The sonic motor nucleus (SMN), a likely homologue of the hypoglossal nucleus, provides the final common pathway for sound production in the oyster toadfish (Opsanus tau). SMN neurons increase in size and number for 7-8 years postnatally, and the swimbladder-sonic muscle complex grows throughout life. This study describes the normal embryonic and larval development of the SMN from its initial differentiation on about day 19 through day 40, when the yolk sac is resorbed and the fish is free swimming.
In contrast to the rapid development of CNS nuclei in mammals, the SMN gradually increased in maturity with more active growth at the beginning and end of the observation period and a relatively static period in the middle. Consistent with a hypoglossal homology, the SMN differentiated within the spinal cord, added cells rostrally, and eventually extended into the medulla. Immature neurons appeared to originate from precursor cells in the ventral portion of the ventricular zone of the central canal. Such cells were initially round with little cytoplasmic development and later added processes and Nissl substance. The number of neurons increased 10-fold from a median of 35 to 322 cells, and no evidence of cell death was observed. Soma area approximately doubled from 20.6 to 41.2 pm2, and cell nucleus area followed a similar pattern. [3H]-thymidine autoradiography demonstrated that neurons were added continuously throughout the nucleus during embryonic and larval development.