Rohon-beard cells and other large neurons in XXXX embryos originate during gastrulation.

@entity1238 @entity665
The time of origin (birthday) of Rohon-Beard cells in @entity1238 was studied by @entity665 autoradiography. Rohon-Beard cells were selected because they are a morphologically identifiable population of neurons in which the development of chemical and electrical excitability has been studied. A single injection of a radioactive DNA precursor was given to animals in successive stages of development from blastula to late tail bud (Nieuwkoop and Faber stages 8--33/34). The label was available throughout the stage of injection and longer. The labeling pattern was examined when animals had reached stage 42, when Rohon-Beard cells are easily recognized. All neurons including Rohon-Beard cells were labeled in animals injected with @entity665 before stage 10 1/2 (early gastrula). Unlabeled Rohon-Beard cells were observed in animals injected with @entity665 in and after stage 10 1/2. The percentage of unlabeled Rohon-Beard cells increased as development progressed. About 80% were born by the completion of gastrulation (stage 13). The other approximately 20% were born during neurulation and early tail bud stages. By stage 27, no Rohon-Beard neuron incorporated @entity665 . In addition to Rohon-Beard neurons, five other neuronal populations begin generation during gastrulation: Mauthner neurons (Vargas-Lizardi and Lyser, '74), trigeminal ganglion cells, large basal plate cells of the medulla, extramedullary neurons, and primary motor neurons. The first birthdays in any of the six populations are temporally close to but appear to be independent of the others.
