S3) Puberty and HPG Axis Flashcards
What is puberty?
Puberty is a stage of human development when sexual maturation and growth are completed and result in ability to reproduce
Identify 3 forms of development that occur during puberty
- Morphological development
- Physiological development
- Behavioural development
Identify the primary sexual characteristics
- Gonads
- Genitals
- development of these sexual characteristics at birth
Identify 4 secondary sexual characteristics
- Pubic hair
- Axillary hair
- Female breast development
- Male voice changes
Briefly, describe the reproductive system before and after puberty
- Primary sexual characteristics are established before birth
- Reproductive system inactive until puberty
Identify 4 events that occur in puberty for males and females
- Accelerated somatic growth
- Maturation of primary sexual characteristics
- Appearance of secondary sexual characteristics
- Menstruation and spermatogenesis begin
Describe the involvement of the pineal gland in puberty
- Triggered by changes in day length
- Varies with secretion of melatonin
- Pineal gland tumours influence puberty
Describe the variation in the menarche of puberty in western countries over the years
- Average age of menarche = 17 years (C 1800)
- Average age of menarche < 13 years (C 2000)
Explain how nutrition has a notable role in the onset of puberty
- Body weight most important factor
- Leptins may be involved in signalling
- Significant weight loss = menstrual cycles stop
What is the critical weight for puberty to begin in girls?
Weight = 47 kg
When does puberty begin in girls?
9 - 13 years
What is thelarche?
Thelarche is the onset of female breast development
What is pubarche?
Pubarche is the appearance of sexual hair
What is adrenarche?
Adrenarche is the onset of androgen-dependent body changes such as growth of axillary and pubic hair, body odor, and acne, awakening of the adrenal glands
Describe the sequence of events in female puberty
Breast bud growth → Pubic hair growth → Growth spurt → Onset of menstruation
Which hormones drive puberty in females?
- Oestrogen
- Testosterone
Describe the sequence of events in male puberty
Genital development → Pubic hair growth → Spermatogenesis → Growth spurt
Which hormone drives puberty in males?
Testosterone
Identify the stages of the following photos in the Tanner Scale:
Identify the stages of the following photos in the Tanner Scale:
Accelerated somatic growth occurs in both sexes. What drives this process?
- Growth hormone
- Sex steroids
What concludes the period of accelerated somatic growth in both sexes?
Epiphyseal fusion
If accelerated somatic growth occurs in both sexes, why are women generally smaller/shorter?
Provide 3 reasons
- Growth spurt in girls is shorter and earlier
- Growth spurt in boys is longer and slightly faster
- Oestrogen closes epiphyses earlier in girls, oestrogen is responsible for fusing the growth plates in the bones
Compare and contrast puberty in males and females in terms of the following:
- Onset
- First sign of puberty
- Growth velocity
- Duration of puberty
Outline the HPG axis in females
Outline the HPG axis in males
Identify the 4 events which succeed increased stimulation of the hypothalamo-pituitary-gonadal axis
- Gradual activation of GnRH
- Increased frequency and amplitude of LH pulses
- Stimulated secretion of sexual steroids
- Extragonadal hormonal changes
GnRH -1 gene is primarily responsible for mammalian GnRH.
What happens when GnRH is blocked?
- Lack of gonadotrophin synthesis and secretion
- Lack reproductive development occurs
Identify 5 characteristic functions of hypothalamic releasing hormones
- Act on specific membrane receptors
- Transduce signals via second messengers
- Stimulate release & synthesis of stored pituitary hormones
- Stimulates hyperplasia & hypertrophy of target cells
- Regulates its own receptor
How are hypothalamic releasing hormones secreted?
Secretion in pulses tied to internal biological clock
GnRH stimulates the anterior pituitary gland gonadotrophs to secrete two gonadotropic hormones.
Identify them
- Luteinising hormone (LH)
- Follicular stimulating hormone (FSH)
What is leptin and what is its broad function?
- Leptin is an adipocyte-derived protein hormone
- It signals information about energy stores to CNS
Describe the release of leptin
- Leptin has pulsatile release patterns significantly associated with the variations in LH
- Its secretion may be influenced by gonadal steroids but appears to be independent of LH control
What role does leptin have in reproductive function?
- regulates menstral cycle
– puberty: the more leptin and fat the earlier someone will start menstruation
– fertility: low levels increase chance of infertility
– pregnancy: levels increase in pregnancy and help develop the placenta and fetal growth
In four steps, explain how GH secretion from the pituitary gland leads to a growth spurt
⇒ Increased TSH
⇒ Increased metabolic rate
⇒ Promoted tissue growth
⇒ Increased androgens
What effect does the increased production of androgens have?
It retains minerals in body to support bone and muscle growth
How often is GnRH released?
Released every 1-3 hrs, don’t want it to be continuous otherwise the body becomes desensitise to it.
Identify the two factors which affect the intensity of the GnRH stimulus
- Frequency of release
- Intensity of release
Where is GnRH released into?
