The Spinal Cord Neural Stem Cell Niche

Abstract

The spinal cord is the caudal portion of the central nervous system (CNS) that extends from the lower part of the brain stem (the medulla) to the cauda equina. It receives several types of sensory information from the joints, muscles, organs and skin and contains the motoneurons responsible for voluntary/reflex movements and for the function of the autonomic nervous system. The spinal cord is divided into i) gray matter, which notably contains motoneurons and interneurons that form the spinal cord circuitry; ii) white matter, which surrounds the gray matter and is made up of ascending and descending longitudinal tracts; and iii) the central canal or ependymal region, which is organized as an oval or round-shaped epithelium whose apical pole abuts the cerebral spinal fluid. The spinal cord is not simply a relay that carries information between the brain and body, but it also contains a complex circuitry that is implicated in the generation and coordination of reflexive responses to sensory inputs. Furthermore, the spinal cord is involved in the formation of rhythmic movements, such as locomotion and swimming in animals. One emerging field of research concerns spinal cord plasticity, as this structure should not be considered a static and hard-wired system. Instead, the spinal cord displays considerable activity-dependent adaptation and, similar to other CNS regions, can learn and remember throughout life (Guertin 2008; Wolpaw 2010; Wolpaw and Tennissen 2001). Plasticity plays an important role in the acquisition and maintenance of motor skills. In pathology, it could be manipulated to alleviate spinal cord lesions that originate from traumas or degenerative diseases. In parallel with spinal cord plasticity, one field of research that is rapidly growing concerns the presence of neural stem cells and progenitor cells in the adult spinal cord. In this review, I will describe recent findings regarding stem cells and attempt to formulate hypotheses concerning their role in spinal cord physiology and plasticity. The presence of stem cells in the spinal of lower vertebrates, such as salamanders and newts, has been reported for decades. These stem cells are at the basis of the phenomenal regeneration capacity of these animals that is observed when the spinal cord is transected. There are excellent reviews on this topic (for instance, see (Tanaka 2003)), and I will thus focus on the adult spinal cord stem cells in mammals.