Disappearance of Rohon-Beard neurons from the spinal cord of larval Xenopus laevis.

Rohon-Beard neurons are primary sensory cells located in the spinal cord of embryonic lower vertebrates. The kinetics of their normal, gradual, but complete disappearance in Xenopus tadpoles has been followed. Levels of acid phosphatase activity, a common histochemical correlate of cell death, were assayed and found to increase at the time of onset of disappearance of Rohon-Beard cells. Ultrastructural examination revealed the presence of numerous secondary lysosomes, swelling of endoplasmic reticulum and mitochondria, and a decrease in nuclear density. The disappearance of Rohon-Beard neurons may be attributed to autophagic cell death involving lysosomal acid hydrolases. This process begins only a few days after the maturation of voltage- and neurotransmitter-dependent membrane conductances and the electrical uncoupling of these neurons. The loss of Rohon-Beard neurons in embryos whose development was arrested by crowding was appropriate for the developmental stage of the animals rather than their chronological age.