Puberty

Puberty
Presentation by:Angela Bauer-Dantoin, Ph.D.
Associate Professor, Human Biology and Women's Studies
University of Wisconsin Green Bay

* defined as transition period between sexually immature child to sexually mature, fertile adult
* involves growth and maturation of many tissues
* timing has much individual variability
* sequence of events doesn’t vary
* timing differs between the sexes:
o growth spurt begins ~2 yrs earlier in females
o females become fertile ~1 yr earlier than males

Typical sequence of pubertal events Females vs. Males
Ovarian changes during childhood
Testicular changes during childhood
* at birth, seminiferous tubules contain only Sertoli cells and spermatogonia
* Leydig cells are present at birth; disappear by 6 months of age
* Leydig cells reappear at 9-10 yrs. > spermatogenesis begins
* mature sperm not produced until 14-15th yr.
* spermatogenesis depends on descent of testes into scrotum
* body temperature prevents spermatogenesis
* descent >months 7-9 of gestation
* under influence of testosterone
* testes “guided” by gubernaculum (cord attached to testes, scrotum)

Cryptorchidism
* in 3-4% of newborn males, testes haven’t descended
* consequences:
o germ cells killed by normal body temperature
o increased incidence of testicular cancer
* usually corrected with surgery or gonadotropins / GnRH by 2 yrs

Hormonal changes at puberty
* 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
* GH secretion from pituitary also increases
* TSH (thyroid stimulating hormone) secretion from pituitary increases in both sexes:
o increases metabolic rate
o promotes tissue growth

Female hormonal changes
* surge of LH release initiates 1st ovarian cycle
* usually not sufficient to cause ovulation during 1st cycle
* brain and endocrine systems mature soon thereafter
* estrogen levels in blood increase, due to growing follicles
* estrogen induces secondary sex characteristics:
o growth of pelvis
o deposit of subcutaneous fat
o growth of internal reprod. organs, external genitalia
* androgen release by adrenal glands increases > growth of pubic hair, lowering of voice, growth of bone, increased secretion from sebaceous glands

Male hormonal changes
* LH and FSH release increases ~10 yrs. of age
* spermatogenesis; androgen secretion
* adrenals also secrete androgens
* androgens initiate growth of sex accessory structures (e.g. prostate), male secondary sex characteristics (facial hair, growth of larynx)
* androgens causes retention of minerals in body to support bone and muscle growth
* Sertoli cells also secrete some estrogen

Gonadostat hypothesis

* “Prepubertally, steroid hormone feedback operates at a very low setpoint; at puberty, set point increases.”
* if setpoint is raised at puberty, then LH and FSH levels would increase
* evidence: smaller amount of estrogen needed to lower gonadotropin levels in children (vs. adult women)
* evidence against hypothesis: in children without gonads, increase in LH, FSH levels occurs at normal age
* conclusion: even though puberty involves a change in setpoint, it is not the driving force for puberty

Hypothalamic Maturation Hypothesis

* “Activation of reproductive system at puberty is due to maturation of hypothalamus”.
* assume GnRH is driving force for puberty
* supporting evidence:
* in monkeys and humans, secretion of GnRH increases at puberty in absence of gonads
* give immature monkey GnRH > will show ovulatory cycles with estrogen, LH surges
* females with Turner’s Syndrome (XO) show normal LH, FSH onset
* tumors secreting GnRH can cause precocious puberty

Timing of puberty
* trend toward earlier puberty exists within W.
Europe and USA
* examination of lifestyle changes may give clues regarding mechanisms inducing onset
* 2 possible contributing factors: photoperiod and nutrition

Photoperiod
* refers to daylength (amount of light to which a person is exposed / day)
* electricity allows us to artificially extend daylength
* typically, we see only 8 hrs of darkness / day
* simulates summer photoperiod (“long days”)
* evidence against photoperiod hypothesis:
o intensity of domestic light not sufficient to affect neural mechanisms regulating GnRH secretion
o little evidence that photoperiod regulates human reproductive activity
Nutrition

* “Critical body weight must be attained before activation of the reproductive system”.
* even though age of menarche is decreasing, the average body weight of menarche remains the same
* earlier puberty due to improvement of nutrition, living conditions, healthcare?
* evidence supporting hypothesis:
o obese girls go through early menarche
o malnutrition is associated with delayed menarche
o primary amenorrhea common in lean female athletes
o “bodyfat” setpoint very noticeable in girls with fluctuating body weight due to anorexia nervosa

Puberty.ppt