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.
