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Pituitary gland

contributed by Dr Abhijit Datir on May 2, 2008

System: Central Nervous System
Section: Anatomy
Tag: pituitary

The pituitary gland is an inferior extension of the hypothalamus.  It sits atop the base of skull in the sella turcica which itself is covered in a dural fold.  This is commonly referred to as the pituitary fossa and is located in the sphenoid bone in the middle cranial fossa.

Embryology

It has dual origin (ectoderm of the primitive mouth cavity and neuroectoderm of the diencephalon) which reflects the two distinct parts in the adult gland.

At approximately 24 days gestation the beginings of the adenohypophysis and neurohypophysis begin to form as Rathke’s pouch forms as an ectodermal outpouching of stomodeum (primitive oral cavity lined by ectoderm) and the infundibulum which forms in the floor of diencephalon (part of the neural tube).

The infundibulum grows ventrally towards the stomodeum while imultaneously Rathke’s pouch grows dorsally. Rathke’s pouch eventually loses its connection with the stomodeum and forms a discrete sac which adheres to the infundibular process. This sac differentiates to form the adenohypophysis of the pituitary: pars distalis, pars tuberalis, and pars intermedia.  The distal part of the infundibulum differentiates to form the posterior pituitary (neurohypophysis) and retains the connection with the hypothalamus as the stalk. 

By the sixth week the connection with the oral cavity has been lost. If persistant then the structure is called pharyngeal hypophysis, occasionally associated with rests of pituitary cells along this course.

Anatomy

Pars distalis

The pars distalis is the largest part of the pituitary gland and is what is usually referred to as the "anterior pituitary". It froms from the aterior wall of Rathke's pouch. It is composed of cords of epithelial cells individually specialized to secrete tropic hormones acting on target organs:

  1. growth hormone (somatotropin)
  2. thyroid stimulating hormone (TSH)
  3. adrenocorticotropic hormone (ACTH)
  4. FSH & LH
  5. prolactin
Pars tuberalis

The pars tuberalis is the part of the adenohypophysis which surrounds the infundibular stalk.

Pars intermedia

Pars intermedia: thin layer of epithelial cells located between pars distalis and neurohypophysis. It arises from the posterior wall of Rathke's pouch. It contains vestigial lumina of Rathke’s pouch which appear as narrow vesicles of variable length, and may give rise to Rathke's celft cysts. How these differ from pars intermedia cysts is unclear to me at the time of writing. [  needs clarification and references ].

Neurohypophysis

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Anterior and posterior pituitary

Commonly used, anterior and posterior pituitary refer to groupings of the above parts - although there is some controversy as to exactly what fits in where.

Everyone agrees the the pars distalis and pars tuberalis are part of the the anterior pituitary. Whether the pars intermedialis  is part of the anterior or posterior is debated (probably ad-nauseum). The neurohypophysis is clearly in the posterior part.

Blood supply

The pituitary has rich blood supply, with both a portal circulation (to the anterior pituitary) and arterial supply (to the posterior pituitary).

Portal circulation

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Arterial circulation

A rich network of small branches supply the stalk and the posterior pituitary, with a total of six arteries described, three from above and three from below / side (from the cavernous carotid artery). These arteries anastamose with each other to form two networks of vessels:

  1. the "circuminfundibular anastomosis" which surrounds the pituitary stalk
  2. the "inferior hypophyseal arterial circle" (also known as the "inferior capsular arterial rete") which surrounds the base of the pituitary.

The branches are:

  • from above:
    • superior hypophyseal artery (from the internal carotid artery)
    • infundibular artery (from the posterior communicating artery)
    • prechiasmal artery (from the ophthalmic artery)
  • from below / side (cavernous carotid artery)

These arteries were anastomosed each other, making the rich vascular networks, called the "circuminfundibular anastomosis" around the pituitary stalk, and the "inferior hypophyseal arterial circle (or inferior capsular arterial rete)" in the base of the pituitary.

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