INTS 11: The Hypothalamic-Pituitary Axes Flashcards
Read and understand the learning objectives of this session
Define endocrinology
- it is the study of endocrine glands and their hormonal products
- these are released directly into the bloodstream
What are the body’s two principal communication systems?
- the endocrine glands
- the nervous system
Define endocrine gland
- a group of cells which secrete ‘messenger’ molecules (hormones) directly into the bloodstream
Define hormone
- the bioactive ‘messenger’ molecule secreted by an endocrine gland into the blood
- i.e. not simply a metabolite or energy substrate
Define the endocrine system
- the endocrine system is comprised of the endocrine glands and their hormonal products
- it is often also used to describe the cells which respond to these hormones
Give examples of some of the endocrine organs
Where are endocrine cells found?
- they are often grouped together in particular endocrine organs
- e.g. thyroid gland or pituitary gland
- some cells can also be found in other tissues that are not primarily endocrine tissues
Discuss the properties of endocrine cells
What about protein hormone-secreting cells?
- endocrine cells are highly metabolically active
- therefore, they typically have many mitochondria
- they have a rich blood supply to allow their secreted products to be effectively transported into the blood stream
- often, secretory granules will be positioned in a cell adjacent to a nearby capillary
- protein hormone-secreting cells will have a lot of RER for protein sysnthesis
- will also contain secretory granules where protein hormones are stored
What are neurosecretory cells?
Briefly descirbe their function and morphology
- a subset of endocrine cells
- they are neurons which release hormones
- so they will have typical neuronal morphology
- hormones are stored in granules at the axonal terminals
- these terminals release their contents into capillaries, allowing the hormone to enter the circulation
Describe this micrograph
What are the three ways hormones can act?
- endocrine:
- a hormone acting on target cells at a distnace from source
- via the bloodsteam
- paracrine:
- a hormone acting on nearby target cells
- i.e. within immediate area around source
- autocrine:
- a hormone acting on its own immediate source
- i.e. on the cell that released it
What are the three classes of hormones?
- protein or peptide hormones
- steroid hormones
- amino acid derived hormones
Describe the synthesis of protein hormones, their properties and hence their function
- synthesised by the same pathways that synthesise other proteins within the cell
- they are subsequently packaged into secretory granules where they are stored until required
- they are typically easy to dissolve in water, so can circulate in the bloodstream without the need for binding proteins
- they do not easily cross cell membranes and act on cell surface receptors instrad
- the conformation of the receptors are altered when they bind a hormone
- this stimulates intracellular signal pathways which then drive the effects of the hormone within the cell
Describe the synthesis of steroid hormones, their properties and hence their function
- synthesised from cholesterol in the mitochondria when required
- usually not stored within the cell
- they are hydrophobic
- so require binding proteins to carry them through the circulation
- due to their hydrophobicity, they can cross cell membranes relatively easily
- then they bind to and activate intracellular or nuclear receptors
- receptor-steroid hormone complexes can then relocate to the nucleus and influence gene transcription
Describe the synthesis of amino acid derived hormones, their properties and hence their function
- they are a more diverse group
- some display properties like steroid, other more like protein hormones
Describe the hypothalamus
- Location
- Function
- Structure
Location:
- found at the base of the brain, below the thalamus
Function:
- forms part of the limbic system: involved in processes such as learning, emotion and behaviousr
- regulates many homeostatic processes:
- heart rate, body temperature, metabolic rate and food intake
- received neuronal input from a number of other brain regions, particularly the brainstem
- hypothalamic neurons themseles signal to other brain regions to regulate behavious
- can regulate the autonomic nervous system and the pituitary gland
Structure:
- composed of individual nuclei and some less well define subregions which are integrated, but can regulate different physiological functions
Describe the pituitary gland
- Alternate name
- Location
- What it is
- also known as the hypothysis
- it is an endocrine organ that sits within a bony pocket underneath the hypothalamus
- composed of anterior, intermediate and posterior lobes
Describe the structure of the pituitary gland
- observe image
- the pituitary gland is linked to the hypothalamus by a short stalk that contain nerve fibres and a specialised netowork of blood vessels, known as a portal system
Where did the anterior and posterior lobes of the posterior pituitary gland develop from?
What are their alternative names?
What do their different developmental origins change?
- Anterior lobe:
- developed from the oral ectoderm: one of the primary germ layers in the early embryo
- also called adenohypophysis
- Posterior lobe:
- develops as an extension of the hypothalamus
- neural in origin, so called neurohypophysis
- their different developmental origins underpins the different ways they are regulated and the different cells they contain
What does the anterior pituitary lobe help regulate?
- metabolic rate
- stress response
- growth
- reproduction
What does the posterior pituitary regulate?
- water balance
- uterine function
- breast function
What is neurosecretion?
- the release of hormones from neurons
What are the neurosecretory cells in the hypothalamus classed as?
- parvocellular
- magnocellular
What are hypothalamo-pituitary axes?
Give examples
Is their any difference between posterior and anterior lobes?
