Times of origin of brachial sensory neurons are not correlated with neuronal phenotype

The times of origin (birthdays) of sensory and motor neurons that innervate the triceps brachii muscles of the bullfrog ( Rana catesbeiana ) were determined to learn whether neurons innervating a specific target are generated at a particular developmental time. 3 H‐thymidine ( 3 H‐TdR) was made available continuously throughout a specific developmental period. All neurons that innervated the triceps muscle in juvenile frogs (identified by filling cells retrogradely with HRP) were generated prior to metamorphosis. Triceps motoneurons were all postmitotic by early limb bud stage V. Triceps sensory neurons were generated over a protracted period of larval development, from stage V through early pre‐metamorphic stage XV. Most large triceps sensory neurons were generated before the majority of the small cells. However, there was considerable overlap in the times of origin of the two populations; both large and small cells were generated at all stages of sensory neurogenesis. There was thus no strict relationship between sensory soma size and birthdate. Late‐generated sensory neurons tended to be located in clusters within ganglia, whereas HRP‐filled triceps neurons were not. These 3 H‐labeled clusters may represent clones of neurons which would indicate that late stage neuroblasts give rise to neurons that supply different peripheral targets. The time course of triceps neuronal generation paralleled that of all other brachial sensory neurons implying that the time of last cell division does not in itself determine either the target a neuron will innervate or the sensory modality to which it will respond.