sex determination and differentiation (P2)

organizational/activational hypothesis

developed to explain how hormones guide behavioral sex differences

behavioral sex differences result from:

1. differential exposure to hormones that act early in development to organize/program neural circuitry underlying sexually dimorphic behaviors

2. differential exposure to sex steriod homones later in life that activate the neural circuitry previously organized 

young’s hypothesis

hormonal events early in development are important for adult reproductive behavior

young’s experiment

organizational: testosterone was given to pregnant guinea pigs (at levels that did not masculinize the genitalia of female offspring) and a control group (no hormone administration)

activational: as adults, hormone-treated and control female offspring were ovarectomized and given estrogen/progesterone (to stimulate female sexual behavior) and testosterone (to stimulate male sexual behavior)

young’s results

females exposed to androgens prenatally - behaved liked males

1. estrogen/progesterone administration: reduced female sexual behaviors 

2. testosterone administration: exhibited male sexual behaviors

control females - behaved like females

1. estrogen/progesterone administration: exhibited female sexual behaviors

2. testosterone: no effect on sexual behaviors 

young’s conclusion

potential for masculine or feminine behavior is organized by early exposure to androgens

• distinction can be made between pre-natal actions of hormones in causing differentiation or organization of the neural substrates for behavior/actions of hormones in adulthood in causing activation of these substrates

sensitive/critical period

a period of time when an animal is maximally sensitive to organizing effects of hormones to permanently change morphology/behavior

• once it is over, hormones do not have organizing effect

presence of androgens

permanently organizes the brain to permit later demonstrate masculine behavior in response to activational effects of hormones

• brain is masculinized and defeminized

absence of androgens 

the brain and behavior develop in a female typical fashion 

• brain is demasculinized and feminized 

development

sensitive period

relatively permanent

characteristics of organization o

organization

hormones organize or program the neural circuitry underlying sexually dimorphic behaviors

adulthood

no sensitive period

transitory

characteristics of activational

activational

hormones act on the neural circuitry that was previously organized 

aromatization

mechanism in rodents that masculinizes the brain

• conversion of testosterone to estrogen/estradiol

alpha-fetoprotein

binds to estrogen and prevents maternal estrogen from mascuilinizing the female brain

cyclic 

pattern of GnRH in females; without androgen exposurem the neural circuitry involved in GnRH surge generation is spared 

tonic

pattern of GnRH in males; exposure to androgens in development abolishes the potential to generate GnRH surges

interuterine environment

maternal stress

materal care

environmental endocrine disruptors

environmental influences on mammalian sexual development

interuterine environment

in female rat pups’ position in the uterine horn can subtly impact her physiology and behavior as an adult → 2M females are more aggressive less attractive to males, have longer ovarian cycles

maternal stress

male offpring of mothers stressed during pregnancy act more feminine, possibly due to androgen production suppression 

maternal care

males require maternal licking to develop normal adult sexual behavior

females who are licked more as pups are more attentive mothers as adults

endocrine disrupting chemicals

mimic the effects of hormones or disrupt hormonal systems can interfere with sexual differentiation

atrazine

commonly used herbicide; affect reproductve development in fish, amphibians, reptiles and mammals

estrogen 

in birth control pills are excreted through urine, which enters the waste system. estrogen is too small to be filtered out of the sewage system, so it remains in drinking water

bisphenol A

BPA; used in synthesis of plastics; has estrogenic effects

triclosan

antibacterial that interferes with estrogen, androgen and thyroid hormones, increases the risk for breast cancer and decreases male fertility

PFAS

endocrine disrupting properties and are linked to neurodevelopment disturbances