Reproductive System Week 3 Flashcards
Define the terms:
- Thelarche
- Pubarche
- Menarche
- Adrenarche
Thelarche - development of breast
Pubarche - development of axillary and pubic hair
Menarche - first menstrual period
Adrenarche - onset of an increase in the secretion of androgens
Define puberty
The series of events leading to sexual maturity
It is a time of accelerated growth, skeletal maturation, development of secondary sexual characteristics and achievement of fertility
Why has the age at which puberty occurs dropped significantly over the past 150 years?
Improved nutrition and living conditions
When are primary sexual characteristics established?
Before birth
What can the sequence of events in puberty be described as?
Age variable - between individuals and between different sexes
Order consistent
Describe briefly the events that occur in puberty
Accelerated somatic growth
Maturation of primary sexual characteristics (gonads and genitals)
Appearance of secondary sexual characteristics (pubic and axillary hair, female breast development, male voice change)
Menstruation and spermatogenesis begin - marks end of puberty
What triggers the start of puberty?
Maturation of the hypothalamic pituitary gonadal axis
Hypothalamic pulsatile release of GnRH
Increased pituitary gonadotropin in response to pulsatile GnRH is the endocrinologist hallmark of puberty (remember LH responds to pulsativity but FSH responds to intensity of GnRH)
If most parts of the reproductive system are able to work before the start of puberty, why dont they under normal circumstances?
Because hormone levels are too low due to low GnRH secretion
Why is the first ejaculation more difficult to date precisely than the first menarche?
Happens nocturnally
Do the menarche and the first ejaculation mean that the individual is fertile?
No, in early puberty the ovary does not ovulate and the ejaculate consists of small quantities of seminal plasma lacking spermatozoa
Just a sign that the gonads have awakened - beginning to assume adult levels of activity
Why do boys end up taller than girls on average?
Because they start their growth spurt later - they are taller when they start the growth spurt
What is the growth spurt?
An acceleration, followed by a deceleration of growth in most skeletal dimensions
The height gain between take off and cessation is similar in boys and girls and therefore the height difference is due mostly to the difference in height at take off rather than in height gained in the spurt
How many years later on average do boys start their growth spurt compared to girls?
2 years
What causes the growth spurt to end?
Closure of the epiphyseal growth plates
What effect does oestrogen have on the growth plates?
Causes them to close earlier in girls
What hormones does the growth spurt depend on?
Sex hormones –> androgens –> retention of minerals by body to support bone and muscle growth
Growth hormone –> secreted from pituitary –> increases TSH –> increases metabolic rate –> promotes tissue growth –> increased androgen release
What other changes occur in body composition during puberty?
Men get more lean mass (1.5 times women)
Women get twice as much body fat as men
Skeletal mass of men 1.5 times that of women
What is responsible for the greater average strength of men compared to women?
Greater number of larger muscle cells - anabolic effects of androgens
At what ages in women and men do the earliest changes in body composition at puberty occur?
6 - women (start getting more body fat)
9 - men (start getting more lean mass)
What secondary sexual characteristics are regulated by oestrogen?
Growth of breast and female genitalia
Maturation of internal reproductive organs
Growth of pelvis
Deposit of subcutaneous fat
What secondary sexual characteristics are regulated by androgens secreted from ovaries and adrenal gland?
Growth of female pubic and axillary hair
Bone growth
Increased secretion from sebaceous glands
Which secondary sexual characteristics are regulated by testicular and adrenal androgens?
Genitalia
Body hair
Deepened voice - enlarged larynx and laryngeal muscles
Growth of sex accessory structures (prostate)
What is the earliest detectable endocrine change associated with puberty?
Progressive increase in adrenal androgens - particularly DHEA and DHEAS (Adrenarche)
Between what ages does Adrenarche occur?
8-15 years
When does most gonadotropin secretion initially occur?
At night
What happens to levels of LH as an individual progresses through puberty?
Initially most secretion occurs at night
In late puberty - daytime pulses also increase but still less than those occurring at night
Until finally the adult pattern of higher basilar levels achieved - no pulsing variation throughout the day
How do testosterone levels change over puberty?
Follows LH in boys
Smaller increases in girls
How do oestrogen levels change over puberty?
In girls, rise consistently from low levels to the levels seen in mature women
Smaller rises in boys
What is thought to stimulate the increased GnRH release that occurs during puberty?
Neuropeptide transmitter Kisspeptin 1 from arcuate nuclei
Released in pulses that Mach the pulsatile secretion of GnRH
What are two hypotheses for the decreasing age of puberty?
Have to reach a critical weight:
- (47kg women and 55kg men) therefore improved nutrition, living conditions, healthcare has contributed
- leptins may be involved - secreted by white adipocytes - marker of fat content - rises during puberty - mutations in gene or receptor gene fail to enter puberty - treatment with leptin can overcome this - but unable to trigger precociously - maybe a background permissive signal essential for triggering signal to operate
- T3 and T4 may also have permissive role
- malnutrition associated with delayed menarche
- primary amenorrhoea common in lean female athletes
- body fat set point noticeable in girls with fluctuating body weight due to anorexia nervosa
Photoperiod:
- light:dark cycle known to affect reproduction in animals
- pineal gland - melatonin
- western world - electricity - prolonged daylight
- little evidence
- pineal tumours can affect puberty
What is the first detectable phenotypic change in puberty?
Breast enlargement
Testicular enlargement
When do levels of LH and FSH increase and what does this cause?
Between 9-12
Amplitude of pulse increases - especially at night
Initiates gonadal development
When does the LH and FSH increase occur in males?
~10
How is pubertal development classified?
Tanner standard :
Girls - breast, pubic hair, axillary hair, menarche
Boys - testicular volume, penis enlargement, pubic hair, axillary hair, spermarche
Monitoring of pubertal growth acceleration (2-3 times greater than prepubertal)
What age does puberty begin in girls?
9-13