Pituitary Gland

Question 1

What is a hormone? What are the 2 types?

Answer 1

• A messenger carried from the organ where they are produced to the organ which they affect by means of blood stream

- Peptide & Steroid

Question 2

Describe the synthesis, storage and receptors of peptide hormones:

Answer 2

Synthesis: Synthesised as prohormones require further processing (e.g. celeavage) to activate (insulin secretes as preproinsulin)

Storage: Stored in vesicles (regulatory secretion; triggered to be released)

Receptors: Bind receptors on cell membrane and transduce signal using 2nd messenger systems

Question 3

Describe the synthesis, storage and receptors of steroid hormones:

Answer 3

Syntheis: Synthesised in series of reactions from precursor cholesterol (e.g. cortisol)

Storage: Released immediately (constitutive secretion)

Receptors: Bind to intracellular receptors to change gene expression directly

Question 4

What part does the pituitary gland sit in?

Answer 4

• The sella turcica of the sphenoid bone
• Boney dish

Question 5

Describe some features of the pituitary gland:

Answer 5

• Sits in sella turcica of sphenoid bone
• Anterior pituitary & Posterior pituitary
• Hangs from stalk (Infundibulum)
• Near to the optic chiasm (controls vision)
• Above it is the hypothalamus

Question 6

What is the origin of the pituitary gland?

Answer 6

Anterior Pituitary: ADENOHYPOPHYSIS

• Derived from upgrowth from oral ectoderm of primitive oral cavity (mouth) called Rathke’s pouch ; epithelial origin

Posterior Pituitary: NEUROHYPOPHYSIS

• Formed from downgrowth of diencephalon that forms floor of third ventricle ; neural origin

Question 7

How does the hypothalamus regulate the aterior pituitary?

Answer 7

• Hypothalamic parvocellular neurones release hypothalamic releasing/inhibitory factors into capillary plexus of median emminance
• Capillaries are leaky (fenestration) ; holes in walls allow factors flow into blood vessels easily
• Flow down blood supply (hyopthalamo pituitary portal circulation) down infundibulum to anterior pituitary

Question 8

What are hypothalamic parvocelullar neurons?

Answer 8

• Regulate anterior pituitary function
• Short, terminate on median eminence
• Release hypothalamic releasing/inhibitory factors into capillary plexus in median eminence
• Factors carried by portal circulation to anterior pituitary

Question 9

What is the anterior pituitary made of?

Answer 9

• Formed from primitive mouth
• Made of endocrine cells (hormone storing cells):

Somatotrophs (make growth hormones) , Lactotrophs (make prolactin), Corticotrophs (regulate adrenal cortex & make adrenocorticotropic hormone) , Thyrotrophs (make thyroid stimulating hormone) , Gonadotrophs (make gonadotropins regulate reproduction)

• Regulated by hypothalamic releasing/inhibiting factors via hypophyseal pituitary portal system

Question 10

Describe the hypothalamo-pituitary portal system:

Answer 10

1) Axon terminals of hypothalamic neurosecretory cells release hormones (RHs & IHs) into hypothalmao-pituitary portal system
2) RHs & IHs travel in portal system to anterior pituitary
3) RHs & IHs stimulate or inhibit release of hormones from anterior pituitary cells
4) Anterior pituitary hormones leave gland via the blood
5) Blood vessels constitute the hypothalama-pituitary portal system

Question 11

What are the anterior pituitary cells & their hormones?

Answer 11

SOMATOTROPHS: Growth hormone (somatotrophin)

LACTOTROPHS: Prolactin

THYROTROPHS: Thyroid stimulating hormone (TSH) (Thyrotrophin)

GONADOTROPHS: Luteinising hormone (LH) , Follicle stimulating hormone (FSH)

CORTICOTROPHS: Adrenocorticotrophic hormone (ACTH, corticotrophin)

Question 12

Which anterior pituitary hormones are RELEASING or INHIBITING?

Answer 12

Growth hormone : Growth hormone releasing hormone (releasing) , Somatostatin (inhibiting)

Prolactin: Dopamine (inhibiting- lots of dopamine, less prolactin)

Thyroid stimulating hormone : Thyrotrophin releasing hormone (releasing)

LH & FH: Gonadotrophin releasing hormone (releasing)

Adrenocorticotrophic hormone: Corticotrophin-releasing hormone (releasing)

Question 13

What are the main targets of the anterior pituitary hormones?

