A human spinal cord cell promotes motoneuron survival and maturation in vitro

Primary cultures of motoneurons represent a good experimental model for studying mechanisms underlying certain spinal cord pathologies, such as amyotrophic lateral sclerosis and spinal bulbar muscular atrophy (Kennedy's disease). However, a major problem with such culture systems is the relatively short cell survival times, which limits the extent of motoneuronal maturation. In spite of supplementing culture media with various growth factors, it remains difficult to maintain motoneurons viable longer than 10 days in vitro. This study employs a new approach, in which rat motoneurons are plated on a layer of cultured cells derived from newborn human spinal cord. For all culture periods, more motoneurons remain viable in such cocultures compared with control monocultures. Moreover, although no motoneurons survive in control cultures after 22 days, viable motoneurons were observed in cocultures even after 7 weeks. Although no significant difference in neurite length was observed between 8-day mono- and cocultures, after 22 and 50 days in coculture motoneurons had a very mature morphology. They extended extremely robust, very long neurites, which formed impressive branched networks. Data obtained using a system in which the spinal cord cultures were separated from motoneurons by a porous polycarbonate filter suggest that soluble factors released from the supporting cells are in part responsible for the beneficial effects on motoneurons. Several approaches, including immunocytochemistry, immunoblotting, and electron microscopy, indicated that these supporting cells, capable of extending motoneuron survival and enhancing neurite growth, had an undifferentiated or poorly differentiated, possibly mesenchymal phenotype.