Puberty Flashcards
What is puberty
Puberty is the preparation for sexual maturity, to enable us to reproduce. It creates sexual dimorphism; in other words it causes a distinct difference between male and female sexes.
When is the onset of puberty?
There are many environmental factors that influence the onset of puberty. The pineal gland is thought to be an important influence on puberty, as disorders of this gland can cause early onset (precocious) puberty. Body weight is a key factor in the onset of puberty in girls, as well as nutrition and leptin (released from adipose tissue). Low body weight can cause cessation of menstruation, even after puberty has occurred. The age at which puberty occurs can vary, however importantly, the sequence of which these changes occur is consistent.
What are primary secondary characteristics
These are the sexual characteristics at birth, i.e. before puberty has begun. This includes the anatomy of the internal and external genitalia.
What are secondary sexual characteristics
These are the characteristics that develop after puberty; i.e. pubic hair, breast or genital development or enlargement, and menstruation in females.
What is the sequence of events in boys
Sequence of events: boys As mentioned, boys typically begin puberty slightly later than girls, around age 10-14.
Genital development The first sign of puberty occurring in boys is testicular volume enlargement.
Pubic hair growth As with girls, under the influence of testosterone, pubic hair starts to develop.
Spermatogenesis This process begins next, however it is difficult to determine exactly when the timing of this occurs. Growth spurt This happens much later in boys, however due to the lower levels of oestrogen, boys typically grow a lot taller than girls as the epiphyseal growth plates are slower to close. Genital enlargement The testes and external male genitalia continue to increase in size and volume until adult external genitalia has developed.
Pubic hair Similarly to girls, the coarseness and pattern of pubic hair will change until an adult distribution has occurred.
What is the tanner scale
A scale to asses the stages o puberty
Describe accelerated somatic growth in both girls and boys
• Occurs in both sexes
– Depends on growth hormone + IGF-1) & sex steroids in both sexes
– Earlier and shorter in girls - less testosterone
– Men larger because growth spurt longer and slightly faster
• Genital development in boys depends on testosterone
• Ended in both sexes by epiphyseal fusion
– Oestrogen closes epiphyses earlier in girls
What is an important factor for the onset o puberty
• In western countries puberty now much earlier
– Average age of menarche was ~17 years (C 1800)
– Now < 13 years
– In girls, critical weight 47 kg
– significantly weight loss = reproductive cycles cease
• Therefore Body weight important factor: nutrition • Leptins may be involved in signalling
What is the age of start o puberty and the first sign
Boys: 10-14, testicular volume up to 4ml
Girls: 9-13, brast buds
What does the hypothalamus-pituitary unit regulate and control
• The output of the hypothalamus-pituitary unit regulates the function of
– Thyroid (hypothalamic–pituitary–thyroid axis HPT)
– Adrenal (hypothalamic–pituitary–adrenal axis HPA)
– reproductive glands (hypothalamic–pituitary–gonadal axis HPG)
• also controls
– somatic growth, lactation, milk secretion and water metabolism
What initiates puberty and what are the first phenotypic changes seen
• Puberty initiated by the brain (H) • Nocturnal GnRH pulsatility (LH secretion) precedes phenotypic changes by several years • Phenotypic changes (breast buds/testicular enlargement seen)
What is the onset of puberty associated with
• Onset of puberty associate with
steady rise in FSH & LH secretion • Most parts of the reproductive
system can work before normal age
of puberty
– Precocious puberty – But don’t because hormone
levels low – Due to low GnRH secretion
What are the characteristics of GnRH
• Secretion in pulses tied to internal biological clock
– Synchronized external signals i.e. by light
• Act on specific membrane receptors
• Transduce signals via second messengers
• Stimulate release of stored pituitary hormones
• Stimulate synthesis of pituitary hormones
• Stimulates hyperplasia and hypertophy of target cells
• Regulates its own receptor
What is GnRH-1 gene
Gonadotrophin-releasing hormone (GnRH-1)
• GnRH -1 gene
– primarily responsible for mammalian GnRH (there are other vertebrate
variants of this gene)
– The GnRH-1 gene is exclusively expressed in a discrete population of
neurons in the hypothalamus
– Treating prepubertal primates with pulsatile GnRH alone induces
puberty
– Lack of gonadotrophin synthesis and secretion and reproductive
development occurs if GnRH is blocked – Treatment of infertile male HPG-mice with synthetic GnRH induces
spermatogenesis and in females similar treatment causes ovarian
maturation and can lead to established pregnancy
– ∴ critical role of GnRH in reproductive maturation established
Describe the secretion of GnRh and what it stimulates
• Secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus stimulates the anterior pituitary gland gonadotrophs to secrete two gonadotropic hormones: – follicle-stimulating hormone (FSH) – luteinizing hormone (LH)
• One releasing hormone – GnRH 10 amino acid peptide • GnRH release is pulsatile – Every 1-3 hrs – Intensity of GnRH stimulus is affected by • Frequency of release • Intensity of release – GnRH travels to pituitary in hypophysial portal system
What is adenohypophysis
• Anterior pituitary
– Not nervous tissue
– connected to the hypothalamus by the superior hypophyseal artery
– an amalgam of hormone producing glandular cells.
