Microanatomy: hypothalamus and pituitary

Question 1

Embryology of hypothalamus and pituitary

Answer 1

• Hypothalamus is part of the diencephalon
• The anterior pituitary is from Rathke’s pouch, an invaginaiton of oral ectoderm (glandular tissue)
• Posterior pituitary is derived from diencephalon ectoderm (more closely related to the hypothalamus)
• Posterior pituitary is neuroendocrine tissue

Question 2

Anatomy of pituitary

Answer 2

• The pituitary is connected to the hypothalamus via the stalk (infundibulum), and the organ sits w/in the sella turcica
• Optic chasm runs just superior to the anterior pituitary
• 3rd ventricle is just superior to stalk
• The anterior aspect of the pituitary stalk is the median eminence (first capillary bed in anterior pituitary portal circulation)
• The pars distalis is the main portion of the anterior pituitary (looks like glandular tissue)
• Pars intermedia separates the anterior and posterior pituitary
• Pars nervosa is main part of posterior pituitary (looks like brain tissue)

Question 3

Posterior pituitary

Answer 3

• There is no portal circulation, only a capillary bed w/in the post pituitary
• Neurosecretory neuron cell bodies reside in hypothalamus (SON and PVN) send axons down the stalk and have terminals directly on the capillary beds
• The cell bodies (hypothalamus) make the hormones, but they are stored in the axons near the terminals (post pituitary- vesicles stored in herring bodies)
• The two hormones released from post pit are ADH (from SON) and oxytocin (from PVN)
• The BBB is absent in the post pit (fenestrated capillaries)

Question 4

Functions of ADH and oxytocin

Answer 4

• ADH released in response to increases in plasma osmolality, stress, hypovolemia
• ETOH inhibits ADH release (functions as a diuretic)
• ADH increases AQP in kidney to facilitate reabsorption of water
• Oxytocin is important in women during parturition and lactation
• It facilitates uterus contractions to expel the fetus/placenta
• It facilitates milk ejection (but not synthesis); suckling causes oxytocin release

Question 5

Regulation of anterior pituitary 1

Answer 5

• Anterior pituitary cells release hormones into the portal circulation in response to hormones coming down the portal circulation from the hypothalamus (specifically the parvicellular neuroendocrine cells)
• The capillaries of the portal system have a weakened BBB
• Parvicellular cells send axons to the median eminence and dump release hormones into portal circulation: GHRH, GnRH, CRH, TRH
• The releasing hormones cause anterior pituitary cells to release their (stimulating) hormones: GHRH-> GH, GnRH-> LH/FSH, CRH-> ACTH, TRH-TSH

Question 6

Regulation of anterior pituitary 1

Answer 6

• 2 compounds from the parvicellular cells inhibit release of hormones: somatostatin inhibits GH release and DA inhibits prolactin release
• The anterior pituitary hormones are made and stored there, the releasing hormones from the hypothalamus are made in the hypothalamus and stored at the axons terminals in the median eminence

Question 7

Cells of the anterior pituitary

Answer 7

• 3 cell types: acidophils, basophils, chromophobes
• Must be able to distinguish each one from each other
• Chromophobes have very little staining and are probably degreanulated acidophils or basophils
• Acidophils are most numerous cells and are pink staining
• Acidophils may be somatotropes (release GH) or lactotropes (release prolactin)
• Excess of GH in childhood results in giantism (enlarged long bones) and GH excess in adulthood results in acromegaly (thickening of bones esp in face and hands)
• Prolactin promotes milk synthesis in breast

Question 8

Basophils in anterior pituitary

Answer 8

• Basophils are less numerous than acidophils, but can release a wider variety of hormones
• Basophils are bluer in appearance
• BFLAT: Basophils can release FSH, LH, ACTH, TSH
• Thyrotropes, corticotropes, gonadotropes are all basophils