Postnatal Sexual Development & Puberty Flashcards
What are the periods of the reproductive system?
- fetal period
- prepubertal period
- reproductive period
- senescence period
Cycle diagram: puberty -> cyclicity -> pregnancy -> post-partum & lactation -> cyclicity -> this repeats until reproductive senescence
What is the fetal period?
- sex determination, differentiation, & tract development & growth
What is the prepubertal period?
- further growth & development, preparation for adulthood
What is the reproductive period?
- sexual activity resulting in conception & production of offspring is possible
What is the senescence period?
- reproductive function is no longer possible
How does hypothalamic development differ btwn females & males?
- key factor in differential sexual development of female vs male is ALPHA FETOPROTEIN (a-FP)
- in female embryos, a-FP BINDS E”2”, preventing it from passing the BBB & entering the brain, so hypothalamus is “FEMINIZED” & GnRH surge center develops
- in males, T freely enters brain since a-FP DOES NOT BIND T, so T in brain is aromatized into E”2” & hypothalamus is “DEFEMINIZED” & surge center does not develop
How does LH secretion differ btwn males & females?
- pubertal females have high amplitude surges of LH every several weeks & pulsatile releases in btwn surges
- pubertal males have sm LH pulses every 2-6 hrs throughout. testosterone immediately follows each LH pulse.
How does the secretion of GnRH change before and after puberty?
- before puberty in both males & females, GnRH neurons in both tonic & surge centers release low amplitude & low frequency pulses of GnRH
- after puberty in females, tonic center controls basal lvls of GnRH, but they are higher than in prepubertal females b/c pulse frequency increases. surge center controls preovulatory surge of GnRH
- males do not develop a surge center
What is the prepubertal period?
- prepubertal is period btwn birth & puberty
- period of secondary sexual growth & development
- varies from a few days or weeks to several months or years, depending on spp
- divided into early-, mid-, late-prepubertal, & peri-pubertal stages
- in the second half of this period, the rate of growth for reproductive tract increases over general body growth
- somatic & behavioural secondary sexual characteristics appear
- culminates w/ ONSET OF PUBERTY
What is the difference btwn puberty and maturity?
- sexual maturation, physical maturation, & fertility potential, all continue beyond onset of puberty
- puberty is defined differently in different spp & from different perspectives
- for animals, puberty = age @ which reproduction & parentage 1st becomes possible
- for humans, puberty = transition from childhood to adulthood
When is the onset of puberty in female animals:
- age @ 1st estrus (most practical, although most 1st ovulations are silent)
- age @ 1st ovulation (scientifically more accurate, but less practical; requires frequent palpation, ultrasonography, or measuring progesterone twice weekly)
- age @ 1st attainment of ability to support pregnancy (can be a while after either 1st estrus or 1st ovulation; for dairy cattle it is usually based on acquisition of a threshold body size
When is the onset of puberty in male animals:
- age when behavioral traits are expressed? - no (humping behaviours in puppies)
- age at first sperm produced? - sperm is produced quite a bit earlier than other signs
- age @ first ejaculate? - 1st ejaculate could or could not have sperm, in beginning sperm are not v good quality
- age when the ejaculate contains a threshold number of spermatozoa? (more practical & meaningful from a breeding standpoint; in bulls, minimum of 50 million sperm w/ @ least 10% motility; has a chance of achieving pregnancy but these numbers increase later)
What factors affect the rate of sexual maturation?
- genetics
- nutrition
- season
- breed/race
- sexual/ social associations
- environmental influences
- general health
- endocrine disruptors
How does genetic /spp variation affect the rate of sexual maturation?
- great variation among different spp
- even among a spp, ~50% of variations are due to genetics
How does breed/race affect the rate of sexual maturation?
- breeds w/in spp can vary greatly in age @ puberty
- genetic selection & geographical locations have affected age @ puberty
- variations also seen among human races, African-American girls reach puberty earlier (mean 12 yr; ~50% by age 9 vs. ~12% Caucasians)
- this mean age is 18 yr for girls in some tribes in high altitude regions in Asia
How does nutrition affect the onset of puberty in dairy heifers?
- age @ 1st parturition should be 24 months & weight 1200 lb
- nutrition is believed to be also a main source in reduction of age @ puberty in human populations
- overweight kids reach puberty earlier
- for each decade for >100 years, the age at puberty has dropped by 4 months
What is the effect of growth rate & male exposure on age at puberty?
