Hypothalamo-adenohypophysial System

Question 1

What is the bony shelf that the pituitary is found in?

Answer 1

Sella Turcica.

Question 2

What is the clinical importance of the Sella Turcica?

Answer 2

• A tumour will be constrained by the walls of the bone cavity.
  Pituitary tumours can:
• protrude out towards the brain
  or
• go through the bone if really malignant

Question 3

Where is the hypothalamus located?

Answer 3

• Just above the pituitary gland

Question 4

Which ventricle of the brain does the hypothalamus surround?

Answer 4

3rd ventricle

Question 5

What are the two areas of the brain that are useful anatomical markers for distinguishing the posterior pituitary from the anterior?

Answer 5

Anterior (front) - optic chiasma
Posterior (back) - mammillary body

Anterior Lobe = adenohypophysis
Posterior Lobe = neurohypophysis

Question 6

Describe the embryological origin of the pituitary gland.

Answer 6

Originates from:
Glandular Tissue:
- Extension of buccal cavity starts growing upwards

Nervous Tissue:
- Downwards movement of tissue from developing hypothalamus

• Both tissues fuse
• Eventually, the connection to the rest of the buccal cavity is cut off.

Adenohypophysis = derived from glandular tissue
Neurohypophysis = derived from neural tissue

Question 7

What are hypothalamic nuclei?

Answer 7

They are groups of neuronal cell bodies that can be grouped functionally

Question 8

Where is the median eminence?

Answer 8

It is located between the top of the pituitary stalk and the hypothalamus.

Question 9

Where is the Primary Capillary Plexus located?

Answer 9

Median Eminence (a mass of capillaries which receives blood from the superior hypophysial artery)

Question 10

Where is the Secondary Capillary Plexus located?

Answer 10

In the anterior pituitary (adenohypophysis)

Question 11

What connects the primary capillary plexus to the secondary?

Answer 11

Connected via the hypothalamo-hypophysial portal system

Question 12

What is the hypothalamo-hypophysial portal system?

Answer 12

• Lots of neurons coming from the hypothalamic nuclei terminate on the walls of the primary capillary plexus
• Primary capillary plexus feeds blood down into the portal vessels which run down through pituitary stalk to terminate within the adenohypophysis

Question 13

What happens to the blood supply after the hypothalamo-hypophysial portal system?

Answer 13

• The blood from the secondary capillary plexus is gathered into the cavernous sinus and out through the jugular veins

Question 14

What is a key feature of the primary and secondary capillary plexus?

Answer 14

The capillaries are fenestrated (there are small holes in it)

Question 15

Describe how the hypothalamus exerts control over the adenohypophysis.

Answer 15

• Some neurons from the hypothalamus terminate on the median eminence at the primary capillary plexus
• The chemicals are released directly into the primary capillaries and so they are actually HORMONES and not neurotransmitters
• There is important hypothalamic control over the anterior pituitary through hormones released by neurons originating in the hypothalamus
• The hypothalamo-adenohypophysial portal system carries these chemical to target cells in the anterior pituitary
• These hypothalamic hormones bind to adenophysial cells and stimulate release of anterior pituitary hormones into the circulation.

Question 16

What are the FIVE types of adenohypophysial cells and what hormones do they produce?

Answer 16

1. Somatotrophs - somatotrophin (growth hormones)
2. Lactotrophs - prolactin
3. Gonadotrophs - LH and FSH (Luteinizing Hormone, Follicle Stimulating Hormone)
4. Thyrotrophs - Thyrotrophin (Thyroid Stimulating Hormone -TSH)
5. Corticotrophs - corticotrophin (ACTH)

Question 17

How are adenohypophysial hormones synthesised?

Answer 17

• Synthesised as prohormones
• Enzymatic cleavage of prohormone = bioactive hormone
• Stored in secretory granules and released by exocytosis

Question 18

What are the THREE types of adenohypophysial hormone? Give examples of each.

Answer 18

Protein
- somatotrophin, prolactin

Glycoprotein have an alpha chain (same in all) and a beta chain (different)
- FSH, LH, thyrotrophin

Polypeptide
- ACTH

Question 19

State the EIGHT hypothalamic hormones and the adenohypophysial hormones that they control.

Answer 19

1. Somatotrophin releasing hormone (SRH) + somatostatin (SS)
   = somatotrophin
2. Dopamine (DA) + Thyrotrophin releasing hormone (TRH)
   = prolactin
3. Gonadotrophin releasing hormone (GnRH) + Gonadotrophin inhibitory hormone (GnIH)
   = FSH + LH
4. TRH
   = thyrotrophin
5. Corticotrophin releasing hormone (CRH) + vasopressin (VP)
   = corticotrophin

Question 20

Which of the above hormones are inhibitory?

Answer 20

Dopamine and Somatostatin are both INHIBITORY

Question 21

Which hypothalamic hormone is dominant over its counterpart?

Answer 21

1. SRH - dominant over somatostatin
2. Dopamine - dominant over TRH
3. GnRH - dominant over GnIH

Question 22

State the tissues that the FIVE adenohypophysial hormones act on.

Answer 22

1. Somatotrophin
   - general body cells
   - (hepatocytes in particular)
2. Prolactin
   - breasts in lactating women

3.Thyrotrophin - thyroid
4. Gonadotrophin
- gonads:
men = testes
women = ovaries
5.Corticotrophin - adrenal cortex

Question 23

Describe the two modes of action of somatotrophin

Answer 23

1. Direct effect:
   Somatotrophin can act directly on tissues by binding to somatotrophin receptors in general cells of the body

OR

2. Hepatocytes:
   Can act on hepatocytes and stimulate hepatocytes to produce IGF 1, which is particularly important in growth

Question 24

What are the metabolic actions of somatotrophin

Answer 24

1. Somatotrophin stimulates:
   - amino acid transport into cells
   - protein synthesis, increases
   - lipid metabolism leading to increased fatty acid production
2. Increased cartilaginous growth
3. Decreased glucose utilisation resulting in increased blood glucose concentration.

Question 25

What stimulates somatotrophin release?

Answer 25

Sleep, stress, oestrogens, exercise, fasting (hypoglycaemia), amino acids, ghrelin (from stomach)

Question 26

What inhibits somatotrophin release?

What has a negative effect on somatotropin release?

Answer 26

Direct - IGF 1 back to the pituitary

Indirect - IGF 1 back to the hypothalamus

Somatotrophin itself and somatomedins have a negative feedback effect on somatotrophin production.

Question 27

What is the main effect of prolactin?

Answer 27

Prolactin stimulated lactogenesis in the breast.

Question 28

What are some other effects of prolactin?

Answer 28

• Prolactin is also a natural contraceptive as it inhibits LH release
• It also upregulates LH receptors in the gonads. It decreases sexual behaviour
• Has effects on the immune system

Question 29

Describe the neuroendocrine reflex of prolactin.

Answer 29

• There are tactile receptors in the nipple that are stimulated by the suckling of the baby
• Has an afferent pathway to the hypothalamus and stimulates:
• Release of thyrotrophin releasing hormone
• Inhibits dopaminergic neurons.
• Leads to release of prolactin
• Prolactin starts the synthesis of milk for the next suckling period.