- how the hypothalalamus controls the pituitary glands
- e.g. hypothalamo-pituitary-gonadal axis: regulates reproductive function
- e.g. hypothalamo-pituitary adrenal axis: regulates stress response
- the hypothalamus control the function of the anterior and posterior lobes of the pituitary by two different modes of neurosecretion
Describe the posterior pituitary lobe
- Structure: from hypothalamus to itself
- Major hormones
- cell bodies are up in the hypothalamus
- their axons actually stretch all the way down, forming the posterior pituitary
- the hormones made in the cell bodies are trafficked down the axon and are stored in nerve terminals in the posterior pituitary, waiting for their stimuli
- major hormones:
- vasopression
- oxytocin
Describe the anterior pituitary lobe
Structure: from hypothalamus to itself
- cell bodies are in the hypothalamus and their axons extend down to the base of the hypothalamus but don’t go into the pituitary
- this base is the median eminence, which has a capillary plexus (so good blood supply)
- neurocrine neurons can release hypothalamic factors into this blood supply
- there is then a portal circulation between the hypothalamus and the anterior pituitary
- the capillary plexus spreads throughout the anterior pituitary so that anything released baths the anterior pituitary endocrine cells
Describe parovellular neurosecretory cell bodies
- Location
- What hormones do they secrete?
- Where do these hormones go?
- located in the hypothalamus
- the paraventricular nucleus
- their relatively short axons project to the median eminence at the base of the hypothalamus
- the neuronal terminals secrete hypothalamic hormones into the primary capillary plexus of the hypothalamo-pituitary portal circulation
- these hormones are carried by long portal veins to the secondary capillary plexus, which supplies the anterior pituitary
- this allows hypothalamic hormones to modulate anterior pituitary hormone release
Describe magnocellular neurosecretory cell bodies
- Location
- What hormones do they secrete?
- Where do these hormones go?
- located in the hypothalamus
- in the paraventricular and supraoptic nuclei
- their long axons pasd through the median eminence, projecting to and forming (along with support cells) the posterior pituitary
- these neurons secrete oxytocin and vasopression (ADH) into the general circulation in response to stimuli
Observe the schematic diagram illustrating the hypothalamus, posterior pituitary and the cell bodies
- note how hypothalamic parvocellular neurons projedt to the median eminence
- and magnocellular neurons project to the posterior pituitary
Observe the schematic diagram illustrating the hypothalamus, anterior pituitary and the cell bodies
- hypothalamic hormones are released from parvocellular neurons at the primary capillary plexus in the median eminence
- they are carried by the portal veins to the secondary capillary plexus where they can act on anterior pituitary cells to regulaye anterior pituitary hormone secretion
What anterior pituitary hormones are these hypothalamic hormones responsible for?
Use diagram to help you
How is homeostasis maintained in the anterior pituitary?
- hormone release from the anterior pituitart is usually controlled by negative feedback
- the products of various axes acting upon the hypothalamus and pituitary suppress their own release
What is special about the regulation of the female hypothalamo-pituitary-ovarian axis?
- it is an example of positive feedback
- the axis is mainly controlled by negative feedback, but the luteinising hormone surge necessary for ovulation is driven by oetrogen acting at the hypothalamus and pituitary
- this stimulates the release of more gonadotrophin releasing hormone and luteinising hormone
- which further drives the release of more oestrogen
What do most anterior pituitary hormones act on?
Give some examples
- Most hormones from the anterior pituitary are responsible for acting on endocrine glands to stimulate the release of further hormones.
- For example:
- TSH stimulates the release of T3 and T4
- ACTH stimulates the release of glucocorticoids such as cortisol
- LH and FSH stimulate the release of testosterone or oestrogen
- GH stimulates the release of somatomedins
What are the two hormones released by the magnocellular neurons that terminate in the posterior pituitary?
Give their hormone type
Function
- vasopressin:
- The principal physiological action of vasopressin is to stimulate the reabsorption of water in the renal collecting ducts
- Vasopressin thus has an anti-diuretic effect, hence its alternative name of anti-diuretic hormone (ADH)
- Vasopressin release is stimulated when blood electrolyte concentration is high:
- it concentrates urine in order to preserve blood electrolyte concentration within healthy limits.
- Oxytocin:
- acts to drive rhythmic contraction of the uterine muscles during childbirth, and contraction of myoepithelial cells in the breast to cause milk ejection during breastfeeding.
- Oxytocin also acts on the brain, promoting, for example, maternal behaviour and social recognition.
Give an overview of where the hormones released by the anterior and posterior pituitary act on
And what hormones
Describe the function of vasopressin and dysfunction caused by the hypersecretion and hyposecretion of it
- Function:
- released in response to high blood osmolality to drive water reabsorption in the kidney
- Hypersecretion:
- syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- hyponatremia: low sodium concentration in the blood, due to increased blood volume
- Hyposecretion:
- diabetes insipidus: producing large volumes of unconcentrated urine, driving thirst and possibly dehydration
Describe the function and dysfunction of glucocorticoids, such as cortisol, caused by the hypersecretion and hyposecretion of it
Describe the function and dysfunction of the thyroid hormones, T3 and T4, caused by the hypersecretion and hyposecretion of it
Describe the function and dysfunction of androgens, such as testosterone, caused by the hypersecretion and hyposecretion of it
Describe the function and dysfunction of oestrogens, such as oestradiol, caused by the hypersecretion and hyposecretion of it
Describe the function and dysfunction of growth hormone caused by the hypersecretion and hyposecretion of it
Describe the function and dysfunction of prolactin caused by the hypersecretion and hyposecretion of it
How are hormones usually measured?
- Hormones occur in μM and pM concentrations and can be measured using antibodies
- which cause a colour change when bound to the hormone etc.
- Measuring hormones can be difficult as they are released in a pulsatile fashion so their concentrations vary greatly with time.
- Dynamic testing involves stimulating the secretion of a hormone and conclusions can be drawn if a patient does not respond as expected.
If you had a disease of the thyroid gland that prevented it from secreting hormones, which choice below best reflects what would happen to the levels of the various hormones involved in the hypothalamo-pituitary thyroid axis?
