Pituitary gland

The pituitary gland, together with its connections to the hypothalamus, acts as the main endocrine interface between the central nervous system and the rest of the body. 

Gross anatomy

The pituitary gland sits atop the base of skull in a concavity within the sphenoid bone called the sella turcica (pituitary fossa), immediately below the hypothalamus and optic chiasm.    

The pituitary is usually divided (in practice) into anterior and posterior parts, which actually refer to groupings of four subparts ^4-5:

• anterior pituitary
  • pars distalis
  • pars tuberalis
  • pars intermedia
• posterior pituitary
  • neurohypophysis

The pituitary gland is covered superiorly by a dural reflection - the diaphragma sellae. This has a variably sized opening for the infundibular stalk. Not uncommonly the diaphragma sellae is incomplete (considered a normal variant) and CSF may enter into the pituitary fossa and may give rise to the "empty sella sign" on neuroimaging. 

Anterior pituitary

The anterior pituitary is by far the largest part of the gland, and is responsible for synthesis and release of most pituitary hormones (with the exception of oxytocin and antidiuretic hormone (ADH) which are released by the posterior pituitary).

Pars distalis

The pars distalis is the largest part of the pituitary gland. It forms from the anterior wall of Rathke 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. follicular stimulating hormone (FSH)
5. leutinizing hormone (LH)
6. prolactin

Pars tuberalis

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

Pars intermedia

The pars intermedia in a thin layer of epithelial cells located between pars distalis and neurohypophysis. It arises from the posterior wall of Rathke pouch and contains vestigial lumina of Rathke pouch which appear as narrow vesicles of variable length. These may give rise to Rathke celft cysts (also known as pars intermedia cysts ^6-7). 

Posterior pituitary (neurohypophysis)

The posterior pituitary (aka neurohypophysis) is a direct extension form the hypothalamus and does not synthesise any hormones, but rather releases oxytocin and ADH (vasopressin) which has travelled down the infundibulum in vesicles termed Herring bodies. The cell bodies reside into hypothalamic nuclei: 

• supraoptic nucleus
• paraventricular nucleus 

The infundibular stalk is also considered part of the neurohypophysis and extends from the tuber cinereum and pierces the diaphragma sellae before being surrounded by the pars tuberalis.  

Relations

• inferiorly: basisphenoid, sphenoid sinus
• superiorly: diaphragma sellae, suprasellar cistern
• laterally: cavernous sinus (and contents)
• anteriorly: anterior intercavernous sinus
• posteriorly: posterior intercavernous sinus

Size and age dependent changes

The pituitary gland volume changes depending on hormonal status. Also, what would be a normal gland in one demographic would be grossly abnormal in another. 

Generally speaking, young adults have larger glands than older individuals, and hormonally active individuals (puberty/pregnancy) have the largest glands. These plump glands completely fill the pituitary fossa, and have a convex upper border, whereas older individuals will have a largely empty pituitary fossa, with a deflated and thinned gland lying in the floor of the sella. 

Although one should always be wary of measurements, they can serve to quantify what may otherwise seem overly subjective impressions. These are reasonable maximal figures for the height of the gland ⁹:

• children (<12 years): 6 mm (upper surface flat or slightly concave)
• puberty: 10 mm (upper surface convex; more striking in females)
• young adult
  • male: 8 mm
  • female: 9 mm
  • pregnancy: 12 mm
• older adult (>50 years): gradually decreases in size

Blood supply

The pituitary has rich blood supply, with both a portal circulation (to the anterior pituitary) and arterial supply (to the posterior pituitary) ¹.

Portal circulation

The anterior pituitary receives blood which descends from the hypothalamus along the infundibulum as a portal system (venous channels connecting two capillary beds); this accounts for the typical pattern of contrast enhancement seen on dynamic MRI imaging. 

The superior hypophyseal arteries give superior branches which supply the superior most infundibulum which receives axons from a number of hypothalamic nuclei. These axons release various releasing and inhibiting factors which are then taken down the infundibulum in the hypophyseal portal venous plexus, and thus delivered to the anterior pituitary where they control the release of its numerous hormones ^2,4-5. 

Arterial circulation

A rich network of small branches supplysupplies the stalk and the posterior pituitary, with a total of six arteries described, three from above and three from below/side (from the cavernous segment internal carotid artery). These arteries anastomose with each other to form two networks of vessels ^1-2:

1. circuminfundibular anastomosis: surrounds the pituitary stalk
2. inferior hypophyseal arterial circle (inferior capsular arterial rete): surrounds the base of the pituitary

Branches

• 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)
  • inferior hypophyseal artery (from the meningohypophyseal trunk)
  • capsular artery
  • artery of the inferior cavernous sinus

Venous drainage

Venous blood drains into the nearby cavernous and intercavernous sinuses.

Variant anatomy

The main variations are in size/number ⁸:

• hypoplasia
• hyperplasia: physiologic prolactin cell hyperplasia due to lactation or pregnancy and in response to end organ failure such as Addison disease, Klinefelter or Turner syndrome, or primary hypothyroidism 
• partially empty sella turcica (see: empty sella sign)
• duplication

Embryology

The pituitary gland has a 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 beginnings of the adenohypophysis and neurohypophysis begin to form as Rathke pouch, which forms as an ectodermal outpouching of stomodeum (primitive oral cavity lined by ectoderm) and the infundibulum which forms in the floor of the diencephalon (part of the neural tube).

The infundibulum grows ventrally towards the stomodeum while simultaneously Rathke pouch grows dorsally. Rathke 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 persistent then the structure is called a pharyngeal hypophysis, occasionally associated with nests of pituitary cells along this course.

Related pathology

• pituitary adenoma
  • pituitary microadenoma
  • pituitary macroadenoma
• pituitary apoplexy
• pituitary stalk abnormal enhancement
• Rathke cleft cyst
• craniopharyngioma
• ectopic posterior pituitary