Pituitary Gland

Question 1

What is a hormone?

What are the 2 main groups of hormones?

What are their properties - synthesis, storage and receptors?

Answer 1

A messenger that is carried by the bloodstream to effect an action elsewhere in the body

Peptide - synthesised as prohormones (inactive), requires further processing to make them active i.e. chopped up and cleaved by various enzymes to make them active; stored in vesicles and only released in response to a signal to bind to receptors on the cell membrane; and activate a secondary messenger system to induce a response

Steroid - synthesised from cholestrol (precursor), released immediately (as soon as they are made), bind to intracellular receptors to change gene expression indirectly

Question 2

What is the pituitary gland?

Where is it located?

What is the name of the bony dish on which the pituitary gland sits on?

Label the diagram:

Which important structure is located near the posterior pituitary gland?

Which important structure is located near the anterior pituitary gland and why is this important clinically?

Answer 2

Master gland

Sits at the base of the brain - hangs from the stalk, known as the infundibulum, and has an anterior and posterior portion - posterior portion contunous with the hypothalamus

Sella turcica of sphenoid bone (literally means turkish saddle)

Hypothalamus

Optic chiasm - partial crossing of nerve fibres that come in from the eyes to the back of the brain; important if there is a tumour located in that region (i.e. pituitary gland tumour)

Question 3

What are hypothalamic parvocellular neurons and from where to where do they travel?

Label the diagram:

What is rich in blood supply?

Why is the hypothalamus important? What is important in regulating pituitary function?

Answer 3

Parvo = short, population of neurons from the hypothalamus, short neurons that project from the hypothalamus to the median eminence (part of the hypothalamus immediately adjacent to the pituitary stalk)

The median eminence

Pituitary cannot function without it - the parvocellular neurons releases factors (inhibitory or excitatory) into the median eminence, from which they are carried via the bloodstream (portal system) to the pituitary gland to influence its function

Question 4

What alternative name is given to the anterior lobe of the pituitary gland?

Why is the anterior pituitary anatomically different from the hypothalamus?

What are the 5 main subtypes of cells in the anterior pituitary and what type of hormone do they produce / secrete?

What structure regulates their release?

Answer 4

Adenohypophysis

AP = glandular tissue, and embryologically originates from elsewhere to the hypothalamus, H = contains neuronal tissue too

Somatotroph - growth hormones (somato = growth)

Lactotroph - prolactin

Corticotroph - ACTH (adrenocoreticotrophin)

Thyrotroph - TSH (thyroid stimulating hormone)

Gonadotroph - FSH / LH (follicle-stimulating hormone / luteinizing hormone)

Hypothalamus

Question 5

How is hypothalamic regulation achieved? What is the blood vessel system called?

How do the parvocellular neurons regulate the anterior pituitary?

How do the factors get into the blood vessel system from the median eminence?

Answer 5

Via the hypothalamo-pituitary portal system (portal as they take blood from one part ot another part)

Stimulate the anterior pituitary to release specific hormonses - the neurons release factors (e.g. inhibitory or stimulatory) into the median eminence to get into the blood vessels, which can transport them down the infundibulum to the anterior pituitary - binds to relevant cells to cause release of the relevant hormones

The portal system contains fenestrations - making the walls leaky so the factors can leak in and travel down into the anterior pituitary

Question 6

What is the factor associated with the thyroid gland?

What is the sequence of events that allows it to cause the anterior pituitary to release TSH (thyrostimulating hormone)?

Answer 6

TRH - thyrotrophin releasing hormone) - released from hypothalamic parvovellular neurons into the portal circulation system

TRH travels via the portal system to the anterior pituitary, finds the thyrotroph cells to stimulate their hormone release, thyrotrophs contain thyroid stimulating hormones (TSH), TSH travels to thyroid gland, causing it to release thyroxine

Question 7

What is the general sequence for the hypothalamus stimulating the pituitary to release hormones to stimulate other glands to release further hormones?

