Experimental evidence for the early commitment of the presumptive adenohypophysis

To investigate the commitment potentialities of the adenohypophyseal tissue, its presumptive territory, localized in the anterior ridge of the neural plate in the chick embryo, was surgically removed from 2- to 4-somite stages of development. At later stages, operated embryos were progressively collected to study the extent (if any) of the adenohypophyseal tissue. The results revealed that Rathke's pouch and later the adenohypophysis were completely absent in 33 out of the 45 studied embryos. The surrounding tissues in the operated embryos developed in a comparable manner to those in normal embryos. The head morphogenesis was not affected by the surgical ablation. Strikingly, no regeneration of adenohypophyseal tissue was observed in the operated embryos. At younger stages, a very small invagination was observed close to the diencephalon 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. This epithelial invagination is believed therefore to be a residuum of the stomodeal ectoderm and/or the stomodeo-adenohypophyseal channel which 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 suggests that the adenohypophysis presumptive territory is already committed prior to neural tube closure and also seems to confirm its origin from the neural primordium in avian embryos. Since close topological relationships link the presumptive adenohypophysis territory with both those of the hypothalamus and ectoderm of nasal cavity, possible developmental influences through such intimate contacts cannot be totally excluded.