Pituitary gland (hypophysis)

Structure and location

The pituitary gland is located in the pituitary fossa (sella turcica) of the sphenoid bone. Superiorly, the pituitary gland is covered by the circular diaphragma sellae of dura mater. Anteroinferiorly, the gland faces the sphenoid sinus, anterosuperiorly the optic chiasm and laterally the cavernous sinus.

The pituitary gland is connected to the hypothalamus by the infundibulum (pituitary stalk), which is a process that extends inferiorly from the tuber cinereum of the hypothalamus. The infundibulum not only connects the two glands physically, but it also enables the passage of the hypothalamic hormones to the hypophysis as it is traversed by the hypophyseal portal system and hypothalamohypophyseal tract.

The hypophysis has two major parts, adenohypophysis (anterior part) and neurohypophysis (posterior part). These parts differ in their embryological origin, and thus have different histological appearance and functions.

Hypophyseal portal system

The hypophyseal portal system is a vascular system made of tiny blood vessels that connect the adenohypophysis to the hypothalamus. The capillaries and venules of the portal system are fenestrated which allows the smooth exchange of molecules between the blood vessels and the cells of the anterior pituitary. The epithelial cells of adenohypophysis are arranged in cords between vascular sinusoids where the hormonal exchange between the neural tissue and blood happens.

The hypophyseal-portal system originates from the superior and inferior hypophyseal arteries, which are the branches of the internal carotid artery. The former provides the supply mostly for the anterior pituitary, while the latter supplies the posterior pituitary.

The superior hypophyseal arteries form a primary plexus within the infundibulum and median eminence. This plexus consists of many fenestrated capillaries which rejoin and form the hypophyseal portal veins that travel to the anterior pituitary. The portal veins divide and form another plexus in the anterior pituitary; the secondary plexus. This vascular system is very important as it is the direct connection between the site of releasing of hypothalamic hormones (median eminence) and the cells of the adenohypophysis.

To summarize, the hypophyseal portal system consists of the primary and secondary capillary plexuses (beds) in the pituitary gland, plus the intervening portal veins.

Hypothalamohypophyseal tract

The hypothalamohypophyseal tract is a bundle of axons that connects the hypothalamic nuclei with the neurohypophysis. Its function is to carry the two hypothalamic neurohormones oxytocin and antidiuretic hormone to the neurohypophysis, where they are stored and released upon the body’s needs.

Anterior pituitary (adenohypophysis)

The anterior pituitary (adenohypophysis) is made of three distinctive parts:

• Anterior part (distal or glandular part) is the part with the strongest secretory activity. It is composed of follicles that vary in size, but essentially contain three types of cells. These cells are classified according to their histological staining and include chromophils (acidophilic and basophilic) and chromophobes.
  • The acidophilic cells further divide into somatotrophs and lactotrophs. The former produce the growth hormone, while the latter produce prolactin.
  • The basophilic cells are divided into gonadotrophs (producing FSH and LH), corticotrophs (ACTH) and thyrotropes (TSH).
  • The chromophobes stain weakly, and they are the progenitor cells.
• Tuberal part extends from the anterior (distal) part of the pituitary gland. The majority of its cells are gonadotrophs, with the small remainder being the thyrotrophs. The tuberal part surrounds the infundibular stem, a collection of axons that project into the pituitary from the hypothalamus. These axons are filled with the accumulated hormones (oxytocin and vasopressin). 
• Intermediate part is located between the anterior part of adenohypophysis and neurohypophysis. It contains follicles that are made up of colloidal matrices. The majority of the cells in this part are the corticotropic basophils, with less abundant pituitary stem cells. These corticotropic cells of this part produce melanocyte-stimulating hormone and endorphins.
  

Posterior pituitary (neurohypophysis)

The posterior portion of the pituitary gland is a specialized neuroendocrine structure. Unlike the anterior pituitary, it doesn’t contain the secretory cells, but rather many unmyelinated axons that originate from the secretory hypothalamic neurons, specifically from the magnocellular neurons of the paraventricular and supraoptic nuclei. The only cellular components of the posterior pituitary are the glial cells called the pituicytes.

The neurohypophysis is divided into several parts:

• Infundibulum consists of the many unmyelinated axons that form the hypothalamohypophyseal tracts. These tracts connect the pituitary gland to the hypothalamus and serve to transport the neurohormones from the hypothalamic nuclei to the neurohypophysis. The infundibulum is continuous with the median eminence, the area where the releasing hormones from the hypothalamus are released in order to control the anterior pituitary.
  
• Neural lobe (pars nervosa, posterior lobe) is a collection of approximate 100,000 unmyelinated axons and their endings. These axon terminals contain the neurosecretory bodies (Herring’s bodies) that are filled with granules that contain the hypothalamic hormones destined to be released by the neurohypophysis. Each granule contains either antidiuretic hormone (ADH) or oxytocin. Within the granules, the oxytocin and ADH are bound to the proteins called the neurophysin I and II, respectively. The neural lobe also houses supportive pituicytes that envelope the axons.