4.1&4.2 - The Pituitary Gland

Question 1

What is a hormone?

Answer 1

• a messenger that is carried from the organ where they are produced to the organ which they affect by means of the blood stream
• two types: peptide hormones and steroid hormones

Question 2

What are peptide hormones?

Answer 2

• synthesised as prohormones requiring further processing (e.g. cleavage) to activate
• stored in vesicles (regulatory secretion)
• bind receptors on cell membrane and transduce signal using 2nd messenger systems

Question 3

What are steroid hormones?

Answer 3

• synthesised in a series of reactions from cholesterol
• released immediately (constitutive secretion)
• bind to intracellular receptors to change gene expression directly

Question 4

Where is the pituitary gland?

Answer 4

• in the brain, below the hypothalamus
• sits in the sella turcica of the sphenoid bone
• anterior and posterior pituitary

Question 5

What is the anterior pituitary gland?

Answer 5

• adenohypophysis
• derived from an upgrowth from the oral ectoderm of the primitive oral cavity called Rathke’s pouch
• epithelial origin

Question 6

What is the posterior pituitary gland?

Answer 6

• neurohypophysis
• formed from a downgrowth of the diencephalon that forms the floor of the third ventricle (directly from brain)
• neural origin

Question 7

What are hypothalamic parvocellular neurons?

Answer 7

• they regulate anterior pituitary function
• short and terminate on median eminence (base of hypothalamus, allows communication between anterior pituitary and hypothalamus)
• release hypothalamic releasing / inhibitory factors into capillary plexus in median eminence
• these hypothalamic regulatory factors are carried by portal circulation (made of leaky capillaries) to the anterior pituitary

Question 8

What are the features of the anterior pituitary (adenohypophysis)?

Answer 8

• anatomically distinct from the hypothalamus
• made up of endocrine cells: somatotrophs, lactotrophs, corticotrophs, thyrotrophs, gonadotrophs
• regulated by hypothalamic releasing/inhibitory factors via hypophyseal-pituitary portal system

Question 9

How does the hypothalamo-pituitary portal system work?

Answer 9

1. axon terminals of hypothalamic neurosecretory cells release hormones (RHs and IHs) into the hypothalamo-pituitary portal system
2. the RHs and IHs travel in the portal system to the anterior pituitary
3. the RHs and IHs stimulate or inhibit the release of hormones from anterior pituitary cells
4. anterior pituitary hormones leave the gland via the blood

• these blood vessels constitute the hypothalamo-pituitary portal system

Question 10

How does the hypothalamo-pituitary portal system work to regulate thyroid hormone?

Answer 10

1. axon terminals of hypothalamic neurosecretory cells release Thyrotrophin Releasing Hormone (TRH) into the hypothalamo-pituitary portal system
2. TRH travel in the portal system to the anterior pituitary
3. TRH stimulates the release of Thyroid Stimulating Hormone (TSH) from anterior pituitary thyrotrophs
4. TSH leaves the gland via the blood to travel to the thyroid gland to stimulate thyroid hormone release of thyroxine

Question 11

Somatotrophs - what hormones regulate them and what hormone do they produce?

Answer 11

• regulated by hypothalamic release of growth hormone releasing hormone (RH) and somatostatin (IH)
• produces growth hormone - somatotrophin
• receptors for growth hormone found in general body tissues (particularly liver) where it causes growth

Question 12

Lactotrophs - what hormone regulates them and what hormone do they produce?

Answer 12

• regulated by dopamine (IH)
• produces prolactin (inhibited by dopamine)
• receptors found in breasts in lactating women

Question 13

Thyrotrophs - what hormone regulates them and what hormone do they produce?

Answer 13

• regulated by thyrotrophin releasing hormone (TRH) - RH
• produce thyroid stimulating hormone (TSH aka thyrotrophin)
• receptors for TSH found in the thyroid

Question 14

Gonadotrophs - what hormone regulates them and what hormone do they produce?

Answer 14

• regulated by gonadotrophin releasing hormone (RH)
• produce luteinising hormone (LH) and follicle stimulating hormone (FSH)
• receptors are in testes for males and ovaries in females

Question 15

Corticotrophs - what hormone regulates them and what hormone do they produce?

Answer 15

• regulated by corticotrophin releasing hormone (RH)
• produces adrenocorticotrophic hormone (ACTH, corticotrophin)
• receptors are in the adrenal cortex

Question 16

What is bitemporal hemianopia?

Answer 16

• the fibres from the nasal (medial) retinae of the eye cross at the optic chiasm
• compression of the optic chiasm by a pituitary tumour/suprasellar tumour prevents transmission of sensory information from lateral (temporal) visual fields (peripheral vision) to the occipital lobe

Question 17

What are the mechanisms of growth hormone action?

