Embryonic origins of auditory brain-stem nuclei in the chick hindbrain

The auditory nuclei of the chick brain stem have distinct morphologies and highly specific synaptic connectivity. Nucleus magnocellularis (NM) and nuc leus angularis receive tonotopically ordered cochlear input. NM in turn pro jects tonotopically to nucleus laminaris (NL), maintaining binaural specifi city with projections to either dorsal or ventral NL dendrites. NM and NL a rise from a common anlage, which differentiates as the cells migrate and ac quire their mature morphologies. NM and NL cells are closely associated dur ing embryogenesis and synapse formation. However, the morphologies of the n uclei and of the cells within the nuclei differ greatly between NM and NL. While later maturation of these nuclei has been described in considerable d etail, relatively little is known about the early embryonic events that lea d to the formation of these nuclei. We examined the embryonic origins of ce lls in brain-stem auditory nuclei with particular emphasis on NM and NL. Li pophilic dyes were injected into small regions of the embryonic hindbrain p rior to the birth and migration of cells that contribute to these nuclei. W e found that NM arises from rhombomeres r5, r6, and r7, and NL arises mostl y from r5 with a few cells arising from r6. NM and NL thus have partially o verlapping rhombomeres of origin. However, we found that the precursors for NM and NL are found in distinct regions within rhombomere 5, with NM precu rsors in medial regions and NL precursors in lateral regions. Our results d o not support a lineage relationship between NM and NL cells and they sugge st that NM and NL are specified prior to migration of precursors to the aud itory anlage. (C) 2000 Academic Press.