15. Hypothalamic Pituitary Axis And Growth Hormone

Question 1

What is the socket of bone that the pituitary gland sits in called?

Answer 1

Sella turcica

Question 2

What processes do the hypothalamus and pituitary gland modulate?

Answer 2

Body growth
Reproduction
Adrenal gland function
Water homeostasis
Milk secretion
Lactation
Thyroid gland function
Puberty

Question 3

What are the 2 parts of the pituitary gland?

Answer 3

Anterior (adenohypophysis)

Posterior (neurohypophysis)

Question 4

Where does the anterior pituitary arise from?

Answer 4

Evagination of oral ectoderm (primitive gut tissue)

Question 5

Where does the posterior pituitary arise from?

Answer 5

Originates form neuroectoderm (primitive brain tissue)

Question 6

How is the posterior pituitary connected to the hypothalamus?

Answer 6

By the infundibulum

Question 7

What is the neurocrine function of the posterior pituitary?

Answer 7

Oxytocin and ADH produced by neurosecretory cells in supraoptic and paraventricular nuclei of hypothalamus
Transported down nerve cell axons to posterior pituitary
Stored and released from posterior pituitary into general circulation to act on distant targets

Question 8

What is the anterior pituitary function?

Answer 8

Hormones synthesised in hypothalamus transported down axons and stored in median eminence before release into hypophyseal portal system
These hormones stimulate/inhibit target endocrine cells in anterior pituitary gland (neurocrine)
Endocrine cells of anterior pituitary secrete a variety of hormones into bloodstream to act on distant target cells

Question 9

What is the role of oxytocin?

Answer 9

Milk let down and uterus contractions during birth

Question 10

What are tropic hormones?

Answer 10

Affect the release of other hormones in the target tissue

Question 11

What are the tropic hormones of the hypothalamus?

Answer 11

TRH - thyrotropin releasing hormones (TSH)
PIH - prolactin release-inhibiting hormone aka dopamine (PRL)
CRH - corticotropin releasing hormone (ACTH)
GnRH - gonadotropin releasing hormone (FSH, LH)
GHRH - growth hormone releasing hormone
GHIH - growth hormone-inhibiting hormone (somatostatin)

Question 12

What are the hormones produced by the anterior pituitary?

Answer 12

TSH (thyroid stimulating hormone) - secretion of thyroid hormone from thyroid gland
ACTH (adrenocorticotropic hormone) - secretion of hormones from adrenal cortex
LH (luteinising hormone) - ovulation and secretion of sex hormones
FSH (follicle stimulating hormone) - development of eggs and sperm
PR (prolactin) - mammary gland development and milk secretion
GH (growth hormone) - growth and energy metabolism, stimulate IGFs

Question 13

How does negative feedback work in hormones in hypothalamus and anterior pituitary?

Answer 13

Hypothalamus.     hormone 1
\+ve -> anterior pituitary.       hormone 2
\+ve -> target endocrine gland
-> hormone 3.   -> -ve
\+ve -> target tissue

Question 14

What is growth influenced by?

Answer 14

Hormones
Genetics
Nutrition
Environment

Question 15

How are growth promoting effects mainly exerted?

Answer 15

Indirectly via insulin-like growth factors (somatomedins)

Question 16

What is the importance of GH in childhood and teenage years?

Answer 16

GGH stimulates long bone growth

IGFs stimulate both bone and cartilage growth

Question 17

What is the importance of GH and IGFs in adults?

Answer 17

Help maintain muscle and bone mass and promote healing and tissue repair as well as modulating metabolism and body composition

Question 18

What can increase or decrease GH secretion?

Answer 18

Increase - deep sleep, stress, exercise, low glucose or fatty acids, fasting
Decrease - REM sleep, high glucose or free fatty acids, obesity

Question 19

What is the long loop negative feedback in GH secretion?

Answer 19

Mediated bu IGFs
Inhibit release of GHRH from hypothalamus
Stimulates release of somatostatin from hypothalamus
Inhibit release of GH from anterior pituitary

Question 20

What is the short loop negative feedback in GH secretion?

Answer 20

Mediated by GH itself via stimulation of somatostatin release

Question 21

What is growth hormone deficiency?

Answer 21

In childhood, results in pituitary dwarfism
Proportionate type of dwarfism
Complete or partial deficiency
Height below 3rd percentile on standard growth chats
Delayed or no sexual development during teen years

Question 22

What can growth hormone excess result in?

Answer 22

In childhood, gigantism - often caused by pituitary adenoma

In adulthood, acromegaly - large extremities (hands, feet, lower jaw)

Question 23

How does GH exert its effects on cells?

Answer 23

Tyrosine kinase receptors
GH receptors activate Janus kinases (JAKs)

Growth hormone binds
Dimerisation from cross phosphorylation of JAK
Phosphorylation of GH receptors
Activation of inhaling pathways
Transcription factor activation and IGF production

Question 24

What are insulin-like growth factors (IGFs)?

Answer 24

IGF2 mainly involved in foetal growth
IGF1 major growth factor in adults
Binding proteins modulate their availability
Actions of IGFs can be paracrine, autocrine or endocrine

Question 25

How to IGFs act?

Answer 25

Through IGF receptors to modulate:

• cell growth
• cell number
• increase rate of protein synthesis
• increase rate of lipolysis in adipose tissue

Question 26

What are the other hormones that influence growth?

Answer 26

Insulin
Thyroid hormones
Androgens
Oestrogens 
Glucocorticoids