The vertebrate somatosensory system recognizes many chemical, mechanical and thermal stimuli but how this sensory diversity is established during development is poorly understood. Caron and colleagues now reveal that neuronal diversity in the zebrafish trigeminal sensory ganglia, which innervate the head, depends on the timing of neurogenesis (see ). To analyze neuronal birthdate (the time of a precursor's last division before it differentiates into a neuron) and cell fate, the researchers developed BAPTISM, a method in which the photoconvertible fluorescent protein Kaede marks neurons born at different times; birthdate is then correlated with fate using other nonconvertible fluorescent markers to label neural subpopulations. Their analyses show that the trigeminal sensory ganglia are formed from both early-born and late-born neurons. However, whereas the early-born neurons give rise to multiple classes of sensory neurons, the late-born neurons are restricted in their fate. The researchers speculate that this rapid multimodal neuronal specification is needed to produce functional trigeminal sensory ganglia by the time zebrafish hatch, two days after fertilization.
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