Hypothalamus-Pituitary Axis Part 1

Question 1

Describe features of endocrine glands

Answer 1

They are ductless and can be discreet organs such as pituitary or adrenal glands or they can be associated with other tissues or be scattered within complex organs.

Question 2

Describe the mode of secretion of endocrine glands

Answer 2

• They are very vascular (fenestrated capillaries) and so hormones can diffuse directly into capillaries to act on target organs (classic endocrine secretion).
• some hormones are secreted and act locally (paracrine secretion),
• Some hormones act directly on the cell that it is produced by (autocrine secretion)

Question 3

What are the classes of endocrine hormones?

Answer 3

• Peptide hormones (synthesised from mRNA and stored in secretory vesicles),
• Steroid hormones (derived form fat or cholesterol),
• Amino acid derivatives (eg, T3/4)

Question 4

Name some examples of major endocrine glands

Answer 4

• Hypothalamus,
• Pituitary gland,
• Pineal gland,
• Parathyroid
• Thyroid,
• Thymus,
• Adrenal glands,
• Ovaries/testies,
• Pancrease/kidney/stomach/adipose/small intestines

Question 5

What are some factors that control hormone release?

Answer 5

Humoral - Capillary blood concentration of ions/metabolites.
Neuronal - Eg, Preganglionic SNS fibres stimulate adrenal medulla cells.
Hormonal - eg, Hypothalamus can secrete hormones that act on pituitary gland

Question 6

Name the three parts of the adenohypophysis (anterior pituitary)

Answer 6

• Pars distalis (main part),
• Pars tuberalis,
• Pars intermedia (quite rudimentary, only a line of cells that lies next to posterior pituitary)

Question 7

Name the two parts of the neurohypophysis (posterior pituitary)

Answer 7

• Pars nervosa (posterior lobe - extension of stalk. Mainly made of neuron tissue),
• Pituitary stalk.

Question 8

Where is the pituitary gland located?

Answer 8

Immediately posterior to optic chiasma and sphenoid sinus. It is enclosed by the bony sella turcica with the diaphragma sella superiorly and the sphenoid sinuses inferiorly.

Question 9

Explain the hypothalamic control of the anterior pituitary lobe

Answer 9

• There is no neuronal control, instead hypothalamic neurons release hormones into hypothalamic-hypophyseal portal vessels which flow into secondary capillary plexus which surrounds the anterior pituitary lobe. The hormones travel through fenestrations in capillaries and bind to their receptors on target troph cells. Hypothalamus releases ‘hormone releasing hormones’ (releases hormones to stimulate the release of certain hormones from the anterior pituitary)

Question 10

Explain the relation of the posterior pituitary and the hypothalamus

Answer 10

Bodies of cells that secrete posterior pituitary hormones are located in the supraoptic and paraventricular nuclei of the hypothalamus. The hormones are then transported along the axons of magnocellular neurons (hypothalamo-hypophyseal tract) into the posterior pituitary. The nerve endings lie on the surfaces of capillaries where they secrete ADH or Oxytocin

Question 11

What are the functions of large/small - bodied neurons of the hypothalamus?

Answer 11

Large-bodies neurons make ADH or OT.

Small-bodied neurons make releasing hormones which are secreted into the primary plexus of the hypophyseal portal system

Question 12

Name the hypothalamic hormones that control secretion of the anterior pituitary gland and their function

Answer 12

• Thyrotropin-releasing hormone (TRH) which stimulates secretion of TSH.
• Gonadotropin-releasing hormone (GnRH) which stimulates secretion of LH and FSH.
• Corticotropin-releasing hormone (CRH) stimulates secretion of ACTH,
• Growth hormone releasing hormone (GHRH), stimulates secretion of growth hormone.
• Growth hormone inhibitory hormone (GHIH), inhibits secretion of growth hormone,
• Prolactin-inhibiting hormone, (PIH) (dopamine) which inhibits synthesis and secretion of prolactin.

Question 13

Explain the embryonic development of the pituitary gland

Answer 13

• Anterior Pouch arises as an upwards growth of oral ectodermal cells from Rathke’s pouch (oral ectoderm origin).
• Posterior pituitary arises as a downward growth of neural tissue from infundibulum (neuroectoderm origin)

Question 14

What can occur to residual tissue of the pharyngeal hypophysis?

Answer 14

It can become neoplastic and form hormone secreting tumours called craniopharyngiomas

Question 15

Name the cell types of the anterior pituitary and the hormones they secrete

Answer 15

Thyrotropes - Thyroid stimulating hormone (TSH),
Somatotropes - Growth hormone,
Corticotropes - Adrenocorticotropic hormone (ACTH),
Lactotropes - Prolactin (PRL).
Gonadotrophes - Follicle-stimulating hormone (FSH) and luteinising hormone (LH)

Question 16

Describe the arrangement of cells within the anterior pituitary gland

Answer 16

Superiorly - Cluster of thyrotropes and then inferior to that is corticotropes.
Laterally - somatotropes.
Inferiorly - Lactotropes.
Gonadotropes don’t have a definitive region and are instead scattered throughout the pituitary.

Question 17

Explain the importance of molecular regulation of anterior pituitary development

Answer 17

There are gene regulatory programmes that give rise to the 5 different anterior pituitary cell types. If there is a genetic defect in one of the genes/transcription factors then there will be an issue with the cell type and therefore hormones produced.

Question 18

Describe the different physiological actions of the different anterior pituitary hormones

Answer 18

GH - Stimulates body growth, secretion of IGF-1, lipolysis and inhibits actions of insulin of carb and lipid metabolism.
ACTH - Stimulates production of glucocorticoids and androgens.
TSH - Stimulates production of thyroid hormones by thyroid follicular cells.
FSH - Stimulates development of ovarian follicles/regulation of spermatogenesis
LH - Causes ovulation, stimulates production of oestrogen and progesterone/stimulates testosterone production.
PRL - stimulates milk production and secretion.

Question 19

What are the neurohormones controlling the anterior pituitary hormones?

Answer 19

• Prolactin releasing hormone,
• Prolactin inhibiting hormone,
• Thyrotropin-releasing hormone,
• Corticotrophin-releasing hormone,
• growth hormone-releasing hormone,
• Somatostatin,
• Gonadotropin-releasing hormone

Question 20

What are some neurohormones released from the posterior pituitary?

Answer 20

• Oxytocin (oxt),

- Vasopressin (AVP)/Anti-diuretic hormone (ADH)