HPG Axis Flashcards
What does successful reproduction require?
- Fertilisation
- Support of conceptus, embryo, and fetus
- Birth at the right time
- Support of the neonate
What is crucial to all the steps of successful reproduction?
Hormones of the hypothalmic-pituitary-gonadal (HPG) axis
What does the output of the hypothalamus-pituitary unit regulate the function of?
- Thyroid
- Adrenals
- Reproductive glands
What does the hypothalamic-pituitary unit control?
- Somatic growth
- Lactation
- Milk secretion
- Water metabolism
What does the hypothalamus always produce, regarding hormone cascades?
A releasing hormone that is specific to the hormone cascade that it follows
What are the common features of the releasing hormones produced by the hypothalamus?
- Secretion in pulses tied to internal biological clock
- Act on specific membrane receptors
- Transduce signals via second messengers
- Stimulate release of one or more pituitary hormones
- Stimulate synthesis of pituitary hormones
- Stimulates hyperplasia and hypertrophy of target cells
- Regulates its own receptor
Does the hypothalamus produce any hormones other than releasing hormones?
No
Give 7 hypothalamic releasing hormones
- Corticotrophin releasing hormone
- Thyrotropin releasing hormone
- Growth hormonne releasing hormone
- Somatostatin
- Gonadotropin releasing hormone
- Prolactin releasing hormone
- Prolactin inhibiting hormone
What is the effect of corticotrophin releasing hormone on the pituitary?
Stimulates ACTH release
What is the effect of thyrotropin releasing hormone on the pituitary?
Stimulates TSH and prolactin secretion
What is the effect of growth hormone releasing hormone on the pituitary?
Stimulates GH secretion
What is the effect of somatostatin on the anterior pitutiary?
Inhibits GH (and other hormone) secretion
What is the effect of gonadotrophin releasing hormone (GnRH) on the pituitary?
Stimulates LH and FSH secretion
What is the effect of prolactin releasing hormone on the pituitary?
Stimulates PRL secretion
What is the effect of prolactin inhibiting hormone on the pituitary?
Inhibits PRL secretion
What is the pituitary gland also known as?
Hypophysis
Where does the pituitary gland lie?
In depression of base of skull known as pituitary fossa, or sella turcica
What does the pituitary gland resemble in terms of size and shape?
A bean
What seperates the anterior and posterior pituitary?
Lumen of Rathke’s pouch
What is the anterior pituitary also known as?
Adrenohypophysis
Is the adrenohypophysis made of nervous tissue?
No
How is the adrenohypophysis connected to the hypothalamus?
By the superior hypophyseal artery
What is the anterior pituitary made up of?
Collection of hormone producing glandular cells
What are the hormone producing cells of the anteiror pituitary under the control of?
The hypothalamus
How does the hypothalamus control the hormones produced by the anterior pituitary?
The hypothalamus tells the pituitary which hormone is needed with specific releasing hormone
What are the cell types of the anterior pituitary?
- Thyrotrophic cells
- Mammotrophic cells
- Corticotrohic cells
- Somatotrophic cells
- Gonadotrophic cells
- Pars intermedia cells
What hormone do thyrotrophic cells produce?
Thyroid stimulating hormone
What does thyroid stimulating hormone act on?
The thyroid gland
What hormone do mammotrophic cells produce?
Prolactin
What does prolactin act on?
- Mammary glands
- Testes
What hormone do corticotrophic cells produce?
ACTH
What does ACTH act on?
Adrenal cortex
What hormone do somatotrophic cells produce?
Growth hormone
What does growth hormone act on?
All body tissues, especially;
- Bone
- Muscle
- Connective tissue
What % of AP cells are gonadotrophic cells?
5-10%
What hormones do gonadotrophic cells produce?
- FSH
- LH
What do FSH and LH act on?
Gonads- testes and ovaries
What hormone do the pars intermedia cells produce?
Melanocyte-stimulating hormone (MSH)
What does MSH act on?
Melonocytes in epidermis
What anterior pituitary hormones are involved in reproduction?
All involved in some way, but LH, FSH, and prolactin are most significant
What is the posterior pituitary also known as?
Pars nervosa
What is the posterior pituitary?
An outgrowth of the hypothalamus
How can the posteiror pituitary be distinguished from the anterior histologically?
Stains lighter
What is the posterior pituitary made of?
Nervous tissue
Describe the relationship between the nerves of the hypothalamus and the posterior pituitary
Hypothalamic neurones pass through neural stalk and end in posterior pituitary
What is the median eminence?