Answer 7

Hypothalamus, factor travels along portal system, stimulates specific cells in the pituitary, causes release of stimulating hormone, end result on another gland for it to release another set of hormones to cause action

Question 8

What are the 5 anterior pituitary cell types and the hormones they produce / release, fill in the table:

Answer 8

Question 9

What is the name of the factor / hormone released by the hypothalamus to regulate anterior pituitary gland function? Fill in the table:

Answer 9

Orange = on / causes release; blue = off / inhibits

1. GHRH (growth hormone releasing hormone)

Off switch = somatostatin (stop growth)

2. Dopamine - prolactin is released by having less inhibition, so reduced dopamine from the yhpothalamus results in reduced inhibition of prolactin release from the pituitary
3. TRH (thyrotrophin releasing hormone)
4. GRH (gonadotrophin releasing hormone)
5. CRH (corticotrophin releasing hormone)

Question 10

What are the target cells / target receptors of the main 5 hormones released from the anterior pituitary gland? Fill in the table:

Answer 10

1. General body tissues, particularly in the liver
2. Mammary glands
3. Thyroid gland (located in the neck)
4. Ovaries / Testes
5. Adrenal gland - adrenal cortex

Question 11

Which important structure could a pituitary tumour compress and why?

What issues arise from this?

How is this tested?

Answer 11

Optic chiasm, because small area for the pituitary gland to sit in

Compromising vision - loss of peripheral vision

Perimetry testing - visual field test where the patient sits still and lights are flashed in different areas in front of them. Everytime they see a light flash, they must press a button. If a patient is suffering from a loss of peripheral vision, then they would fail to press the button when lights are flashed in the temporal parts of their visual field

Question 12

What is bitemporal hemianopia and why does it arise?

What is found in the optic chiasm?

How might a patient present with bitemporal hemianopia?

Answer 12

Loss in peripheral vision - often from compression of the optic chiasm due to a pituitary tumour. Often called a pituitary adenoma, macro = bigger than 10mm, micro = smaller than 10 mm

Nerve fibres from the nasal (medial) retina, which receive informational from the temporal visual field, cross at the optic chiasm

Patient may say they have lately been bumping ot things, struggling to drive etc.

Question 13

What is the neuroendocrine reflex arc to cause milk production?

Answer 13

1. Infant latches onto mother’s breast, stimulating many sensory and touch receptors, which travels to the dopaminergic neurons, inhibiting their function
2. Results in less dopamine released from the neurons, hence more prolactin can be produced by the anterior pituitary (prolactin production is suppressed by dopamine)
3. Increased prolactin production results in increased milk secretion in the mammary glands

Question 14

Where is growth hormone (somatotrophin) released from and where does it act in the body?

What are the 2 mechanisms of growth hormone action? And what are the 2 types of IGFs and what are they responsible for?

Answer 14

Released from the anterior pituitary gland by somatotrophs. Acts on all the bodily tissues e.g. muscle, bone etc. Also acts on the liver, as the liver produces an intermediate molecule, a hormone called: insulin-like growth factor

1. Directly binds to growth hormone receptors - e.g. on the skeleton
2. Causes IGF release from liver: IGF-1 is produced in adults and also causes growth, IGF-2 is produced mainly in foetuses and also causes growth

Question 15

What is the difference between gigantism and acromegaly?

Growth Hormone = GH

Answer 15

Gigantism - too much GH during childhood, causes the individual to grow taller as the growth plates have not yet fused

Acromegaly - too much GH during adulthood, leads to enlargement of features e.g. hands, feet, facial features etc. Growth plates have fused, so cannot grow taller

Question 16

What are the symptoms of acromegaly?

How might these present clinically / what to ask to notice them clinically?

Answer 16

Coarsening of facial features - macroglossia (big tongue), prominent nose

Prognathism - large jaw

Increased hand an feet size

Sweatiness

Headaches

Have you noticed enlargements of your hands, feet or facial features? - e.g. Do your rings still fit? Has your shoe size gone up?

Are you sweating more than normal?

Any headaches?

Have you experienced poor quality of sleep, i.e. waking up gasping for air (obstructive sleep apnea)?

