Flashcards in Jones - taught Deck (155)
What is the HPG axis
The hypothalamic–pituitary–gonadal axis
multiple endocrine glands working together as a system
What does the HPG axis do?
regulate development, reproduction, and ageing in animals
What do endocrine glands do?
secrete hormones into the bloodstream; the hormones travel to and act on other organs or tissues
What are the components of the HPG?
• Anterior pituitary
What is the hypothalamus?
Component of forebrain; part of the diencephalon
What does the hypothalamus do?
Regulates many core body functions (homeostatic functions) eg. Metabolism, growth, reproduction, stress.
How is the hypothalamus in contact with the anterior pituitary gland?
Secretes a peptide hormone: gonadotrophin releasing hormone (GnRH)
Critical component of the reproductive system
What are the two parts of the pituitary gland?
• Posterior and anterior (distinct lobes); derived from separate types of cells during embryogenesis; different functions.
What is the anterior pituitary gland?
An endocrine gland
What is the anterior pituitary gland made up of?
different groups of cells:
o Thyrotropes (Thyroid-stimulating hormone; TSH)
o Somatotropes (Growth hormones)
o Gonadotrophs (Follicle-stimulating hormone; FSH and Luteinising hormone; LH)
o Corticotropes (adrenocorticotropic hormone; ACTH)
o Lactotropes (prolactin; PRL)
How does GnRH connect the three systems?
GnRH from the hypothalamus travels in the portal blood to the anterior pituitary where it acts on gonadotrophs
How does GnRH positively regulate cells proliferation and hormone production?
through a G-protein-coupled receptor (GnRHr)
What does GnRHr signalling stimulate?
Production of FSH and LH
What is GnRH?
What do FSH and LH do?
act on different types of cells and these cells ten produce different hormones
What are the three main types of sex steroid?
progestagens, androgens and oestrogens
What are sex steroids derived from?
a common precursor: cholesterol
Interconversion of sex steroids is via?
a biosynthetic network
Action of these steroids is dictated by tissue-specific receptors
Once inside a cell a steroid ...
steroid-receptor complexes bind to steroid response elements on DNA and impact on transcription
What affect does oestrogen have on gonadotrophs?
1. Oestrogen binds to the oestrogen receptor (ER) on the gonadotroph
2. The hormone-receptor complex translocates to the nucleus and mediates negative transcriptional control of target genes through oestrogen response elements (EREs)
3. Suppression of transcription
4. This regulates the production of FSH and LH
What family are th inhibins and activins from?
What do inhibins/activins do on gonadotrophs?
bind to inhibin and activin receptors on the gonadotroph cell
They act to regulate FSH and LH expression
What is LH and FSH output predominantly regulated by in females?
secretory products from the ovary
In females Negative feedback on LH and FSH is by?
: oestrogens, progestogens and inhibins
in females Positive feedback on LH and FSH is by?
activin; oestradiol (an oestrogen)
What is the effect of oestradiol in females?
at low concentrations acts to negatively regulate LH expression but at high levels it acts to positively regulate expression
What is the effect of progesterone in females?
o High concentration seen in luteal phase of menstrual cycle (after ovulation) enhances the negative feedback of oestradiol
o At certain levels, the positive feedback effect of oestradiol is blocked
What level does feedback act on the HPG axis?
both the levels of the pituitary and at the level of the hypothalamus