Development of the Mauthner Neurons in Embryos and Larvae of the Zebrafish, Brachydanio rerio

After 1 day of development, the large size of the Mauthner neurons enables them to be distinguished from other immature neurons of the hindbrain. By 1V2 days the neurite of the M-neuron extends at least to the midline of the brain. At this time the marginal layer of the brain and spinal cord may be seen, and tactile stimulation of embryos removed from the chorion elicits a coil response. Slightly older embryos respond with weak swimming behavior. At 2 days of development, the dendrites are beginning to form on the M-neuron and the axon extends well down the length of the spinal cord. Tactile stimulation causes sustained swimming which moves the animal many times its own length. At 3 days the M-neurons have many adult morphological characteristics. The axon extends to the caudal end of the spinal cord, the ventral and lateral dendrites are present and branched, and the axon cap can be distinguished. Sensory organs important in M-neuron functioning also develop in correspondence with the early maturation of the giant cells. By 2 to 2? days, neuromasts of the head and body are innervated, and fibers from the eye begin innervation of the midbrain. By 3 days dendrites of the eighth cranial ganglion cells are present in the sensory maculae of the otic vesicles and axons from these cells extend to the lateral dendrite of the M-neurons. Most animals hatch at 4 days, and at this time sudden auditory stimulation causes an abrupt darting movement, a startle-reflex which appears similar to that mediated by M-neurons in adults. These findings suggest that, as in the adult, the M-neurons mediate startle-reflexes in larvae.