Distinctive structural and cytoskeletal properties of the long-surviving neurons in cell cultures of embryonic spinal cord

A distinctive population of neurons survives for longer than 3 months in cell cultures of chick or rat spinal cord. These neurons form a minor proportion (1%) of the neurons initially developing in the cultures, but are the only ones to survive longer than 30 days in vitro. In addition to their longevity, they share important morphological and cytoskeletal characteristics, which render them distinctive as a group even in early cultures which contain numerous other neurons of short-term viability. Each long-surviving neuron projects one neurite of great length relative to its other neurites, or to those of the shorter-lived neurons, and the length of this neurite is maintained constant for many weeks in vitro. This well-defined morphological feature may indicate the lineage(s) of these neurons. Structurally these cells are very different to the shorter-lived neurons. They are rich in neurofilaments and contain very few microtubules, whereas the shorter-lived neurons contain few neurofilaments but many microtubules. These differences in cytoskeleton coincide exactly with the distinction between limited and prolonged survival in vitro, and the possibility is considered that cytoskeletal stability in the presence of numerous small inflows of calcium might underlie "hardiness" in vitro. The state of development of the long-lived neurons is considered in the context of their shared features, and it is suggested that they may provide a model in which the regulation of development of neuronal function can be analysed.