DEVELOPMENT OF THE PITUITARY GLAND
The pituitary gland, also termed the hypophysis, consists of two major components, the adenohypophysis and the neurohypophysis. The adenohypophysis (anterior lobe) is derived from the oral ectoderm, and the neurohypophysis (posterior lobe) is derived from the neural ectoderm of the ﬂoor of the forebrain.
A pouchlike recess—Rathke pouch—in the ectodermal lining of the roof of the stomodeum is formed by the fourth to ﬁfth week of gestation and gives rise to the anterior pituitary gland. Rathke pouch extends upward to contact the undersurface of the forebrain and is then constricted by the surrounding mesoderm to form a closed cavity. The original connection between Rathke pouch and the stomodeum—known as the craniopharyngeal canal—runs from the anterior part of the pituitary fossa to the undersurface of the skull. Although it is usually obliterated, a remnant may persist in adult life as a “pharyngeal pituitary” embedded in the mucosa on the dorsal wall of the pharynx. The pharyngeal pituitary may give rise to ectopic hormone secreting pituitary adenomas later in life.
Behind Rathke pouch, a hollow neural outgrowth extends toward the mouth from the ﬂoor of the third ventricle. This neural process forms a funnel-shaped sac—the infundibular process—that becomes a solid structure, except at the upper end where the cavity persists as the infundibular recess of the third ventricle. As Rathke pouch extends toward the third ventricle, it fuses on each side of the infundibular process and subsequently obliterates its lumen, which sometimes per- sists as Rathke cleft. The anterior lobe of the pituitary is formed from Rathke pouch, and the infundibular process gives rise to the adjacent posterior lobe (neurohypophysis). The neurohypophysis consists of the axons and nerve endings of neurons whose cell bodies reside in the supraoptic and paraventricular nuclei of the hypothalamus, forming a hypothalamic–neurohypophysial nerve tract that contains approximately 100,000 nerve ﬁbers. Remnants of Rathke pouch may persist at the boundary of the neurohypophysis, resulting in small colloid cysts.
The anterior lobe also gives off two processes from its ventral wall that extend along the infundibulum as the pars tuberalis, which fuses to surround the upper end of the pituitary stalk. The cleft is the remains of the original cavity of the stomodeal diverticulum. The dorsal (posterior) wall of the cleft remains thin and fuses with the adjoining posterior lobe to form the pars intermedia. The pars intermedia remains intact in some species, but in humans, its cells become interspersed with those of the anterior lobe, and it develops the capacity to synthesize and secrete proopiomelanocortin (POMC) and corticotropin (adrenocorticotropic hormone [ACTH]). The part of the tuber cinereum that lies immediately above the pars tuberalis is termed the median eminence.
Both the adenohypophysis and the neurohypophysis are subdivided into three parts. The adenohypophysis consists of the pars tuberalis, a thin strip of tissue that surrounds the median eminence and the upper part of the neural stalk; the pars intermedia, the portion posterior to the cleft and in contact with the neurohypophysis; and the pars distalis (pars glandularis), the major secretory part of the gland. The neurohypophysis is composed of an expanded distal portion termed the infundibular process; the infundibular stem (neural stalk); and the expanded upper end of the stalk, the median eminence of the tuber cinereum.