The pituitary gland is located right below the optic chiasm in the sella of the brain
How is the pituitary gland connected to the hypothalamus?
What is the posterior lobe (neurohypophysis) of the piutitary gland derived from?
Right: adenohypophysis (derived from the oral ectoderm (Rathke pouch))
Hormones of the posterior lobe?
ADH and oxytocin
How is the hypothalmus connected to the posterior pituitary? What are Herring bodies?
The posterior pituitary is actually a collection of nerve axons whose cell bodies are in the supraoptic and paraventricular nuclei of the hypothalamus that terminate near capillaries. Thus ADH and oxyotcin are actually neuropeptides, that is, they are peptides released from (magnocellular) neurons
Herring Bodies: axonal swellings packed with hormonal granules from the hypothalamic nuclei
Pitucytes: supportive glial cells
Although both posterior hormones are synthesized in both nuclei of the hypothalamus, ADH is primarily associated with the _____ nuclei and oxyotcin is primarily associated with _______ nuclei.
Describe the transmission ans secretion of ADH and oxytocin once synthesized.
The hormones are transported down the axons of the hypothalmic-hypophyseal tract in neurosecretory vesicles and stored in bulbous nerve terminals in the posterior pituitary. Once stimulated, the neurosecretory vesicles are exocytosed and the secreted hormones enter nearby fenestrated capillaries originating from the capillary plexus of the infundibular process
NOTE: The capillary plexus of the infundibular process originates from the inferior hypophysial artery (from the internal carotid artery) and empties into the posterior hypophysial veins. (to the internal jugular vein). NOTE: a small portion of the hypophysial veins connect to the anterior pituitary so traces of these hormones may be seen there
How are the posterior pituitary hormones transported in blood?
This venous blood enters circulation and Oxytocin is transported in free form along side Neurophysin I and ADH is carried with Neurophysin II (these are peptides so no carriers)
NOTE: The peptide precursor for ADH is prepropressophysin and the precursor for oxytocin is prepro-oxyphysin. In the Golgi, the signal peptides of each of these are cleaved to form prohormones, which are packaged into secretory vesicles to travel down the axon through the hypothalamic-hypophysial tract, during which the neurophysins are cleaved from the prohormones
Where does the anterior lobe of the pituitary originate from?
oral ectoderm (Rathke pouch)
What are the anterior pituitary hormones?
FSH, LH, ACTH, TSH, Prolactin, and Growth hormone
The hypothalamus and the anterior pituitary, unlike the posterior lobe, are related both neurally AND in an endocrine fashion. How are they physically connected?
Directly by the hypothalamic-hypophysial portal blood vessels
Describe the hypothalamic-hypophysial portal blood vessels
Arterial blood is delivered to the hypothalamus via the superior hypophysial arteries which distribute the blood into a capillary network in the median eminence, called the primary capillary plexus (thus, in this set of capillaries, hormones from the hypothalamus enter). These capillaries converge to form long hypophysial portal vessels, which travel down the infundibulum to the anterior pituitary. In the lobe, another set of capillaries, the secondary capillary plexus arises from the inferior hypophysial artery (this is where the hormones of the anterior pituitary enter) to be drained to the internal jugular vein
NOTE: in contrast to most organs, the anterior pituitary receives primarily venous blood
How are hormones from the hypothalamus transported to the anterior pituitary?
Hypothalamic-releasing hormones are synthesized in the cell bodies of hypothalamic neurons in the ventral hypothalamus and travel down the axons of these neurons to the median eminence of the hypothalamus. Upon stimulation, the hormones are secreted into surrounding hypothalamic tissue and enter the nearly capillary plexus to be delivered to the anterior lobe
TSH is secreted by ____.
FSH and LH are secreted by ____.
ACTH is secreted by ____.
GH is secreted by _____.
Prolactin is secreted by _____.
TSH- thyrotrophs (5%)
FSH and LH- gonadotrophs (15%) (co-secretion)
ACTH- corticotrophs (15%)
GH- somatotrophs (20%)
Prolactin- lactotrophs (15%) (higher in females and in pregnancy)
NOTE: These hormones are synthesized in the anterior lobe in the standard preprohormone, prohormone, and maturation process
The hormones of the anterior pituitary are organized into 'families' based on structural similarities. What are these families?
TSH, FSH, and LH- family I
ACTH- family II (found in the middle of the lobe)
GH and prolactin- family III (acidophils) found in the peripheral of the anterior lobe (Peripheral hormones -especially GH- are more likely to be impacted by injury)
How are the posterior pituiatry hormones relased?
An action potential is transmitted from the cell body in the hypothalamus and the secretory vesicle, with the hormone and their respective neurophysin, is exocytosed to a nearby capillary when Ca2+ levels rise
What are the main roles of oxytocin?
Produces milk ejection (aka 'letdown') from the lactating breast by stimulating contraction of myoepithelial cells lining the milk ducts
Promotes uterine contraction- can be used to induce labor and reduce postpartum bleeding
What factors induce secretion of oxytocin from the posterior pituitary?
suckling; the sight, smell, and sound of an infant; orgasm; and dilation of the cervix
T or F. Oxytocin primarily utilizes negative feedback mechanisms
F. It is actually one hormone that works commonly via positive feedback
Why are women in labor so response to oxytocin but others are not?
receptors are upregulated via estrogen
What stimulates ADH release?
small increases in serum osmolarity causes
larger decreases in ECF volume (10+%)
Pain and Nausea
How does ADH work?
1) binds to V2 receptors on the principle cells of the distal tubules of the CD which is coupled to a Gs protein to increase water reabsorption by inserting more aquaporin 2 channels in the luminal membranes.
Thus, urine will be more concentrated
2) Causes smooth muscle contraction by binding the V1 receptor which is coupled to a Gq protein and acts via phospholipase C to cause increased BP
Inhibitors to ADH release?
Decreased serum osmolarity
Describe how ADH is stimulated to be released via increased osmolarity?
Increases in serum osmolarity is sensed by osmoreceptors in the anterior hypothalamus.
Describe how ADH is stimulated to be released by depleted ECF volume
- For ECF depletion to stimulate ADH release, there must be a 10+% drop, which is sensed by peripheral baroreceptors and is then transmitted via the vagus nerve to the hypothalamus
NOTE: Drops in ECF override osmolarity changes (i.e. if ECF is low and osmolarity is low, ADH will still be secreted)
What is central diabetes insipidus?
Failure of the posterior pituitary to secrete ADH, causing serum osmolarity to rise and large volume of dilute urine to be produced
What is nephrogenic diabetes insipidus?
Failure of ADH to bind to the V2 receptor of the principle cells in the collecting duct (or a defect in the Gs protein of adenylyl cyclase). This differs from central DI in that serum ADH will still be high even though serum osmolarity will be high and urine will be dilute
How is nephrogenic DI treated? Why?
thiazide diuretics work by inhibiting Na+ reabsorption in the easly distal tubule
How is SIADH treated?
demeclocycline or water restriction
Why would vasopressin be upregulated in heart failure?
circulating blood decreases to activate ADH (and additional renin release causes systemic vasoconstriction which further increasing afterload of the heart)