Answer 17

• insulin-like growth factor - IGF-1 is the main one made in adults and children, and IGF-2 is main in developing foetus
• somatotrophin can either act directly on body tissues or can act on liver which releases IGF which also acts on body tissues - both produce same effects on growth and development

Question 18

What is acromegaly and the symptoms?

Answer 18

• excess growth hormone
• symptoms include:
• coarsening of facial features
• macroglossia - bigger tongue
• prominent nose
• prognathism - large jaw
• increased hand and feet size
• sweatiness
• headaches
• carpal tunnel due to growth of soft tissues around median nerve in wrist, compressing it

Question 19

Suggest some treatments for acromegaly?

Answer 19

• somatostatin analogues / agonists
• growth hormone receptor antagonist

Question 20

When does acromegaly cause gigantism (increased height)?

Answer 20

• if there is excess growth hormone before puberty is finished, you get gigantism because the epiphyseal growth plates have not fused yet
• if there is excess growth hormone later in life after puberty, you do not get gigantism because growth plates have fused but you are affected in other ways

Question 21

What is the posterior pituitary gland?

Answer 21

• neurohypophysis
• posterior pituitary hormones - arginine vasopressin (AVP) - AKA anti-diuretic hormone
• oxytocin

Question 22

What are hypothalamic magnocellular neurons?

Answer 22

• long, originate in supraoptic (AVP) and paraventricular (oxytocin) hypothalamic nuclei
• pass down from nuclei through pituitary stalk to posterior pituitary

Question 23

What are the features of the posterior pituitary?

Answer 23

• it is anatomically continuous with hypothalamus
• does not make hormones itself, stores arginine vasopressin and oxytocin produced in the hypothalamus
• made of only neuronal tissue, not endocrinal tissue like anterior pituitary

Question 24

How do AVP and oxytocin regulate posterior pituitary gland action?

Answer 24

1. two sets of hypothalamic neurosecretory cells produce AVP and oxytocin and transport them to the posterior pituitary
2. excitation of these hypothalamic magnocellular neurons stimulates release of AVP / oxytocin into the posterior pituitary where they diffuse into blood capillaries
3. they then leave the posterior pituitary via the blood

Question 25

What are the physiological actions of AVP?

Answer 25

• anti-diuretic hormone
• diuresis is the production of urine
• main physiological action = stimulation of water reabsorption in the renal collecting duct = concentrates urine
• acts through the V2 receptor in the kidney
• also a vasoconstrictor via V1 receptor
• also stimulates ACTH from anterior pituitary

Question 26

How does AVP concentrate urine at a cellular level?

Answer 26

• AVP binds to V2 receptor on collecting duct cell which results in intracellular signalling cascade
• this inserts aquaporin-2 molecules (water transport molecules) into apical membrane of duct cell
• water diffuses down its concentration gradient through aquaporin-2 from urine in nephron into collecting duct cell
• it then moves through aquaporin-3 into plasma

Question 27

What are the physiological actions of oxytocin?

Answer 27

• contraction of myometrial cells in uterus at parturition (in labour) which leads to delivery of baby
• contraction of myoepithelial cells in breast during lactation which leads to milk ejection

Question 28

What is the neuroendocrine reflex arc for milk production (anterior pituitary)?

Answer 28

1. mechanical stimulation of nipple and surrounding area activates afferent pathways
2. afferent signals integrated in the hypothalamus and inhibit dopamine release from dopaminergic neurons
3. less dopamine in the hypothalamic-pituitary portal system causes less inhibition of anterior pituitary lactotrophs
4. increased plasma prolactin increases milk secretion in mammary glands

Question 29

What is the neuroendocrine reflex arc for milk ejection (posterior pituitary)?

Answer 29

1. mechanical stimulation of nipple and surrounding area activates afferent pathways
2. afferent signals integrated in the hypothalamus and stimulate oxytocin-releasing neuron activity
3. action potentials travel down oxytocin neurons and oxytocin is secreted into the bloodstream
4. increased plasma oxytocin increases milk ejection in mammary glands

Question 30

Summary - adenohypophysis vs neurohypophysis

Answer 30

• ORIGIN: oral ectoderm (epithelial) vs appendage of the hypothalamus (neural)
• REGULATION: receives signalling hormones from the hypothalamus via hypothalamo-pituitary portal circulation vs does not produce any hormones of its own; stores and secretes two hormones made in the hypothalamus
• HORMONES: LH&FSH, prolactin, TSH, GH, ACTH vs oxytocin, AVP/ADH