Answer 13

Growth hormone: General body tissue (mainly liver)

Prolactin : Breasts (lactating women)

Thyroid stimulating hormone: Thyroid

Gonadotrophins(LH & FSH): Testes males, Ovaries females

Adrenocorticotrophic hormone: Adrenal cortex

Question 14

What happens when there is a pituitary tumour?

Answer 14

• Normally benign
• Compresses optic chiasm
• Causes bitemporal hemianopia (can’t see temple side of eyes)

Question 15

Why is bitemporal hemianopia caused?

Answer 15

• Fibres from nasal retinae cross at optic chiasm
• Compression of optic chiasm by pituitary tumour prevents transmission of sensory info from LATERAL (TEMPORAL) VISUAL FIELDS to occipital lobe

Question 16

What are the mechanisms of growth hormone action?

Answer 16

DIRECT:
Hypohalamus -> growth hormone releasing hormone// Somatostatin -> hypothalamo pituiatry portal system -> anterior pituitary somatotrophs -> release growth hormone// stop growth hormone

INDIRECT:

• Growth hormone tells liver to make INSULIN LIKE GROWTH FACTOR (IGF-1 & IGF-2)
• IGF-1 binds to own receptors in muscle and bone and causes growth and development

Question 17

What happens in people whos anterior pituitary (tumour) makes too much growth hormone?

Answer 17

Before puberty ends: Grow tall GIGANTISM

After puberty: Growth plates fused so do not grow taller, growth of soft tissue instead (hands, jaws etc) - ACROMEGALY

Question 18

What is acromegaly?

Answer 18

• Pituitary tumour in adults making too much growth hormone
• Doesn’t make taller, soft tissue bigger (jaw, hands, tongue, feet)
• Can get sweatiness , headaches, coarsening of facial features

Question 19

What are the posterior pituitary hormones?

Answer 19

• Arginine vasopressin (AVP) aka Anti-diuretic hormone
• Oxytocin

Question 20

Describe hypothalamic magnocellular neurons:

Answer 20

• Long, orientate in supraoptic (AVP) anf paraventricular (oxytocin) hypothalamic nuclei
• Nuclei -> stalk -> posterior pituitary
• Posterior pituitary does not make hormones itself, stores AVP and oxytocin produced in hypothalamus

Question 21

Describe the regulation of the posterior pituitary gland:

Answer 21

1) 2 sets of hypothalamic neurosecretory cells produce AVP & oxytocin and transport them to posterior pituitary
2) Excitation of these hypothalamic magnocellar neurons stimulates release of AVP or oxytocin into posterior pituitary where they diffuse into blood capillaries
3) Leave posterior pituitary via blood

Question 22

What is the physiological action of vasopressin?

Answer 22

AKA = Anti- Duretic hormone Diuresis = production of urine

• Main role= stimulation of water reabsorption in renal collecting duct
• This concentrates urine via V2 receptor in kidney
• Also a vasoconstrictor (via V1 receptor)
• Stimulates ACTH release from anteripr pituitary

Question 23

How does vasopressin concentrate urine?

Answer 23

• AVP binds to V2 receptor on basolateral membrane & stimulates intracellular signalling cascade
• Results in insertion of aquaporin-2 into apical membrane of collecting duct
• Aquaporins are water channels that allow transport of water across cell along conc gradient from urine across collecting duct cell into circulation exits via aquaporin-3 water channel
• AVP triggered reabsorption of water from urine to circulation hence concentrated the urine

Question 24

What are the physiological actions of oxytocin?

Answer 24

• Makes uterus contract at time of delivery (contraction of myometrial cells)
• Breast during lactation -> Myoepithelial cells -> Contraction -> Milk ejection

Question 25

What is the role of the anterior pituitary in lactation?

Answer 25

1) Mechanical stimulation of nipple & surrounding area activates afferent pathways
2) Afferent signals integrated in 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

MILK SECRETION

Question 26

What is the role of the posterior pituitary gland in lactation?

Answer 26

1) Mechanical stimulation of nipple & surrounding area activates afferent pathways
2) Afferent signals integrated in the hypothalamus and stimulate oxytocine-releasing neuron activity
3) Action potentials travel down oxytocine neurons and oxytocin is secreted into the bloodstream
4) Increased plasma oxytocine increases milk ejection in mammary gland