EXPERIMENTAL-EVIDENCE FOR THE EARLY COMMITMENT OF THE PRESUMPTIVE ADENOHYPOPHYSIS

To investigate the commitment potentialities of the adenohypophyseal t issue, its presumptive territory, localized in the anterior ridge of t he neural plate in the chick embryo, was surgically removed from 2- to 4-somite stages of development. At later stages, operated embryos wer e progressively collected to study the extent (if any) of the adenohyp ophyseal tissue. The results revealed that Rathke's pouch and later th e adenohypophysis were completely absent in 33 out of the 45 studied e mbryos. The surrounding tissues in the operated embryos developped in a comparable manner to those in normal embryos. The head morphogenesis was not affected by the surgical ablation. Strikingly, no regeneratio n of adenohypophyseal tissue was observed in the operated embryos. At younger stages, a very small invagination was observed close to the di encephalon of 9 out of the 45 studied embryos. Its extent was reduced to less than 20% when compared to normal embryos at the same stages. T his epithelial invagination is believed therefore to be a residuum of the stomodeal ectoderm and/or the stomodeo-adenohypophyseal channel wh ich normally regresses at later stages of development. Therefore, the anterior ridge of the neural plate seems irrevocably destined to form adenohypophyseal tissue even as early as 2- to 4-somite stages. This s uggests that the adenohypophysis presumptive territory is already comm itted prior to neural tube closure and also seems to confirm its origi n from the neural primordium in avian embryos. Since close topological relationships link the presumptive adenohypophysis territory with bot h those of the hypothalamus and ectoderm of nasal cavity, possible dev elopmental influences through such intimate contacts cannot be totally excluded.