Early Development of the Hagfish Pituitary Gland: Evidence for the Endodermal Origin of the Adenohypophysis

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Abstract
The adenohypophysis in early head fold stage hagfish embryos is at first a distinct differentiated thickening of the archenteric roof where it contacts the developing infundibular portion of the brain. This portion of the archenteron eventually becomes the dorsal epithelium of the nasopharyngeal duct. The later development of the adenohypophysis involves formation of multiple acinar outgrowths of the dorsal nasopharyngeal epithelium which together form a layered mass of follicular tissue adjacent to the neurohypophysis. This mode of origin of the adenohypophysis by endodermal polyinvagination and delamination differs from all other vertebrates, including the lampreys. The pertinence of this new information for considerations of monophyletic versus diphyletic evolutionary origins of the modern cyclostome groups is pointed out. The unusual development of the hagfish adenohypophysis presents some new and unanticipated puzzles within the general question of pituitary evolution.

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