• The anterior pituitary produces six peptide hormones:
– prolactin
– growth hormone (GH)
– thyroid stimulating hormone (TSH)
– adrenocorticotropic hormone (ACTH) • FSH, LH & Prolactin most significant for reproduction
What initiates gonadal development
• in young children, LH and FSH levels insufficient to initiate gonadal
function
• between 9-12 yrs., blood levels of LH, FSH increase
• amplitude of pulses increases, especially during sleep
• high levels of LH, FSH initiate gonadal development
Levels so low in young children . Once gnrh released, then fsh and lh increase.t here hormones go on to then stimulate gonadal function
Describe sleep dependent nocturnal rise in LH
• In adolescent boys the sleep related LH increase • Stimulates a nocturnal rise of Testosterone • Androgen levels increase could account for some of the early pubertal changes seen in males • Similar pattern seen in females with concomitant increase in oestrogen
Describe the negative feedback loop for the HPG axis
See slide
What does LH stimulate in males
• LH stimulates Leydig cells in testis • Produce Steroid hormone ( from cholesterol ) Testosterone • Greatest amount produced from testis • Once production starts in the medium long term testosterone levels remain constant • There is an effect of : – Circadian rhythm • Highest early morning – Effects of environmental stimuli • Both driven by brain
Describe the testicular lobules
• Each lobule contains 1-4 tightly coiled tubes –(Seminiferous tubules (ST)) • Each ST ~60 cm long – 90% of the testicular volume – location for spermatogenesis • In between there is loose connective tissue and blood vessels • Interstitial tissue contains interstitial cells (Leydig cells) – Produce testosterone – cells function independently of ST • ST need functioning Leydig cells
What types of cells are thes seminiferous tubules lined by
Lined by complex epithelium made
of 2 cell types
– Supporting cells (Sertoli Cells)
– Spermatogenic cells/germ cells
Describe Sertoli cells function
Sertoli cells
– provide nutrition and hormonal support to germ cells allowing sperm formation
– Sensitive to FSH (increase sperm
production)
– Secreted inhibin (-) feedback on AP FSHTight junction
What do fsh and lh act on in females
• Act primarily on gonads via Gα s PCR →adenylate cyclase
• Female: Target cells Ovarian granulose cells, Theca interna,
– Stimulate sex hormone
synthesis (steroidgenesis)
• ( i.e. oestrogen, progesterone, inhibin)
– Control gamete production
(folliculogenesis and ovulation)
What do theca and granulosa cells respond to
- Granulosa cells responding to FSH
* Theca cells responding to LH
What are the effects of oestrogen an dprogesterone on GnRH in females
• Female
– Moderate titres of oestrogen reduce GnRH secretion
• Negative feedback
– High titres of oestrogen alone promote GnRH secretion
• Positive feedback
• LH ‘surge’
What are the effects of oestrogen and progesterone on fsh and lh in females
• Female
– Progesterone increases inhibitory effects of moderate oestrogen
– Progesterone prevents positive feedback of high oestrogen
• No LH surge – Oestrogen reduces GnRH per pulse, progesterone frequency of pulses
What are the effects of inhibin on FSH
- From granulosa cells of corpus luteum
- Inhibits the secretion of FSH (same as male)
- Has a small inhibitory effect on LH
What are the hormonal changes resulting in a growth spurt
GH secretion from pituitary • Increases TSH • increases metabolic rate • promotes tissue growth • ↑androgens = retention of minerals in body to support bone and muscle growth • = Growth spurt
What are the effects of lepton
• Leptin is an adipocyte-derived protein hormone
• Signals information about energy stores to CNS
• important role in regulating neuroendocrine function.
– reproductive dysfunction associated with leptin deficiency
– leptin can accelerate the onset of reproductive function
• Leptin has pulsatile release pattern significantly associated with the variations
in LH.
• leptin can regulate GnRH levels, and its secretion may, in turn, be influenced
by gonadal steroids but appears to be independent of LH control
What is the sequence of events in girls
Typically age 9-13
Thelarche - breast bud development - first sign of puberty
Pubic hair Pubic hair starts to develop under the influence of testosterone, in both boys and girls. This is the next stage.
Growth spurt The growth spurt happens next over a wide age range. The levels of oestrogen are important in fusion of the epiphyseal growth plate, which will result in the end of the growth spurt. Growth hormone (GH) has a key role in both boys and girls to cause the growth spurt to occur.
Menarche Menarche is the initiation of the menstrual cycle, with a girl