Have you noticed any coarsening of your features - compare old photo to now

Carpal tunnel syndrome - e.g. median nerve runs through that, can be compressed due to swelling of the carpal tunnel tissues (found in the wrist of the hands), causing numbness / tingling in the first 3 and a half fingers of their hands

IGF-1 produced by the liver in response to the growth hormone - can cause increased insulin resistance (i.e. diabetes), hyperglycemia, e.g. do you drink / pee a lot? and hypertension

Polydipsia - excessive drinking

Polyuria - excessive peeing

Prognathism - bulging of lower jaw (mandible)

hypopituitarism - swelling of the gland can squish other cells in the pituitary gland causing irregularities in other hormone release e.g. hypothyroidism

IGF1 - causes metabolic effects

Question 17

Clinically, to diagnose acromegaly, why are GH levels not measured?

How else can a diagnosis of acromegaly be made?

Answer 17

GH levels fluctuate throughout the day, and there is overlap in normal region and acromegaly region, so there is not a distinction between positive and negative results

To try and suppress GH activity in the patient - if the methods do not lower GH levels = acromegaly

Question 18

What is another word for the posterior pituitary?

What are the 2 hormones released from the posterior pituitary glands?

What is the posterior pituitary continuous from?

What nerve fibres connect / travel from the hypothalamus to the pituitary gland?

Answer 18

Neurohypophysis

1. AVP - arginine vasopressin (AKA ADH - anti-diuretic hormone)
2. Oxytocin

Hypothalamus

Magnocellular neurons - much longer and bigger cell bodies, travelling from the nuclei in the hypothalamus, down the infundulum to the posterior pituitary

Question 19

What type of tissue is the posterior pituitary also made up of?

How does the hypothalamus and posterior pituitary work together to produce / release AVP / ADH?

What is the physiological action of AVP / ADH?

Answer 19

Neural tissue as it is continuous from the hypothalamus

2 distinct populations of the magnocellular neurons originate from the hypothalamus, one that produces AVP and the other, oxytocin. These are transported into the pituitary gland. Stimulation of the megnocellular neurons causes the release of these 2 hormones from th epituitary gland into the bloodstream, from which they diffuse into the blood to access their target organs

Main function: Stimulates H2O reabsorption in the renal collection duct. Othe functions: vasoconstricts via the V1 receptor and stimulates ACTH release from the anterior pituitary

Question 20

How does AVP concentrate urine?

Answer 20

1. AVP travels through the bloodstream, and when it reaches the collecting duct cell, it binds to V2 receptors in the collecting duct
2. Stimulates an intracellular cascade (series of reactions), that eventually results in the movement of aquaporins (water channels) so they can be inserted into the cell membrane of the collecting duct cells
3. Water in the urine can flow along a concentration gradient through the aquaporins (water channels) into the blood

Question 21

What are the physiological actions of oxytocin? (2 main functions)

Answer 21

1. Contraction of uterus - childbirth
2. Lactation - involved in ejection of milk from the breast (prolactin is involved in the production of the milk)

Question 22

What is the neuroendocrine reflex arc for milk ejection via oxytocin?

Answer 22

1. Latching of the baby onto the brain (mechanical stimulation) activates afferent pathways to the hypothalamus
2. This stimulates oxytocin neurons resulting in a release of oxytocin from the posterior pituitary into the bloodstream (circulation)
3. Increased plasma oxytocin results in ejection of milk from the mammary gland

Question 23

Describe this diagram to summarise this lecture:

Answer 23

Anterior pituitary - releases 5 main hormones :

GH - 2 mechanisms, either directly or via the liver (IGF-1); Prolactin = milk production, gonadotrophins = LH / FSH, which stimulates the gonads = ovaries and testes; TSH stimulates thyroid gland for it to produce thyroid hormones, ACTH stimulates the adrenal cortex to produce cortisol

All of these regulated by factors from the hypothalamus that travels down the portal system

Posterior pituitary - stimulation of the long, hyhpothalamic neurons (magnocellular neurons) = release of AVP (reabsorption of water, and also stimulates cortisol release) or oxytocin (milk production and childbirth)

Question 24

Fill in this table about the chain of command for the release of these consequent hormones:

Answer 24

1. GHRH - growth hormone releasing hormone, somatostatin
2. Dopamine
3. TRH - thyrotrophin releasing hormone
4. GRH - ganodotrophin releasing hormone
5. CRH - corticotrophin releasing hormone, AVP (slightly)