3 The Hypothalamo-Adenohypophysial Axis Flashcards
Q: Describe the anatomical nature of the pituitary gland. What is it attached to? What does it lie within and why is this structure clinically important? How do pituitary tumours form?
A: The pituitary is attached to the base of the brain
It lies within a bone lined cavity - Sella Turcica
Sella Turcica is important from a clinical point of view because a tumour will be constrained by the walls of the bony cavity
Pituitary tumours may protrude out towards the brain or it may go through the bone (if it is really malignant)
Q: Draw a basic diagram of the pituitary and its surrounding structures. How do you determine the orientation? (2)
A: brain
3rd ventricle (surrounded by hypothalamus)
hypothalamus
optic chiasm mammillary body
anterior lobe (adenohypophysis) // posterior lobe (neurohypophysis)
posterior gland (hypophysis)
There are TWO areas of the brain which are good for distinguishing front from back:
Front - Optic Chiasma
Back - Mammillary Body
Q: Why is the pituitary gland known to be the master gland?
A: regulates other glands
Q: What is the optic chiasm?
A: group of nerves for eyes (close relationship to pituitary)
Q: What are hypothalamic neurones?
A: groups of cell neurones in hypothalamus, cell bodies in hypothalamus and axon going down
have different effects
Q: Pituitary gland formation. At what point does development start? What are the 2 types of tissue that make it up? Describe them.
A: First few weeks after conception:
Glandular Tissue - upward growth from oral ectoderm of Rathke’s pouch (oral cavity)
Nervous Tissue - there is a downwards movement of tissue from the developing hypothalamus
These two tissues then fuse and normally lose contact with the rest of the buccal cavity.
So one part of the pituitary is derived from glandular tissue (adenohypophysis) and the other part is derived from neural tissue (neurohypophysis)
Q: How does the 3rd ventricle relate to the hypothalamus?
A: The hypothalamus surrounds one of the ventricles of the brain - 3rd Ventricle
Q: What are the names of the two parts of the pituitary based on?
A: embryological origin of the tissue
Q: Illustrate the link between the hypothalamus and the pituitary gland?
A: In the hypothalamus, lots of bundles of neurones can be grouped functionally into HYPOTHALAMIC NUCLEI (2)
Median Eminence = an area that lies between the top of the pituitary stalk and the hypothalamus
The neurohypophysis- posterior pituitary (mainly made up of nerve axons)
The cell bodies of these nerve axons lie in the hypothalamus
Some of the axons coming from the hypothalamic nuclei terminate in the median eminence and some within the posterior pituitary
on left = anterior pituitary
Q: What is the median eminence?
A: where the hypothalamus and pituitary meet
Q: What is the pars distalis?
A: bulk of glands in the anterior pituitary
Q: What is the pars tuberalis?
A: part of the anterior lobe (growth) of the pituitary gland, and wraps the pituitary stalk in a highly vascularized sheath
Q: Illustrate the hypothalamic-hypophysial portal circulation (pituitary blood supply).
A: The median eminence is essentially a MASS OF CAPILLARIES receiving blood form the superior hypophysial artery
Lots of neurones coming from the hypothalamic nuclei terminate on the walls of the PRIMARY CAPILLARY PLEXUS (fenestrated)
The primary capillary plexus feeds blood down into long portal vessels which run down through the pituitary stalk to terminate within the adenohypophysis - form secondary capillary plexus
From here the blood is gathered into the Cavernous sinus (vein) and out through the Jugular veins
Primary Capillary Plexus - in the median eminence
Secondary Capillary Plexus - in the anterior pituitary
Q: What does being fenestrated mean?
A: having perforations- makes it leaky
Q: Where does the median eminence lie in terms of the blood-brain barrier?
A: outside
Q: Illustrate the hypothalamo-adenohypophysial axis. (5)
A: hypothalamic neurone with short axon that ends in median eminence releases hormone out of axon (not neurotransmitter) ->
when it comes out of axon is diffuses freely into capillary (hypothalamic neurosecretion- hormone is released into hypothalamo-hypophysial portal system)
said target cells are packed full of hormone containing vesicles ->
release of adenohypophysial hormone into general circulation ->
elicit response
Q: What is an important link between the brain and the endocrine system?
A: the hypothalamus is a source of release hormones and release inhibiting hormones
In this case, the action of these hormones controls the anterior pituitary
Q: Draw a simple flow diagram for the hypothalamo-adenohypophysial axis. (5)
A: hypothalamic nuclei (clusters of cell bodies of neurones)
neurones to median eminence (short axon)
neurosecretions (hypothalamic releasing/inhibiting hormones)
adenohypophysis
adenohypophysial hormones
Q: What are the 5 types of adenohypophysial cells and what do they produce?
A: somatotroph- somatotrophin = growth hormone
lactotroph- prolactin
thyrotroph- thyroid stimulating hormone (TSH, thyrotrophin)
gonadotroph- lutenising hormone, follicle stimulating hormone (LH, FSH)
corticotroph- adrenocorticotrophic hormone (ACTH, corticotrophin)
other cells of undefined structure are present too
Q: What process requires prolactin?
A: lactation (breastfeeding)
Q: What structure are FSH and LH important for?
