The pituitary gland has two functional components (Fig. 21.3):  Anterior lobe (adenohypophysis), the glandular epithe lial tissue  Posterior lobe (neurohypophysis), the neural secretory
These two portions are of different embryologic origin. The anterior lobe of the pituitary gland is derived from an evagination of the ectoderm of the oropharynx toward the brain (Rathke’s pouch). The posterior lobe of the pituitary gland is derived from a downgrowth (the future infundibulum) of neuroectoderm of the foor of the third ventricle (the diencephalon) of the developing brain (Fig. 21.4).
The anterior lobe of the pituitary gland consists of three derivatives of Rathke’s pouch:  Pars distalis, which comprises the bulk of the anterior lobe of the pituitary gland and arises from the thickened anterior wall of the pouch Pars intermedia, a thin remnant of the posterior wall of the pouch that abuts the pars distalis  Pars tuberalis, which develops from the thickened lateral walls of the pouch and forms a collar or sheath around the infundibulum
The embryonic infundibulum gives rise to the posterior lobe of the pituitary gland. The posterior lobe of the pituitary gland consists of the following:  Pars nervosa, which contains neurosecretory axons and their endings.
 Infundibulum, which is continuous with the median eminence and contains the neurosecretory axons forming the hypothalamohypophyseal tracts
Knowledge of the unusual blood supply of the pituitary gland is important to understanding its functions. The
FIGURE 21.4 • Development of the pituitary gland. This diagram shows sequential stages (a to c) in the development of the pituitary gland.
FIGURE 21.3 • Pituitary gland. a. Photomicrograph of a pituitary gland. The pituitary gland lobes can be identified on the basis of their appearance, location, and relation to each other. 7. b. Drawing of a pituitary and related regions of the hypothalamus. The anterior lobe of the pituitary gland consists of the pars distalis, pars tuberalis, and pars intermedia; the posterior lobe of the pituitary gland consists of the infundibulum and pars nervosa.
pituitary blood supply is derived from two sets of vessels The hypothalamohypophyseal portal system provides the (Fig. 21.5): crucial link between the hypothalamus and the pituitary  Superior hypophyseal arteries supply the pars tuber-gland. alis, median eminence, and infundibulum. These vessels The arteries that supply the pars tuberalis, median eminence, arise from the internal carotid arteries and posterior com-and infundibulum give rise to fenestrated capillaries (the primunicating artery of the circle of Willis. mary capillary plexus). These capillaries drain into portal  Inferior hypophyseal arteries primarily supply the pars veins, called the hypophyseal portal veins, which run nervosa. These vessels arise solely from the internal carotid along the pars tuberalis and give rise to a second fenestrated arteries. An important functional observation is that most sinusoidal capillary network (the secondary capillary plexus). of the anterior lobe of the pituitary gland has no direct This system of vessels carries the neuroendocrine secretions arterial supply. of hypothalamic nerves from their sites of release in the
FIGURE 21.5 • Diagram of the blood supply to the pituitary gland. The hypophyseal portal veins begin in the capillary beds of the median eminence and infundibulum and end in the capillaries of the pars distalis.
median eminence and infundibulum directly to the cells of the pars distalis.