- growth rate & bull exposure both decrease age @ puberty in beef heifers
- similarly in human populations, girls exposed to older males reach puberty earlier
How does group size & male exposure influence the age at puberty?
- larger groups reduce age at puberty
- effect of male exposure can be stronger than group size
- effect of male exposure on reducing age @ puberty does not have to be physical; visual or olfactory cues are sufficient
- girls living in dorms may have synchronized periods
- role of pheromones cannot be ruled out
How does season of birth influence the age of puberty?
- for seasonal breeders, age @ puberty is strongly influenced by season of birth
- spring-born ewe lambs reach puberty @ a much younger age but less synchronously
- fall-born ewe lambs take much longer time to reach puberty but are more synchronous
How does glucose affect GnRH release?
- in OVX ewe lambs, low amplitude LH pulses occurred hourly before treatment with 2-deoxyglucose (2DG - is a glucose oxidation inhibitor -> more glucose in the bloodstream, not being oxidized into cells)
- LH frequency & amplitude dropped after treatment
- but LH still could respond to exogenous GnRH
- so moment-to-moment regulation of GnRH happens when glucose is available
How do metabolism signals potentially effect GnRH?
blood glucose, fatty acids, & leptin levels, as indicators of nutritional & metabolic status, stimulate kisspeptin neurons that in turn stimulate GnRH neurons
How is puberty regulated by the endocrine system?
- gonads, pituitary, or even capacity to produce GnRH are not the limiting factors
- lack of high frequency & amplitude GnRH pulses is key
- role of an oscillator, GnRH pulse generator
- puberty depends on reactivation of GnRH secretion
What leads to the release of GnRH?
- gonadostat theory
- removal of other central inhibitors: ex: opioids, gamma amino butyric acid (GABA)
- activation of a central driver: ex: NE, excitatory AAs, neuropeptide Y, kisspeptin
What is the gonadostat theory?
- before puberty, hypothalamus is highly sensitive to negative feedback of E”2” & GnRH will not be released even w/ low lvls of E”2” present
- high lvl of estrogen receptors (ER) are present @ this time in the hypothalamus
- during the transition to puberty, negative sensitivity of the hypothalamus to E”2” is reduced, allowing higher release of GnRH
- lvls of hypothalamic ER are also reduced
How do we know that the hypothalamus is sensitive to estradiol before puberty?
- OVX causes LH pulses to be presents @ a similar frequency as in adults
- replacing E”2” in OVX animals prevents LH pulses during the 1st part of the prepubertal period
What are the hypothalamic E”2” receptor levels in a prepubertal animal?
high
What are the hypothalamic E”2” receptor levels in an early peripubertal animal?
medium
What are the hypothalamic E”2” receptor levels in a peripubertal animal?
low
What are the hypothalamic E”2” receptor levels in a pubertal animal?
varies
What is the E”2” feedback on LH in a prepubertal animal?
strongly negative
What is the E”2” feedback on LH in an early peripubertal animal?
less negative than in prepubertal animal
What is the E”2” feedback on LH in a peripubertal animal?
less negative than in an early peripubertal animal
What is the E”2” feedback on LH in a pubertal animal?
positive (when estradiol lvls pass a certain threshold, it then becomes positive feedback; follicle becomes so big it bursts open)
What are the E”2” levels in a prepubertal animal?
low
What are the E”2” levels in an early peripubertal animal?
low
What are the E”2” levels in a peripubertal animal?
medium
What are the E”2” levels in a pubertal animal?
high
What is the LH secretion pulse frequency in a prepubertal animal?
low
What is the LH secretion pulse frequency in an early peripubertal animal?
slight increase from prepubertal
What is the LH secretion pulse frequency in a peripubertal animal?
increasing from early peripubertal
What is the LH secretion pulse frequency in a pubertal animal?
LH surge observed
What is the reproductive tract score (1 = low development, 5 = high) in a prepubertal animal?
1
What is the reproductive tract score (1 = low development, 5 = high) in an early peripubertal animal?
2
What is the reproductive tract score (1 = low development, 5 = high) in an peripubertal animal?
3
What is the reproductive tract score (1 = low development, 5 = high) in an pubertal animal?
4 or 5
