Puberty and associated changes

Puberty

• Sexual maturity– Physical– Behavioral– Physiological

• Signs of puberty– Females

• Ovulation• Menstruation

– Males• Somewhat unclear• First ejaculation with viable sperms

– Minimum number required to achieve successful conception

• Puberty and fertility– First menstruation/ejaculation

• Not the sign of fertility• Signs of gonadal development

– Changes in gonadotropin secretion– Changes in gonadotropin responsiveness– Changes in steroid hormone production– Changes in gametogenesis

Physical changes

• Rapid growth– Phases

• Minimum growth• Peak growth velocity

(PGV)– Rapid growth

• Epiphseal fusion– End of growth

– Earlier in females

• Involvement of skeletal and muscular system– Regional differences in rate of growth

• Dimorphism

• Changes in body composition– Amount of fats within the body– Distribution of fats– Changes take place before puberty

• Earliest difference in males and females during puberty

• Endocrine regulation– Synergism between growth hormone (GH)

and steroid hormones (estradiol or testosterone)

• Increased secretion of GH• Increased responsiveness to GH

– GH receptors

Activation of gonads

• Development of secondary sex characteristics– Breasts– External genitalia– Hair distribution

• Pubic hair• Facial hair• Axillary hair

– Larynx (vocal cord) and laryngeal muscles• Deepening of voice

• Role of steroid hormones– Estradiol

• Breast development– Development of mammary alveolar tissue

• Female external genitalia

– Progesterone• Breast development

– Development of mammary ducts

• Role of steroid hormones– Androgens

• Male genitalia• Hair growth and distribution

– Male and female

• Larynx and laryngeal muscles

• Events associated with puberty– Sequential

• Critical for clinical examination

Endocrine control of puberty

• Secretion of gonadotropin– Very low during childhood– Increase in FSH and LH secretion

• Stimulation of follicular development• Stimulation of steroidogenesis

– Circadian pattern of gonadotropin secretion

• Secretion of prolactin– Increased in females

• Response to increased estradiol level

• Steroidogenesis– Increased testosterone production

• Follows LH pattern• Increase by 12 folds (0.2 ng/ml to 2.4 ng/ml)

• Production of estradiol– Increase during puberty in females

• Similar level with adult

• Production of adrenal steroids– Androgens (DHEA)

• Specific– No increase in glucocorticoids or mineralocorticoids

– Adrenarche• Starts around 8 years of age and continues until

13-15 years old

• Production of adrenal steroids– Androgens (DHEA)

• Specific– No increase in glucocorticoids or mineralocorticoids

– Adrenarche• Starts around 8 years of age and continues until

13-15 years old• Very high concentrations compared to gonadal

steroids

– Promotion of hair growth and distribution in both sexes

Changes in secretion of gonadotropins

• Two theories– Gonadostat theory

• Progressive maturation of feedback action of steroids

• Changes in responsiveness of the anterior pituitary gland to GnRH

– Central maturational role to the CNS• Hypothalamus

Gonadostat theory

• Prepubertal period– Ovaries

• Some antral/tertiary follicles– Estradiol production (very low)

– Negative feedback of estradiol on gonadotropin secretion

• Greater sensitivity to estradiol– Low threshold

• Pubertal period– Changes in sensitivity to estradiol

• Increased threshold• Decreased sensitivity• Increased responsiveness of pituitary gland to

GnRH stimulation• All of these are secondary response

– Leads to increased production of steroid hormones

• Increased secretion of gonadotropins

• Delayed appearance of positive feedback– Reproductive cycle during early puberty

• May not accompanied by ovulation

– Capacity to evoke LH surge• Requires repeated exposure to high

concentrations of estradiol• Ovulatory LH surge

– Later portion of puberty– Estradiol concentrations may be too low

GnRHHypothalamus

Pituitary gland

FSH

E (-)

GnRH

Hypothalamus

Pituitary gland

FSH

E (+)

Prepubertal period Puberty

Hypothalamus maturation model

• Pubertal activation of hypothalamus– Increased output of GnRH

• The CNS rather than gonadal axis• Changes in GnRH pulsatility

– No alteration in pituitary or ovarian response

• Independent of steroid exposure

Effects of environment

• Decreasing age to the first menstruation and male puberty– Western countries

• Advanced health care• Economics• Wellness

– Majority of women will experience menopause• Increased life expectancy• Other health issues

Role of light exposure

• Critical for animals with breeding seasons– Requires exposure to increasing length of

daylight (long-day breeders)• Horses

– Requires exposure to decreasing length of daylight (short-day breeders)

• Sheep

– Wild species

Role of nutrition

• Attaining critical body weight– Critical for initiation of the reproductive cycle– Domestic species (i.e. cattle)

• Body weight rather than actual age determines when animals reach puberty

– Humans• Same as the domestic species

– Activation of hypothalamus-pituitary-gonad axis

• Growth spurt

• Body size (weight and height)– May be critical

• Pregnancy

– Women with eating disorder (anorexia)• Maintenance of critical body weight (47 kg)• Irregular/cessation of menstrual cycle

• Actual body weight vs. fatness of the body– Lean people (i.e. athletes)

• Later initiation of menstrual cycle compared to moderately obese individuals

• Low body fat content– Irregular menstrual cycle– Cessation of menstrual cycle

• Metabolic signals– Leptin

• Hormone produced by fat cells• Satiety factor

– Decreases appetite– Energy levels within the body

• Unclear linkage