Neural tissue engineering using embryonic and induced pluripotent stem cells

The human nervous system can be divided into two major components: the central nervous system (CNS) and the peripheral nervous system [1]. The CNS consists of the brain and the spinal cord with the blood-brain barrier restricting the types of biomolecules that can reach these organs [2]. The majority of neural tissue found in the CNS consists of two cell types: neuronal cells and glial cells. Neurons serve as the main information transmitting unit of the nervous system, which can be classified as either sensory, allowing them to detect stimuli from the environment, or motor, responsible for the generation of movement through signaling with muscle tissue [3]. Glial cells encompass a number of different types of support cells, including astrocytes and oligodendrocytes found in the CNS [4,5]. This review will discuss the use of embryonic stem (ES) cell therapy as a method of treating injuries and diseases that inflict damage to the CNS. These studies are particularly relevant now as the first human ES-cell-derived therapy is currently being evaluated in clinical trials as a potential method for treating spinal cord injury (SCI) [6].ES cells possess two hallmark characteristics: the ability to self-renew and pluripotency [7]. The pluripotent nature of ES cells allows them to generate the cells found in neural tissue, including neurons and glia. As a result, ES-cell-based regeneration strategies have been investigated for a number of diseases as well as for repairing mechanically damaged nerve tissue. While many other types of stem cells have been evaluated for their potential to promote neural repair, this review will focus specifically on the attempts made with ES cells as this work will be most applicable to developing therapies using induced pluripotent stem (iPS) cells. First generated in 2006, iPS cells are produced from adult somatic cells, such as skin cells, by inducing specific factors that restore pluripotency [8-10] (Figure 1). The recent generation of these