Zusammenfassung der Ressource
L01 - The HPG Axis
- Hypothalamus
- Found at the base of the brain,
between the midbrain and forebrain
- Collection of brain nuclei/centres
- Controls endocrine
function via pituitary gland
- Gonadotrophin releasing hormone (GnRH)
- Short-life peptide
- Pulsatile GnRH secretion at puberty
triggers gonad activation
- Continuous secretion leads to down regulation
of GnRH receptors on gonadotroph cells
- Causes lack of LH/FSH release
- Can block GnRH effects using antagonist or agonist
(due to resulting downregulation of receptors)
- Pituitary Gland
- Pea-sized gland at the base of the brain
- Hormone secretion regulated by thalamus and
by feedback loops from circulating hormones
- Anterior pituitary hormones in reproduction
- Gonadotrophins
- FSH
- Secreted by gonadotroph cells (ant. pit.)
- Acts on sertoli (testes) and granulosa (ovary) cells
- Has an alpha chain, a beta chain and two carbohydrate chains
- Receptor LHCGR
- LH
- Secreted by gonadotroph cells (ant. pit.)
- Acts on leydig, theca and granulosa cells
- Has an alpha chain, a beta
chain and a carbohydrate chain
- Receptor LHCGR
- hCG
- Human corionic gonadotrophin
- Secreted by trophoblast cells
- Acts of luteal cells in the corpus luteum
- Has an alpha chain, a beta chain
and two carbohydrate chains
- FSHR
- Bind to cell surface receptors
and signal via G-proteins
- Proteins and Peptides
- ACTH - corticotrophs
- GH - somatotrophs
- Prolactin - lactotrophs
- Communication
- Direct - neural to post. pit.
- Indirect - vascular to ant. pit.
- GnRh produces in
parvocellular neurons and
travels to ant.pit. in portal blood
- Stimulates FSH and
LH production
- Hypophyseal portal system
- Gonads
- Different for males and females
- Sex steroids and regulatory factors produced
- Sex steroids
- Progestagens
- Pregnancy
- Progesterone
- 17alpha-OHP
- 20alpha-OHP
- Functions
- Preparation of the uterus
- Maintenance of the
uterus during pregnancy
- Growth of mammary glands
- Suppression of lactation
- Catabolic effects
- Regulation of gonadotrophins
- Receptors
- PR-A
- PR-B
- All derived from cholesterol
- Androgens
- Male
- DHT
- Testosterone
- Androstenedione
- DHEA
- Functions
- Development and maintenance of
the male reproductive system
- Secondary sexual characteristics
- Sexual function
- Support spermatogenesis
- Regulation of gonadotrophins
- Receptors
- AR - polymorphic with
differing sensitivites to
androgens
- Oestrogens
- Female
- Oestradiol 17beta
- Puberty to menopause
- Oestriol
- Pregnancy
- Oestrone
- Post-menopause
- Functions
- Secondary sexual characteristics
- Stimulate growth and
activity of mammary glands
- Stimulation proliferation of
endometrium for progesterone action
- Regulate gonadotrophins
- Receptors
- ERalpha
- ERbeta
- Lipid soluble, act via steroid
response elements (SREs)
- Bound to carrier proteins
- Albumin
- Sex hormone binding
globulin (SHBG)
- Androgen binding protein (ABP)
- Steroid action
- Potency
- Depends on affinity of steroid to receptor
- Varies within each class
- Cross-occupancy
- Some bind receptors from other classes
- Side-effects result
- Some bind but don't activate
- Antagonists
- Regulation
- Alter amount of steroid produced
- Regulated by and regulate
gonadotrophins of ant. pit.
- Feedback loops
- Alter amount or
structure of receptor
- Important in reproduction - gives
better picture than hormone levels
- Negative feedback in males, complicated
feedback in females
- Clinical aspects
- Fertility
- For females, endocrine problems
are a common cause of infertility
- Central Hypogonadism
- Hypothalamus/pituitary problem
- Low FSH/LH
- Primary Hypogonadism
- Ovary/testes problem
- Lack of feedback
- High FSH/LH
- Prolactin
- Inhibits HPG axis when in high doses
- Levels increase in pregnancy
and breast feeding
- Inhibits gonadal activity through
central suppression of GnRH
- Induces lactational amenorrhea
- The Menstrual Cycle
- Cyclic gametogenesis,
once per month
- Hormonal changes cause folliculogenesis
and endometrium changes
- Three phases
- Follicular/proliferative
- Ovulation
- Luteal/secretory
- Duration
- 26-32 days
- Follicular phase varies
- Luteal phase
- 14 days
- Fertile period
- Oocyte - 24 hours
- Sperm - 5 days
- Oligomenorrhea
- <9 cycles in last 12 months
- Amenorrhea
- No bleed in 6 months
- Primary - since birth
- Secondary - ceased
- Menopause
- Females - finite number of oocytes
- Ovulation ceases around 50
- Lack of progesterone/oestrogen
production by ovary
- Removal of -ve feedback leads to high FSH/LH levels
- Higher FSH closer to menopause
- Test for ovulation on day 21
- Treatments/uses
- Hormonal contraception
- Synthetic progestagens and oestrogens
- Suppress ovulation
- Thickens cervical mucous
- Thins endometrium
- Fertility treatment
- IVF/ICSI
- Down-regulate HPG axis with GnRH
agonist/antagonist, stimulate with FSH,
induce oocyte maturation with hCG
- Prostate cancer treatment
- GnRH agonists suppress T cell production