THE HYPOTHALAMO-HYPOPHYSIAL (PITUITARY) AXIS Flashcards
Describe the dimensions of the pituitary gland
Pituitary gland tiny – size of your thumbnail – attached to the base of the brain. Lies in a bony dip – shaped like a Turkish saddle – hence its name sella turcica
What is the pituitary gland also known as
The master gland. It is the source of a number of hormones, of which some have, as their major effect, the hormone production by other endocrine glands elsewhere in the body. The hypothalamus is therefore the overall conductor.
Describe the basic structure of the pituitary gland
Lies within a depression in the sphenoid depression beneath it (sella turcica)
In adults, the two lobes of the gland are quite distinct, with the anterior lobe being made up of various types of secretory cells; it is also called the adenohypophysis. The posterior is composed in large part of nerve axons from the hypothalamus, and is called the neurohypophysis.
Optic chiasm is near the anterior- tumour and visual disturbance
Mammillary body- nearer to the posterior
How does the pituitary gland appear on the MRI
We only see one entity- but functionally it has two distinct lobes.
Describe the differences in the embryological developments of the anterior and posterior lobes
The 2 components of the pituitary gland develop from different structures. There is a upward growth (ectodermal extension) from the buccal cavity called Rathke’s pouch towards a downward growth (ectodermal extension) from the base of the brain - these two growths combine to make one gland (5th week). So the pituitary gland has 2 components, 1 secretory part derived from the buccal cavity the adenohypophysis and 1 neural part, the neurohypophysis, made up of nerve axons derived from the base of the brain.
Describe the different regions of the anterior lobe of the pituitary gland
Pars distalis is the body of the anterior pituitary- contains the neurosecretory cells, the pars tuberalis is the part of the anterior pituitary which wraps around the pituitary stalk (infundibulum). We know that the regulation of anterior pituitary function is via the bloodstream - a special circulation.
What happens to the pituitary gland during pregnancy
It increases in size
Describe the hypothalamic nuclei
The hypothalamus is part of the brain, so is made up of neurones – neurones start off as cell bodies and then their axonal projections to various parts of the brain including the neurohypophysis. Groups of neuronal cell bodies are called nuclei. There are a number of hypothalamic nuclei – and some of these are very important as they send axons down the pituitary stalk to the posterior pituitary (neurohypophysis). A different population of neurons have shorter axons and these terminate in the part of the hypothalamus which abuts the pituitary gland – the median eminence.
Describe the median eminence
It is devoid of the blood-brain barrier and so can communicate with the systemic circulation
Describe the HYPOTHALAMIC – HYPOPHYSIAL PORTAL CIRCULATION
There is a very special blood system which flows to the anterior pituitary. It starts with blood arriving in the SUPERIOR hypophyseal artery and this enters a capillary network in the median eminence. From this capillary network (primary capillary plexus), blood drains to a second capillary network in the anterior pituitary via the portal vein – a portal network. A portal network is where blood from the systemic circulation drains from one capillary network to another before returning to the heart. From the secondary capillary system, blood would flow out into the venous system (jugular vein) via the cavernous sinus. This system is critical for control of anterior pituitary function.
What is the importance of the capillaries being fenestrated
Neurosecretions from hypothalamic nuclei can enter the capillaries and be transported to the pars distalis, and act on secretory cells.
What is a neurosecretion
Hypothalamic neurone
What does the adenohypophysis require
Blood Circulation
What is the portal system called
Hypothalamo-adenohypophysial portal system
Describe the steps that lead to the release of adenohypophysial hormone into the circulation
Axons from hypothalamic nuclei end at the primary capillary plexus in the median eminence. The neurosecretions from these nuclei enter the primary capillary plexus (granules released by exocytosis in nerve terminals upon depolarisation) and travel to the secondary capillary plexus via the portal system. Neurosecretions leak out of the fenestrations in the secondary capillary plexus, act on secretory cells- release of hormones into circulatory system
Describe the innervation of the adenohypophysis
There is no appreciable innervation of the adenohypophysis, although a few peptidergic nerve fibres do terminate within it and could have a controlling influence. There is also evidence for some innervation by sympathetic fibres, specifically to the vasculature. It is quite possible that regulation of the adenohypophysial blood flow could have some influence on hormone release into the circulation
What does trophin mean
means stimulates growth and development of’
List the different types of adenohypophysial cells and the hormones that they produce
SOMATOTROPHS produce Growth Hormone (Somatotrophin)
LACTOTROPHS produce PROLACTIN
THYROTROPHS produce Thyroid Stimulating Hormone (TSH, Thyrotrophin)
GONADOTROPHS produce Luteinising Hormone, Follicle Stimulating Hormone (LH and FSH)
CORTICOTROPHS produce AdrenoCorticoTrophic Hormone (ACTH, Corticotrophin)
What are two interesting characteristics of adenohypophysial hormones
They are greatly influenced by the CNS through the hypothalamus and its hormones.
Most of them have other endocrine glands as their targets
Describe the production and secretion of the hypophysial hormones
Precursor molecules called PROHORMONES
Enzymatic cleavage of prohormone yields
the bioactive HORMONE molecule
Adenohypophysial hormones stored
in secretory granules
Released by exocytosis into the bloodstream
Where are the enzymes that cleave the prohormones found
In the anterior pituitary cells.
Us the example of corticotrophin to describe how hormones are synthesised
POMC = ProOpioMelanoCorticotrophin) — CORTICOTROPHIN + Pro-gMSH + bLPH
Describe what constitutes the different types of adenohypophysial hormones
All made of amino acids
PROTEINS:
GROWTH HORMONE (SOMATOTROPHIN ) 191 aa
PROLACTIN 199 aa
GLYCOPROTEINS: consisting of a and b sub-units (92 aa a
sub-unit common to all )
THYROID STIMULATING HORMONE (TSH, THYROTROPHIN) b-sub-unit 110 aa
2 GONADOTROPHINS both have 115 aa b-sub-unit
- LUTEINISING HORMONE (LH)
- FOLLICLE STIMULATING HORMONE (FSH)
POLYPEPTIDE: ADRENOCORTICOTROPHIC HORMONE (ACTH, CORTICOTROPHIN) 39 aa
What do glycoprotein hormones contain
Glycoprotein molecules – are protein molecules to which CHO molecules have been attached. These have 2 strands – alpha and beta subunits, the alpha subunit is common to all of them, the beta subunit distinguishes them