The upper portion of the neural stalk that extends into the hypothalamus
What kind of gland is the neurohypophysis?
Neuro-secretory gland
What does the neurohypophysis secrete?
- ADH
- Oxytocin
What is oxytocin important in?
Reproduction
What does the HPG axis comprise of?
Interaction between hypothalamus, pituitary gland, and gonads
What does the HPG axis work together to do?
Regulate;
- Development
- Reproduction
- Aging
- Many other body processes
What does the regulation of the HPG axis rely on?
A number of complex positive and negative feedback loops
Why is it important that the HPG axis is regulated?
Don’t want the system to run away with itself
What regulates the HPG axis?
The product of the system- LH, FSH, and gonadal hormones
What does the loss of the regulation of the HPG axis result in?
Disease
What does a major share of the control of sexual functions in both male and female begin with?
Secretion of gonadotrophin-releasing hormone (GnRH) by the hypothalamus
What does GnRH do?
Stimulates the anterior pituitary gland to secrete two gonadotrophic hormones;
- Luteinising hormone (LH)
- Follicle-stimulating hormone (FSH)
What produces LH and FSH?
Gonadotrophs
How much FSH and LH do gonadotrophs secrete in the absence of GnRH?
Little or none
What is the result of gonadotrophs secreting little or no FSH or LH in the absence of GnRH?
They are very sensitive to pulsatile nature and concentration
Describe a GnRH molecule
10aa peptide
Of what nature is the release of GnRH?
Every 1-3 hours, in a discrete pulse
What is the intensity of GnRH stimulus affected by?
Frequency of release
What does LH release mirror?
The pulsatile nature
What happens if we perfuse someone with constant level of GnRH?
Loose LH completely
Does FSH mirror the pulsatile nature of GnRH?
No, it is more generally released
What does an increased intensity of release of GnRH lead to?
Increased FSH
How does GnRH travel to the pituitary?
In the hypophysial portal system
What control is gonadotroph synthesis and release under?
Positive and negative feedback by gondal steriods and gondal peptides
How do gonadal hormones decrease gonadotrophin release?
- Decreasing GnRH release from hypothalamus
- Affecting ability of GnRH to stimulate gonadotrophin secretion from the anterior pituitary
Draw a diagram illustrating the HPG axis
How do FSH and LH primarily act on the gonads in the male?
Via G-alphas PCR affecting adenylate cyclase
Where does LH act in the male?
On the Leydig cells of the testis
What does LH stimulation of the Leydig cells cause?
The cells to release testosterone
What controls testesterone release from the Leydig cells by LH?
Negative feedback
How does negative feedback control the release of testosterone from the Leydig cells?
- Testosterone releases LH from AP
- Testosterone reduces GnRH secretion
What is the result of testosterone reduced LH and GnRH secretion?
Reduces LH and FSH secretion, as they are both released from the same cells
What happens to testosterone levels in the medium/long term?
They are constant
Why are testosterone levels not constant in the short term?
- Circadian rhythm
- Effects of environmental stimuli
What time of day are testosterone levels highest?
In the morning
Where does FSH act in the male?
On Sertoli cells of the seminiferous tubules
What does the stimulation of Sertoli cells by FSH cause?
- Sertoli cells to grow and secrete spermatogenic substances
- Sertoli cells to secrete ABG and inhibin
What is also needed for the function of Sertoli cells?
Testosterone
How does testosterone get to the Sertoli cells?
It diffuses into the seminiferous tubules
What does ABG do?
Binds testosterone and keeps some within the seminiferous tubules, to allow the Sertoli cells to work
What is the function of inhibin produced by the Sertoli cells?
Reduce FSH secretion selectively
Where is inhibin imoprtnat in males?
In spermatogenesis
What is the function of inhibin in spermatogenesis?
Regulates production of sperm cells, so ensures spermatogenesis doesn’t continue all the time
Draw a diagram illustrating the HPG axis in the male
How do FSH and LH act on the gonads in the female?
Primarily via G-alphas PCR affecting adenylate cyclase
What are the target cells for FSH and LH in the female?
Ovarian granulose cells and theca interna
What do FSH and LH act to do in the female?
- Stimulate sex hormone synthesis
- Control gamete production
What sex hormones are produced in the female?
- Oestrogen
- Progesterone
- Inhibin
What aspects of gamete production are controlled by the HPG axis?
Folliculogenesis and ovulation
Why is the control of gamete production more complex in the female than the male?
Because release of gametes is pulsatile over 28 days
