Embryonic differentiation of the pituitary in a snake (Thamnophis brachystoma).

The adenohypophysis of Thamnophis is produced from the stomodeal epithelium in two steps: a diverticulum, enlarging by addition of epithelium to its basal end, defines the posterior end of the gland, and a subsequent infolding promoted by mesenchymal movements occurs in epithelium anterior to the original diverticulum and forms the anterior end of the anlage and the hypophyseal stalk. Immediately thereafter the pars intermedia (PI) is demarcated, first by a luminal, subsequently by an external constriction, and secretion granules are found in the gland. At this time granulated cells are rare in the PI, and in the pars distalis (PD) they are more frequent in the anterior end. Secretion granules occur in cells away from the surface of the residual lumen; the lumen is lined by presumptive stellate cells. The early appearance of secretion granules in cells of the embryonic pituitary, and the presence in the hypophyseal stalk of both mucus-secreting cells and cells with granules similar to those of the PD suggest that some differentiation occurs in the stomodeal epithelial cells before the definitive pouch is formed. The absence of lateral lobes in the embryonic hypophysis precludes the development of the pars tuberalis in Thamnophis.