The hypophysis is an amalgam of two tissues. Early in gestation a finger of ectoderm grows upward from the roof of the mouth. This protrusion is called Rathke's pouch and will develop into the anterior pituitary or adenohypophysis.
At the same time that Rathke's pouch is developing, another finger of ectodermal tissue evaginates ventrally from the diencephalon of the developing brain. This extension of the ventral brain will become the posterior pituitary or neurohypophysis. Ultimately, the two tissues grow into one another and become tightly apposed, but their structure remains distinctly different, reflecting their differing embryological origins.
Both the adenohypophysis and neurohypophysis can be subdivided based on gross and histologic features. The terminology used is as follows:
- Pars distalis - the largest section
- Pars tuberalis - a collar of tissue that usually surrounds the infundibular stalk
- Pars intermedia - a narrow band that is usually separated from the pars distalis by a hypophyseal cleft
- Pars nervosa - the bulk of the posterior pituitary
- Median emminence - the upper section of the neurohypophysis above the pars tuberalis
- Infundibular stalk - the "stem" that connects the pars nervosa to the base of the brain