The hypophysis stays in connection with the hypothalamus via the pituitary stalk. Here an unpaired neuroendocrine gland is involved that - similar to other endocrine glands (thyroid, epiphysis, adrenal) – consists of two parts, each with its own embryonic origin. The hypophysis is composed from a bulging of the hypothalamus (neurohypophysis) and from the added anterior pituitary (adenohypophysis). Over the course of embryonic development, it comes to lie in front of the neurohypophysis.
The adenohypophysisanlage can be seen from stage 10 as a local epithelial thickening in the roof of the stomodaeum (ca. 28 days). Since this thickening lies in front of the oropharyngeal membrane, an ectodermal structure. This anlage lies on the rostral end of the notochord and thus in the immediate neighborhood of the diencephalon floor. In stage 13 (ca. 32 days) the originally flat anlage pushes its way into the mesenchyma and so forms a diverticulum, Rathke's pouch. In stage 20 (ca. 49 days) this lengthens and remains connected with the stomodaeum only via the slinder craniopharyngealcanal.
This runs through the cartilaginous anlage of the sphenoid bone, and after stage 21 (ca. 51 days), it disappears again due to atrophy. From the 3rd month the anterior hypophysis lobe(pars distalis) arises out of Rathke's pouch through cell proliferation and its extension along the hypophysis stalk (pars tuberalis). An intermediate hypophysis lobe is missing in humans. It is replaced by the basophilic invasion into the neurohypophysis.
The neurohypophysisanlage or posterior hypophysis lobe arises as a bulging of the neuroepithelial floor of the diencephalon. This funnel, termed the infundibulum, penetrates into the mesenchyma that lies below it and encounters there Rathke‘'s pouch. The infundibulum forms a diverticulum, that in stage 16 (ca. 38 days)16 thickens to become the posterior hypophysis lobe (neurohypophysis) and adheres to the back side of Rathke's pouch. The neurohypophysis remains connected over the whole lifetime with the diencephalon via the hypophysis stalk. At the beginning of the fetal period, axons sprout from the hypothalamus into the neurohypophysis whereby the neuroendocrine hypothalamus-hypophysis axis is laid down.