Travels to the anterior pituitary gland in the hypophyseal portal system, it stimulates the anterior pituitary gland to release LH and FSH
How is the anterior pituitary connected to the hypothalamus?
Anterior pituitary is connected to the hypothalamus by the superior hypophyseal artery
Identify the six peptide hormones found in the anterior pituitary
- Prolactin
- Growth hormone (GH)
- Thyroid Stimulating Hormone (TSH)
- Adrenocorticotropic Hormone (ACTH)
- Follicle-Stimulating Hormone (FSH)
- Luteinizing Hormone (LH)
Most parts of the reproductive system can work before the normal age of puberty.
Why does puberty not occur earlier?
Puberty is under hormonal control:
- LH and FSH levels insufficient to initiate gonadal function
- Between 9-12 yrs, blood levels of LH & FSH increase to trigger puberty
Which cells does LH act on in males?
Leydig cells: release testosterone
causes a negative feedback on hypothalamus and pituitary gland
Where in the body is the most testosterone produced?
Testes
Which factors affect the production of testosterone?
- Circadian rhythm (highest early in morning)
- Environmental stimuli
Describe the structure of the testes
- Each lobule contains 1-4 seminiferous tubules
- Interstitial tissue contains Leydig cells
- Contains loose connective tissue
- Contains blood vessels
Describe the structure and function of the seminiferous tubules
- Structure:
I. Tightly coiled tubes
II. Composes 90% of testicular volume
- Function: location for spermatogenesis
Explain the relationship between leydig cells and seminiferous tubules
- Leydig cells function independently of seminiferous tubules
- Seminiferous tubules need functioning Leydig cells
Identify the two cells found in the epithelium of the seminiferous tubules
- Sertoli cells
- Spermatogenic cells/germ cells
What do Sertoli cells do?
Sertoli cells provide nutrition and hormonal support to germ cells allowing sperm formation
Describe the hormonal control and release of Sertoli cells
- Sensitive to FSH (increase sperm production)
- Secrete inhibin (feedback on Anterior Pituitary to stop release of FSH)
What do LH & FSH do in the female HPG axis?
They act primarily on gonads via Gαs PCR → adenylate cyclase
Identify the target cells of FSH and LH in the female
- Granulosa cells
- Theca interna
What are the roles of FSH and LH in the female HPG axis?
- Stimulate sex hormone synthesis (steroidgenesis)
- Control gamete production (folliculogenesis and ovulation)
Identify some sex hormones in females
- Oestrogen
- Progesterone
- Inhibin
Describe the hormonal control of ovarian cells
- Granulosa cells respond to FSH
- Theca cells respond to LH
Describe the control of oestrogen on GnRH secretion
- Moderate titres of oestrogen reduce GnRH secretion (negative feedback)
- High titres of oestrogen alone promote GnRH secretion (positive feedback LH ‘surge’), only positive feedback during ovulation
Describe the control of progesterone on GnRH
- Progesterone increases inhibitory effects of moderate oestrogen
- Progesterone prevents positive feedback of high oestrogen (no LH surge)
- it prevents more eggs to mature and so reduces the frequency of GnRH production
- it has the opposite effect of oestrogen
Describe the function and location of inhibin secretion in females
- Location: secreted from granulosa cells of corpus luteum
- Function: inhibits the FSH secretion & small inhibitory effect on LH
- stops more follicles being stimulated
what is the definition of gonadarche
- activation of the gonads
what is the definition of menarche
- onset of menstruation at puberty
what age defines central precocious puberty in boys and girls and what is its relationship with GnRH
-GnRH → elevated GnRH levels, premature activation of HPA
boys - before 9
girls - before 8
- in girls its more common to start puberty early whereas, with boys its more pathological
causes for central precocious puberty
- idiopathic
- CNS lesion
- `pituitary secreting GnRH tumours
- obesity
how can you test for precocious puberty
- basal LH and FSH increased
- girl will have high oestrogen
- boy will have high testosterone
what is peripheral precocious puberty
- not to do with GnRH
- high oestrogen and testosterone peripherally due to problems with ovaries, testicles, adrenal glands or pituitary gland
causes of peripheral precocious puberty
- increased androgen production due to cysts
- increased oestrogen due to HCG germ cell tumours
- increased HCG (pregnancy hormone) due ti hypothyroidism
reasons for delayed puberty
- constitutional growth delay
- malnutrition
- chronic disease
- hypogonadism
What products do the granulosa cells produce and the theca cells produce
granulosa: oestrogens
theca: androgens
by what age should the earliest sign of puberty be observed in boys and girls
9
11
does variation in the luteal phase account for varying duration of a woman’s menstrual period
no
which cells are the primary site of actions for LH in males
leydig
what are the functions progesterone
- inhibit FSH and LH
- initiation of the secretory phase
- increase basal body temp
where is inhibit produced
Sertoli cells in the male
granulosa cells in the female
