possible in vivo. it is hopeful that these psa-expressing schwann cells will be able to migrate throughout the cns without loss of myelinating abilities and may become useful for regeneration and myelination of axons in the central nervous system. astrocytes astrocytes are glial cells that are abundant in the central nervous system. they are crucial for the metabolic and trophic support of neurons; additionally, astrocytes provide ion buffering and neurotransmitter clearance. growing axons are guided by cues created by astrocytes; thus, astrocytes can regulate neurite pathfinding and subsequently, patterning in the developing brain. the glial scar that forms post-injury in the central nervous system is formed by astrocytes and fibroblasts; it is the most significant obstacle for regeneration. the glial scar consists of hypertrophied astrocytes, connective tissue, and ecm. two goals of neural tissue engineering are to understand astrocyte function and to develop control over astrocytic growth. studies by shany et al. (2006) have demonstrated that astrocyte survival rates are increased on 3d aragonite matrices compared to conventional 2d cell cultures. the ability of cell processes to stretch out across curves and pores allows for the formation of multiple cell layers with complex 3d configurations. the three distinct ways by which the cells acquired a 3d shape are: adhering to surface and following the 3d contour stretching some processes between 2 curvatures extending processes in 3d within cell layers when located within multilayer tissue in conventional cell culture, growth is restricted to one plane, causing monolayer formation with most cells contacting the surface; however, the 3d curvature of the aragonite surface allows multiple layers to develop and for astrocytes far apart to contact each other. it is important to promote process formation similar to 3d in vivo conditions, because astrocytic process morphology is essential in guiding directionality of regenerating axons. the aragonite topography provides a high surface area to volume ratio and lacks edges, which leads to a reduction of the culture edge effect. crystalline matrices such as the aragonite mentioned here are allowed for the promotion of a complex 3d tissue formation that approaches in vivo conditions. olfactory ensheathing cells the mammalian primary olfactory system has retained the ability to continuously regenerate during adulthood. olfactory receptor neurons have an average lifespan of 6–8 weeks and therefore must be replaced by cells differentiated from the stem cells that are within a layer at the nearby epithelium's base. the new olfactory receptor neurons must project their axons through the cns to an olfactory bulb in order to be functional. axonal growth is guided by the glial composition and cytoarchitecture of the olfactory bulb in addition to the presence of olfactory ensheathing cells (oecs). it is postulated that oecs originate in the olfactory placode, suggesting a different developmental origin than other similar nervous system microglia. another interesting concept is that oecs are found in both the peripheral and central nervous system portions of the primary olfactory system, that is, the olfactory epithelium and bulb. oecs are similar to schwann cells in that they provide an upregulation of low-affinity ngf receptor p75 following injury; however, unlike schwann cells they produce lower levels of neurotrophins. several studies have shown evidence of oecs being able to support regeneration of lesioned axons, but these results are often unable to be reproduced. regardless, oecs have been investigated thoroughly in relation to spinal cord injuries, amyotrophic lateral sclerosis, and other neurodegenerative diseases. researchers suggest that these cells possess a unique ability to remyelinate injured neurons. oecs have properties similar to those of astrocytes, both of which have been identified as being susceptible to viral infection. stem cells stem cells are characterized by their ability to self-renew for a prolonged time and still maintain the ability to differentiate along one or more cell lineages. stem cells may be unipotent, multipotent, or pluripotent, meaning they can differentiate into one, multiple, or all cell types, respectively. pluripotent stem cells can become cells derived from any of the three embryonic germ layers. stem cells have the advantage over glial cells because they are able to proliferate more easily in culture. however, it remains difficult to preferentially differentiate these cells into varied cell types in an ordered manner. another difficulty with stem cells is the lack of a well-defined definition of stem cells beyond hematopoietic stem cells (hscs). each stem cell 'type' has more than one method for identifying, isolating, and expanding the cells; this has caused much confusion because all stem cells of a 'type' (neural, mesenchymal, retinal) do not necessarily behave in the same manner under identical conditions. adult stem cells adult stem cells are not able to proliferate and differentiate as effectively in vitro as they are able to in vivo. adult stem cells can come from many different tissue locations, but it is difficult to isolate them because they are defined by behavior and not surface markers. a method has yet to be developed for clearly distinguishing between stem cells and the differentiated cells surrounding them. however, surface markers can still be used to a certain extent to remove most of the unwanted differentiated cells. stem cell plasticity is the ability to differentiate across embryonic germ line boundaries. though, the presence of plasticity has been hotly contested. some claim that plasticity is caused by heterogeneity among the cells or cell fusion events. currently, cells can be differentiated across cell lines with yields ranging from 10% to 90% depending on techniques used. more studies need to be done in order to standardize the yield with transdifferentiation. transdifferentiation of multipotent stem cells is a potential means for obtaining stem cells that are not available or not easily obtained in the adult. mesenchymal stem cells mesenchymal stem cells are adult stem cells that are located in the bone marrow; they are able to differentiate into lineages of mesodermal origin. some examples of tissue they form are bone, cartilage, fat, and tendon. mscs are obtained by aspiration of bone marrow. many factors promote the growth of mscs including: platelet-derived growth factor, epidermal growth factor β, and insulin-like growth factor-1. in addition to their normal differentiation paths, mscs can be transdifferentiated along nonmesenchymal lineages such as astrocytes, neurons, and pns myelinating cells. mscs are potentially useful for nerve regeneration strategies because: their use is not an ethical concern no immunosuppression is needed they are an abundant and accessible resource they tolerate genetic manipulations keilhoff et al. (2006) performed a study comparing the nerve regeneration capacity of non-differentiated and transdifferentiated mscs to schwann cells in devitalized muscle grafts bridging a 2-cm gap in the rat sciatic nerve. all cells were autologous. the transdifferentiated mscs were cultured in a mixture of factors in order to promote schwann cell-like cell formation. the undifferentiated mscs demonstrated no regenerative capacity, while the transdifferentiated mscs showed some regenerative capacity, though it did not reach the capacity of the schwann cells. ectomesenchymal stem cells the difficulty of isolating schwann cells and subsequently inducing proliferation is a large obstacle. a solution is to selectively induce cells such as ectomesenchymal stem cells (emscs) into schwann cell-like phenotypes. emscs are neural crest cells that migrate from the cranial neural crest into the first branchial arch during early development of the peripheral nervous system. emscs are multipotent and possess a self-renewing capacity. they can be thought of as schwann