Development of the hypophysis
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.
embryo in stage 13
embryo in stage 13 (ca. 32 days)
- 1
- Floor of the diencephalon (neuroepithelium)
- 2a
- Bulging of the neuroepithelial floor
- 3a
- Rathke's pouch
- 4
- Stomodaeum
Cross section through an embryo in stage 13, overview sketch
Fig. 83Note the ectodermal thickening in the region of the stomodeum (yellow) as well as the somewhat later appearing neuroepithelial bulging of the floor of the diencephalon (blue).
stage 16 (ca. 38 days)
- 1
- Floor of the diencephalon
(neuroepithelium) - 2b
- Infundibulum
- 3a
- Rathke's pouch
- 4
- Ectodermal epithelium of the stomodaeum
in stage 20 (ca. 49 days)
- 2c
- Anlage of the neurohypophysis
- 3b
- Detached Rathke's pouch
- 5a
- Adenohypophyis stalk
(cranio-pharyngeal canal)
The increasing invagination of the ectodermal lining of the stomodeum leads to the formation of Rathke's pouch. This penetrates the mesenchyma that lies in front of the bulging of the diencephalon floor (infundibulum). The two structures slowly lie on each other.
Fig. 85The distal part of the infundibulum forms a diverticulum that thickens to become the posterior hypophysis lobes. Rathke's pouch extends lengthwise and loses its connection to the stomodeum (adenohypophyis stalk, cranio-pharyngeal canal). Normally, this strand atrophies (figure 87).
in stage 21 (ca. 51 days)
- 2d
- The developing neurohypophysis
- 3c
- Adenohypophysis (the developing anterior hypophysis lobes)
- 3e
- Adenohypophysis (between hypophysis lobes - only temporarily present in humans)
- 5b
- Adenophypophysis stalk (atrophying)
- 6
- Cartilage anlage of the future sphenoid bone
the end of the fetal period
- 2e
- Hypophysis (pituitary) stalk
- 2f
- Neurohypophysis (posterior pituitary lobe)
- 3d
- Adenohypophysis (anterior pituitary lobe)
- 3f
- Adenohypophysis (pars tuberalis)
- 5c
- Juxtaglandular parahypophysis
- 5d
- Sphenoid parahypophysis
- 5e
- Pharyngeal Parahypophysis
- 6
- Cartilagenous anlage of the future sphenoid bone
- 7
- Mesenchyma of the pharynx roof
- 8
- Optic chiasm
The distal part of the infundibulum forms a diverticulum that thickens into posterior hypophysis lobes. Rathke's pouch lengthens and its connection to the stomodeum (adenohypophysis stalk, cranio-pharyngeal canal) disappears.
Fig. 87The posterior hypophysis lobe connects with the rear of Rathke's pouch. It remains connected with the diencephanlon via the hypophysis stalk. Two components develop from Rathke's pouch: the anterior hypophysis lobe and its extension along the hypophysis stalk, the pars tuberalis. The middle lobe is not formed in humans.
The adenohypophysis anlage 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 craniopharyngeal canal.
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 embryonic developmental stages of the human hypophysis (at 3 to 8 weeks).
The neurohypophysis anlage 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.