Aspects of Mauthner Cell Differentiation in the Axolotl, Ambystoma mexicanum

SYNOPSIS. In premetamorphic amphibians, the Mauthner cells (M-cells),a single pair of large neurons, are present in the medulla.M-cells differentiate early, are easily recognized morphologically,and in the axolotl embryo, may be approached experimentally:This system is a unique one for the study of neuronal development. The withdrawal of a neuron from the cell division cycle is anearly event in its differentiation. Gastrulae, neurulae andtailbud embryos were each given a single injection of ³H-thymidine.Radioautographs of larvae showed label over M-cell nuclei wheninjections were made before the end of gastrulation, but notwhen injections were made at later stages. Thus, the cells thatgive rise to M-cells cease DNA synthesis during late gastrulation. Unilateral rotations of prospective hindbrain through 180°were performed to see if M-cell axes are specified during neurulation.Rigid axial polarization of the M-cell does not appear to occurin the neurula: The rotated cell regulates and develops normallywith respect to its axes. A major source of input to the M-cell is from the ipsilateralvestibular system. To study the interaction of the M-cell withingrowing axons, unilateral implants of otic vesicles were madeanterior to the otic vesicle in host midtailbud embryos. Preliminarydata suggests a mechanism for the formation of specific neuronalconnections not dependent upon position-time relationships:The ectopic vestibular axons enter the medulla and course caudadto terminate in the region of the ipsilateral M-cell. Whetherthese axons actually form synapses on the M-cell remains tobe established.