The pituitary gland (hypophysis) is an endocrine gland that lies in a bony cavity in the skull. This cavity lies in the sphenoid bone and is called the sella turcica. The gland is attached to the base of the brain by a stalk (infundibular stalk) and is contained in a capsule that is continuous with the dura mater (a tough membrane surrounding the brain).
The gland is composed of two main lobes: an anterior lobe (the adenohypophysis) and a posterior lobe (the neurohypophysis). Between the two lobes is an intermediate lobe (pars intermedia). This intermediate lobe is almost non-existent in the adult, although it may increase in size during pregnancy.
The normal adult pituitary gland is a reddish-grey bean shaped gland. The average gland weighs between 500-900mg (with the female gland usually being slightly heavier than the male). The dimensions of the pituitary are 10-17mm from side to side, 5-15mm from front to back and 5-10mm from top to bottom.
An important anatomical relation to the pituitary gland is the optic chiasm, which lies just above the pituitary fossa. Therefore, any expanding lesion of the pituitary or hypothalamus can present with visual field defects.
The blood supply to the hypothalamus and pituitary is derived from the circle of Willis at the base of the brain. The most important blood supply is from the superior hypophysial arteries which arise from the internal carotids. The blood from this artery enters a primary capillary plexus at the median eminence. Blood from this plexus flows down long portal vessels to a secondary capillary plexus in the adenohypophysis. The portal vessels run down the pituitary stalk (infundibulum) to arrive at the pituitary gland. This system is known as the hypothalamo-hypophysial portal system.
The pituitary gland develops as a fusion of two groups of cells. There is an upgrowth of ectodermal cells from the roof of the primitive pharynx (known as Rathke's pouch), and a down-growth of neural tissue cells from the hypothalamus. These two distinct areas form the anterior (the adenohypophysis) and posterior (the neurohypophysis) lobes respectively.