A: male and female gonads
Q: What does ACTH do?
A: signals to adrenals (cortex) to make hormone-cortisol
Q: What are the precursor molecules for adenohypophysial hormones?
A: prohormone (enzymatic cleavage leads to bioactive hormone molecule)
Q: Where are adenohypophysial hormones stored?
A: cytoplasmic secretory granules (released via exocytosis, not lipid soluble- are proteins)
Q: What is the precursor for ACTH? How does this molecule breakdown (equation)?
A: POMC ProOpioMelanoCorticotrophin
POMC -> corticotrophin (ACTH) + Pro-gammaMSH + betaLPH
Q: How many types of adenohypophysial hormones are there? What are they? Provide examples. (5)
A: 3
proteins (are large)- somatotrophin; 191aa (growth hormone) and prolactin; 199aa
glycoproteins (consist of alpha and beta subunits where 92 aa alpha subunit is common to all)- TSH; beta subunit= 110aa and 2 gonadotrophins; LH, FSH; both have 115aa beta subunit
polypeptides (are small)- ACTH; 39aa
Q: Name 7 hypothalamic hormones. Which ones inhibit/stimulate hormone release?
A: -growth hormone releasing hormone (GHRH/somatotrophin releasing hormone)
-somatostatin (SS) INHIBIT
- dopamine (DA) INHIBIT (part of system where regulation is predominantly inhibition)
- thyrotrophin releasing hormone (TRH) MINOR PLAYER
-gonadotrophin releasing hormone (GnRH)
- corticotrophin/ACTH releasing hormone (CRH) MAIN REGULATOR
- vasopressin
Q: Name the 6 adenohypophysial hormones. What are their main target tissues?
A: growth hormone (somatotrophin) = general body tissues, particularly liver (metabolic effects)
prolactin = breasts in lactating women
thyroid stimulating hormone (TSH/thyrotrophin) = thyroid
luteinising hormone (LH) and follicle stimulating hormone (FSH)- both are gonadotrophins = testes (men), ovaries (women)
ACTH (cortocotrophin) = adrenal cortex (outer area)
Q: When are growth hormones particularly important?
A: childhood, role is less clear in adults
Q: Describe the relationship between the hypothalamic hormones and the adenohypophysial hormones. (9)
A: hypo ————-role————> adeno
GHRH ———stimulates———>
SS ————-inhibits————–> somatotrophin/GH
DA ————-inhibits————–>
TRH ———-stimulates———–> prolactin (part of system where regulation is predominantly inhibition- breastfeeding occurs at specific times)
TRH ———-stimulates———–> TSH
GnRH ———-stimulates———> LH, FSH
CRH ———-stimulates———–>
VP ————-inhibits—————> ACTH
Q: How do somatotrophins act (diagram)? Explain
A: adenohypophysis
|
\ /
growth hormone (somatotrophin)
| \
\ / -> body tissues (metabolic actions)
liver ^ |
| / |
\ / / |
somatomedians (IGF I and IGF II) (produced by liver) |
/
growth and development
Q: What is IGF I? (3)
A: mediator, particularly important for growth
insulin like growth factor
binds to IGF I receptor -> affects growth
Q: Describe the 2 ways in which somatotrophins act?
A: Direct Effect - binding to somatotrophin receptors in general cells of the body
Hepatocytes - stimulating hepatocytes to produce IGF I
Both these pathways mean that somatotrophin controls large aspects of metabolism
Q: What effect do growth hormones have in excess?
A: negative metabolic effects
Q: What are the metabolic growth actions of growth hormones via IGF I? (4)
A: (both direct and indirect)
Stimulation of amino acids transport into cell (eg. muscle) and protein synthesis
increased gluconeogenesis (too much glucose = predispose you to diabetes mellitus)
Stimulation of lipolysis leading to increased fatty acid production (lots= adverse lipid profiles= bad for blood vessels)
Increased cartilaginous growth and somatic cell growth
Q: How do SS and GHRH act in relation to GH? Diagram including negative feedback.
A: 2 different hypothalamic neurones with short axons release SS and GRH which inhibit and promote production of GH (somatotrophin)-> into systemic circulation -> liver -> somatomedians (mainly IGF I)
negative feeback loops from somatomedians to pituitary (direct) and hypothalamus (indirect) separately as well as GH to hypothalamus
Q: What stimulates somatotrophin production? (7) How? (2)
A: sleep (stages III and IV)
stress (eg systemic illness)
oestrogen
exercise
fasting (hypoglycaemia)
amino acids (eg arginine)
ghrelin (from stomach)
and except ghrelin affect hypothalamus while ghrelin acts on anterior pituitary
Q: How does prolactin act? Diagram. (5)
A: Control is brought about by the baby suckling on the breast
There are tactile receptors on the nipple which are associated with an afferent nerve pathway - this goes back to the hypothalamus
((Two hypothalamic hormones controlling prolactin release:
- Dopamine
- Thyrotrophin Releasing Hormone))
When the afferent neural pathway is stimulated, dopaminergic neurones are inhibited and there is direct stimulation of thyrotrophin releasing hormone
- this leads to release of prolactin (efferent endocrine pathway)
Prolactin starts the synthesis of milk for the next suckling period
