Embryology of the Pituitary Gland

Dual Embryonic Origin

Pituitary gland uniquely develops from two distinct ectodermal sources.
Morphogenesis requires maintained apposition and interaction between oral ectoderm and neuroectoderm.

Feature	Anterior Pituitary (Adenohypophysis)	Posterior Pituitary (Neurohypophysis)
Origin	Rathke's pouch (invagination of primitive oral cavity)	Downward extension of hypothalamus
Germ Layer	Stomodeal (oral) ectoderm	Neural ectoderm (floor of forebrain)
Components	Pars distalis, pars intermedia, pars tuberalis	Infundibular stem, median eminence, infundibular process
Cell Types	Somatotropes, lactotropes, thyrotropes, corticotropes, gonadotropes	Terminal axonal projections of magnocellular neurons

Evaginates in front of buccopharyngeal membrane at roof of developing buccal cavity.
Rathke's pouch identifiable in 3-mm embryo during 3rd week of gestation.
Establishes direct connection with downward extension of hypothalamus.
Detaches completely from oral epithelium by 6th week of gestation.
Evolves into individual structure of rapidly proliferating cells.
Pars distalis houses majority of hormone-producing cells.
Pars intermedia remains rudimentary; completely disappears or consists of cystic cavities lined by cuboidal epithelium.
Vestigial cleft separates pars distalis and intermedia; persistence forms Rathke's cleft cysts.
Craniopharyngeal duct remnants may develop into adamantinous craniopharyngiomas or pharyngeal hypophysis.

Differentiation driven by sequentially expressed transcriptional activation factors (homeobox genes).
Cell specification dictated by spatiotemporal expression of signaling molecules and opposing gradients.
Signaling Molecules:
- Intrinsic (from oral ectoderm): Sonic hedgehog (Shh).
- Extrinsic (from ventral diencephalon/neuroectoderm): Fibroblast growth factors (FGF8), Bone morphogenic proteins (BMP4), Nkx2.1.
Transcription Factors:
- Early Organogenesis: Hesx1, Otx2, Pitx1/2/3, Sox2, Lhx3, Lhx4.
- Lineage Determination: Prop1, POU1F1 (Pit1), NR5A1 (SF1).
- POU1F1 (Pit1): Regulates differentiation of thyrotropes, somatotropes, lactotropes.
- Prop1: Regulates differentiation of gonadotropes, thyrotropes, somatotropes, lactotropes.
All anterior lobe cell types begin differentiation concurrently rather than temporally discrete.

Originates from neural ectoderm of ventral diencephalon.
Forms directly from downward extension of hypothalamus.
Contains terminal axonal projections of magnocellular neurons originating in paraventricular and supraoptic nuclei.
Regulated via direct neural connections, completely bypassing hypophyseal portal circulation.

Gestational Age	Developmental Milestone
3rd - 4th Week	Appearance of Rathke's pouch from oral ectoderm.
5th Week	Pouch visible; downward extension of neural ectoderm begins.
6th Week	Rathke's pouch disconnects from oral ectoderm.
7th Week	Floor of sella turcica in place; separates anterior pituitary from epithelial origins.
8th Week	Capillaries interdigitate among mesenchymal tissue of Rathke's pouch and median eminence.
9th Week	Growth hormone (GH)-producing cells identifiable.
12th - 15th Week	Hypothalamic-hypophyseal vascular system establishes; mature pituitary morphology attained.
14th Week	Gonadotropin release detectable in response to GnRH.
20th Week	Fetal pituitary contains LH and FSH; peak gonadotropin secretion occurs.