Naturally Occurring Neuronal Death in Vertebrates

Abstract

Neuronal death during the development of the vertebrate nervous system is composed of a variety of categories, some of which are shared with other developing organ systems, while others are unique to the nervous system. Among the former is the disappearance of neural structures in the course of tail resorption in frog larvae at metamorphosis. At the same time the Mauthner neurons in the medulla, which are involved in the startle response during aquatic life, also regress or disappear. In early embryos of lower vertebrates one finds primitive sensory neurons in the dorsal part of the spinal cord, the Rohon-Beard cells. They undergo regression when the spinal ganglia take over their function (Hughes, 1957; Lamborghini, 1981). This is comparable to the disappearance of the mesonephros in mammalian development, which degenerates when the metanephros becomes functional. The term “phylogenetic death” has been applied to these phenomena (Glücksmann, 1951); but this designation with its implied inevitability should not distract one from the analysis of the proximate causes of these events. For instance, the thyroid hormone has been implicated in the disappearance of the Mauthner cells, but its actual role is still controversial (Kimmel and Model, 1978).