Ganglion formation from the otic placode and the otic crest in the chick embryo: mitosis, migration, and the basal lamina.

We have studied the morphogenesis of the cochleo-vestibular (CVG) and distal cranial ganglia in the early chick embryo (White Leghorn embryos). Light microscopy and immunocytochemical staining for fibronectin and laminin were used to trace the cellular contributions to these ganglia from the otic placode and otocyst. Serial semi-thin plastic sections (3-5 microns) stained with toluidine blue at each Hamburger-Hamilton stage (St.) from 10 to 21 were used. We were able to trace individual groups of cells derived from these epithelial structures into the anlagen of the CVG and the distal parts of cranial n. ganglia VII, IX, and X. For immunostaining, antisera were used to visualize the basal lamina in 15-microns cryostat sections from St. 14 to 21 embryos. Described here for the first time is the otic crest, a ridge of epithelium surrounding the placode. Cells migrate from the otic crest (St. 11 to 14) during the period when the otocyst is forming. These cells become continuous spatially with those derived from the epibranchial placodes and the presumptive ganglia of cranial nerves VII, IX, and X. Furthermore, rostral otic crest cells merge with neural crest cells, which appose the myelencephalon, and they join with the newly formed neuroblasts of the CVG, which migrate from the ventral epithelium of the otocyst at St. 14 to 21. This region of the epithelium forms the bulk of the CVG; it also has many more mitotic figures than the rest of the otocyst. Cells in the rostralmost CVG (vestibular part) are the first to complete their migration and send axons into both the medulla and incipient crista ampullaris. Immuno-staining for fibronectin and laminin shows that these two basal-lamina-associated glycoproteins appear in a continuous layer beneath the otic epithelium just prior to CVG migration. Thus there is no evidence that the migration is launched by a prior decomposition of the basal lamina. The cells migrating from the epithelium bridge the basal lamina with their leading processes while the trailing processes are withdrawing from the epithelium. These trailing processes must withdraw after the neuroblast migrates, since most of the neuroblasts undergo mitosis in subsequent stages. The migrating cells appear to push out of the epithelium by displacing immunostained fragments of the basal lamina ahead of their leading processes. This suggests that the exodus of cells is accompanied by forces within the epithelium itself. Whether this is generated by the migratory neuroblasts themselves or by other sources remains to be determined.