The Pituitary Gland Flashcards
What is a hormone?
A messenger
Carried in the blood from the organ where they are produced to their effector organ
What are the two classes of hormones?
Peptide
Steroid
What are some properties of peptide hormones? (Synthesis, storage, receptors)
Synthesised as pro hormones that require further processing (cleavage) to activate
Stored in vesicles (regulatory secretion)
Bond to receptors on cell membrane and transducer signals using second messenger systems
What are some properties of steroid hormones? (Synthesis, storage, receptors)
Synthesised from cholesterol
Released immediately (constitutive secretion)
Bind to intracellular receptors to change gene expression directly
Where is the pituitary found?
Beneath the optic chiasm
Sits in the sella turcica of the sphenoid bone
Hangs from the pituitary stalk below the hypothalamus
It is roughly the size of a thumb nail
Split into the anterior and posterior pituitary
Which cells regulate the anterior pituitary function? And what are their properties?
Hypothalamic parvocellular neurones
Short, terminate on median eminence
Release hypothalamic releasing/inhibitory factors (hormones) into capillary plexus in median eminence
These regulatory factors are carried in the blood to the anterior pituitary by the hypothalamo-pituitary portal system
Which cells make up the anterior pituitary (adenohypophysis)?
Endocrine cells:
Somatotrophs
Lactotrophs
Corticotrophs
Thyrotrophs
Gonadotrophs
The anterior pituitary is anatomically different from the hypothalamus
How does the hypophyseal-pituitary portal system work?
- Axon terminals of hypothalamic neurosecretory cells release hormones (releasing and inhibitory hormones) into the portal system
- The RHs and IHs travel in the portal system to the anterior pituitary
- In the pituitary the RHs and IHs stimulate or inhibit the release of hormones from anterior pituitary cells
- Anterior pituitary hormones leave the gland via the blood
The portal system contains fenestrated blood vessels making them leaky so the regulatory hormones can diffuse out
Eg. Thyrotrophin releasing hormone (TRH) released from hypothalamus. Travels through portal system to anterior pituitary. TRH stimulates the release of TSH (theo is stimulating hormone) from thyrotrophs. TSH leaves the pituitary to go to the thyroid gland to stimulate the release of thyroxine
What are the 5 anterior pituitary cells and what hormone do they release?
Somatotrophs - growth hormone (somatotropin)
Lactotrophs - prolactin
Thyrotrophs - thyroid stimulating hormone
Gonadotrophs - LH and FSH
Corticotrophs - adrenocorticotrophic hormone (ACTH/ corticotropin)
What are the regulatory hormones of somatotrophs?
Growth hormone releasing hormone - releasing
Somatostatin - inhibitory
Relates to growth hormone
What is the regulatory hormone of lactotrophs?
Dopamine - inhibitory
Relates to prolactin
What is the regulatory hormone of thyrotrophs?
Thyrotropin releasing hormone (TRH) - releasing
Relates to TSH
What is the regulatory hormone of gonadotrophs?
Gonadotropin releasing hormone - releasing
Relates to LH and FSH
What is the regulatory hormone of corticotrophs?
Corticotropin releasing hormone - releasing
Relates to adrenocorticotropic hormone (ACTH)
What are the effector organs of each cell in the anterior pituitary?
Somatotrophs - growth hormone - the liver and all body tissues
Lactotrophs - prolactin - breasts of lactating women
Thyrotrophs - TSH - thyroid
Gonadotrophs - LH and FSH - gonads (ovaries and testes)
Corticotrophs - ACTH - adrenal cortex
How might a pituitary tumour affect the visual field?
Bitemporal hemianopia
Loss of the peripheral vision in both eyes
Due to compression of the optic chiasm above the pituitary
Fibres from the medial retinae (which have crossed from the lateral visual field) cross at the optic chiasm
The tumour prevents transmission of sensory information from the lateral visual field to the occipital lobe
What is the reflex arc of anterior pituitary lactotrophs?
- Mechanical stimulation of nipple
- Afferent signals integrated in the hypothalamus and inhibit dopamine release from dopaminergic neurones
- Less dopamine in the hypothalamo-pituitary portal system causes less inhibition of anterior pituitary lactotrophs
- Increased plasma prolactin increases milk SECRETION in mammary glands
What are the mechanisms of growth hormone action?
Growth hormone releasing hormone released from hypothalamus
Goes via hypothalaomo-pituitary portal to the anterior pituitary somatotrophs
Stimulates the release of growth hormone into the blood
GH goes to general body tissues and causes growth and development
GH also goes to liver and binds to GH receptors, stimulating the release of IGF-1 (insulin like growth factor 1)
IGF-1 binds to IGF-1 receptors on bone and muscle etc and causes growth
What is the difference between gigantism and acromegaly?
Both may be caused by excess levels of growth hormone that could be caused by a pituitary tumour
Gigantism causes longer limbs and happens before/during puberty
Acromegaly does not result in increased height as it occurs after puberty. After the epiphyseal growth plates have fused.
What are some symptoms of acromegaly?
Macroglossia
Prominent nose
Large jaw (prognathism)
Increased hand and feet size
Sweatiness
Headache
What are the posterior pituitary hormones?
Arginine vasopressin (AVP) (anti diuretic hormone)
Oxytocin
Which cells are found in the posterior pituitary?
Magnocellular neurones
These are long, originate in supraoptic (AVP) and paraventricular (oxytocin) hypothalamic nuclei
Nuclei —> stalk —> posterior pituitary
What is the anatomical difference between the anterior and posterior pituitary?
Anterior is not confine with the hypothalamus, it is vascularised in the form of the portal
The posterior IS continuous with the hypothalamus as it’s is made completely of neuronal tissue and has no blood supply
How does regulation of the posterior pituitary gland work?
- Two sets of hypothalamic neurosecretory cells produce AVP and oxytocin and transport them to the posterior pituitary
- Excitation of these hypothalamic magnocellular neurones stimulates release of AVP and oxytocin into the PP where they diffuse into the blood
- They then leave the pituitary via the blood
What is the physiological action of arginine vasopressin?
Aka ADH
Duiresis = production of urine
Stimulates water reapsorbtion in the collecting ducts of the kidney
This concentrates urine
Acts through the V2 receptor in the kidney
Also a vasoconstrictor via the V1 receptor
Also stimulates adrenocorticotrophic hormone release from anterior pituitary
How does vasopressin concentrate urine?
AVP binds to V2 receptors on basolateral section of collecting duct
Acts through a G protein coupled receptor
Moves aquaporin-2 vesicles to the apical membrane of the collecting duck where the aquaporins allow the diffusion of water back through the wall of the collections duct
Water goes into the blood plasma through aquaporin-4 channels
What are the physiological actions of oxytocin
- Uterus at parturition (labour)
Myometrial (muscle) cells
Contraction
Delivery of baby
(Oxytocin analogues can be used to help progress labour)
- Breast during lactation
Myoepithelial cells
Contraction
Milk ejection
What is the reflex arc or oxytocin relating to milk ejection?
- Mechanical stimulation of nipple
- Afferent signals integrated in the hypothalamus and stimulate oxytocin releasing neurone activity
- Action potentials travel down oxytocin neurones and oxytocin is secreted into the blood stream
- Increased plasma oxytocin increases milk ejection in mammary glands
What is the difference between prolactin and oxytocin in terms of lactation?
Prolactin = produces milk
Oxytocin = ejection
