Neural ectoderm, neural crest, and placodes: Contribution of the otic placode to the ectodermal lining of the...

Development of neural ectoderm, neural crest, and otic placode with special reference to a new placodal derivative, the ectodermal lining of the opercular cavity, is described in a teleost fish, the Atlantic cod Gadus morhua, from a stage-by-stage examination of embryonic development. The ectodermal lining of the opercular cavity forms by invagination of the otic placode. The neural plate "infolds" by a wave of cellular rearrangement that transforms the neural plate into a neural rod. This transformation creates a distinct dorsal ectodermal cell layer. When the neural rod is arranged as monostratified columnar cells in the forebrain and midbrain, dorsal ectoderm at the midbrain level thickens lateral to the neural rod to form a cell cluster-the presumptive neural crest and placode. Upon migration of the neural crest from the postoptic midbrain, the dorsolateral area of the dorsal ectoderm thickens and segregates from the neural crest as a placode that is continuous with the presumptive lens placode. As the neural crest migrates from the hindbrain, this placode extends along the hindbrain as a single continuous cluster of cells. At the onset of formation of the lens placode, this continuous placode becomes the placode in the postoptic area of the midbrain and separates into the otic placode at the hindbrain. The otic placode gives rise to the otic neuromast and probably the otic lateral line nerves rostrally and to the ectodermal cell lining of the opercular cavity and otic vesicles caudally. The opercular cavity forms by invagination of the otic placode, creating an internal lumen lined by ectoderm that becomes continuous with evaginated endodermal pharyngeal cells. Free neuromasts are observed along the trailing edge of the external opening of the opercular cavity, which lies horizontally, ventral to the otic vesicles. As embryos develop to hatching, the opening rotates and takes up a vertical position. The adult opercular apparatus, including associated bones and muscles, forms during larval stages. The otic neuromast may be a remnant of neuromasts in the spiracle organ. The spiracle opening lies between the mandibular and hyoid arches, whereas the opercular cavity opens between the hyoid and the first branchial arches. The spiracle opening is, therefore, not homologous with the external opening of the opercular cavity, although the cell lining of the spiracle opening may be of placodal origin. J Morphol 231:231-252, 1997. © 1997 Wiley-Liss, Inc.