chapter 6 - puberty

puberty

process that occurs over time, not an event

• onset depends on the ability of specific hypothalamic neurons to produce GnRH in sufficient quantities to promote and support gametogenets

  • females hypothalamic GnRH neurons develop ability to respond to estradiol

hypothalamus

inherently female

teststerone

defeminizes the hypothalamus during embryogenesis and “eliminates” the GnRH surge center in males

Females

no testosterone

• develops GnRH Surge center in hypothalamus

• fetal estradiol in female is unable to cross blood-brain barrier of hypothalamus

alpha-fetoprotein

binds estradiol and prevents it from crossing the blood brain barrier

• allows surge center to develop in females

• glycoprotein synthesized by embryonic yolk sac and later the fetal liver

males

testosterone crosses blood brain barrier

• converted to estradiol in brain

• estradiol “defeminizes” the hypothalamus

  • minimizes surge center function

• testosterone is aromatized into estradiol

female hypothalamus

contains surge center and tonic center

male hypothalamus

does not contain surge center

• no surge of LH

• Maintains consistent day in and day out in pulsatile patters of secretion

Puberty

ability to accomplish reproduction successfully

• occurs over time, not a single event

• fundamental requirement = secretion of GnRH at the appropriate frequency and quantities to stimulate gonadotropin released by the pituitary

age at first estrus

the age that the female becomes sexually receptive and displays her first estrus

age at first ovulation

age when first ovulation occurs (hard to determine)

onset of puberty

• age at first estrus

• age at first ovulation

• age at which a female can support pregnancy without deleterious effects

onset puberty males

• age when behavioral traits are expressed

• age at first ejaculation

• age when spermatozoa first appear in ejaculate

• age when the ejaculate contains a threshold number of spermatozoa

age when behaviors expressed

most species acquire reproductive behavioral traits before acquiring ability to ejaculate and produce spermatozoa

age at first ejaculation

requires closely coordinated development of nerves, muscles, and secretion of seminal fluid from accessory sex glands

• substantially precedes ability to produce sufficient spermatozoa to achieve fertilization

spermatozoa first appear in ejaculate

male acquire ability to produce seminal fluid and to ejaculate before spermatozoa are available to be ejaculated

ejaculate contains threshold number of spermatozoa

minimum number of spermatozoa required for fertilization

• minimum seminal characteristics required to achieve pregnancy following copulation

males

have a small LH episode that occurs every 2-6 hours

• testosterone secreted after each LH episode

females

have high amplitude preovulatory episodes of LH once every several weeks and basal pulsatile episodes between the large preovulatory surges

females

must reach threshold body size before puberty can be achieved

external factors influencing puberty

1. season during which animal is born

2. photoperiod that the animal is experiencing during the onset of puberty

3. presence or absence of the opposite sex during the peripubertal period

4. density of the groups

breed

can influence age of puberty

limiting factor to puberty

the failure of the hypothalamus to secrete sufficient quantities of GnRH to cause gonadotropin release

surge center

before ovulation can occur, full neural activity of this must be achieved

male onset

brought about because of the decreased hypothalamic sensitivity to negative feedback by testosterone/estradiol

pubertal female

the surge center is quite sensitive to the positive feedback of estradiol

• surge center cannot release “ovulatory quantities” of GnRH because ovary cannot secrete high levels of estradiol

fatness

certain degree is required for the onset of puberty in the female

• reproduction is low priority because it is non-essential as a neonate

hypothalamic neurons

regulate GnRH secretion detect “moment to moment” changes in blood glucose and fatty acids

• “moment to moment” regulation of GnRH occurs only when significant glucose is available for metabolism

leptin

hormonal peptide secreted by adipocytes (fat cells)

• amount is directly related to the amount of fat in the body

• receptors are found in the liver, kidney, heart, skeletal muscle, and pancreas

• some receptors present in the anterior lobe of the pituitary and hypothalamus

  • important signal notifies key hypothalamic neurons that influence GnRH secretions that nutritional status is adequate because threshold degree of “fatness” has been achieved

kisspeptin neurons

may act directly on GnRH neurons

• in the hypothalamus

• send dendritic arborizations into hypothalamic areas containing high population of GnRH cell bodies

delay onset of puberty

small groups of gilts housed together

hastens onset of puberty

the presence of males