The pituitary gland is a pea-sized endocrine gland that sits at the base of the brain. Often referred to as the “master gland”, the pituitary gland synthesizes and releases various hormones that affect several organs throughout the body (see the images below).
Hormones secreted by adenohypophysis (anterior pituitary).
Hormones secreted by neurohypophysis (posterior pituitary).
The pituitary gland is entirely ectodermal in origin but is composed of 2 functionally distinct structures that differ in embryologic development and anatomy: the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary).
The adenohypophysis develops from Rathke’s pouch, which is an upward invagination of oral ectoderm from the roof of the stomodeum; in contrast, the neurohypophysis develops from the infundibulum, which is a downward extension of neural ectoderm from the floor of the diencephalon (see the image below). The oral ectoderm and neural ectoderm that form the pituitary anlagen are in close contact during early embryogenesis, and this connection is critical for pituitary development.
Development of pituitary gland. A: Infundibulum and Rathke’s pouch develop from neural ectoderm and oral ectoderm, respectively. B: Rathke’s pouch constricts at base. C: Rathke’s pouch completely separates from oral epithelium. D: Adenohypophysis is formed by development of pars distalis, pars tuberalis, and pars intermedia; neurohypophysis is formed by development of pars nervosa, infundibular stem, and median eminence.
Over several weeks, Rathke’s pouch undergoes constriction at its base until it completely separates from the oral epithelium and nears its final position as the adenohypophysis.
The transition from Rathke’s pouch to the adenohypophysis involves the formation of the pars distalis from the rapidly proliferating anterior wall, the pars intermedia from the less active posterior wall, and the pars tuberalis from an upward outgrowth of the anterior wall. The incomplete obliteration of Rathke’s pouch can lead to remnants that form Rathke’s cleft cysts.
The neurohypophysis develops from the differentiation of neural ectoderm into the pars nervosa, the infundibular stem, and the median eminence. The infundibular stem is surrounded by the pars tuberalis.