Development of the otic Placode and the statoacoustic ganglion in rat embryos

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To investigate the mechanism of otic development increase of cell number and changes of the distributional pattern of microfilaments during otic development were examined in in vivo and cultured rat embryos (Sprague-Dawley strain). Embryos (9.5 days of gestation) were cultured for 48 hours in rat serum that contained cytochalasin D (2 X 10^<-8>M) or 5-bromo-2'-deoxyuridin (BrdU,1.6 X 10^<-4>M), in which malformed otocysts were observed. Otic development was also analyzed by using a collagen gel culture.The otic placode develops into the otocyst within 24 hours from day 10.5 of gestation. The otic placode, the otic pit, and the otocyst were constituted by 700,2800, and 6000 cells respectively. The results of this experiments indicated that increse of cell number may not play roles but microfilaments may play roles on formation of the otocyst and migration of statoacoustic ganglion cells. Statoacoustic ganglia were formed from the cells which were emigrated from the neural crest and the lateral part of the neural tube in the rhombencephalon, the otic crest, and the dorso-lateral portion of the otic placode.In the collagen gel culture, statoacoustic ganglia seemed to be formed in the same position at the otocyst as in control embryos, but neurites outgrew from the entire statoacoustic ganglion, although the axon of the statoacoustic ganglion was seen in the restricted route in control embryos.