2. Sexual Determination and Differentiation Flashcards
Girls and boys are different
Reproductively and behaviorally
What causes these differences?
Nature AND nurture
Biology AND environmental factors (learning, society, culture)
Strategies to explore questions about the origin of human behavior
Studying sexual differentiation in animal models
Studies of people or animals with anomalous development
Sampling amniotic fluids and correlating hormone levels with later behaviors
Looking for universal commonalities in the behavior of all children
Where else do we see sex differences?
Aggression
Cognition
Mental health disorders
Cardiovascular and immune function
The gonads serve 2 purposes
Production of gametes
Production of steroid hormones
Steroid hormones are required for:
Gamete production, development of secondary sex characteristics, behaviors that bring gametes together
Male gonads: testes
Primary steroid produced is testosterone, which is an androgen
Testosterone production begins as soon as the testes are formed
Female gonads: ovaries
Primary steroids produced are estrogens (ex: estradiol) and progestins (ex: progesterone)
Steroids are not produced from the ovaries until puberty
Sex determination
The process by which the decision is made for a fetus to develop as a male or female
Chromosome driven
Sex differentiation
The process by which individuals develop either male or female bodies and behaviors
Hormone driven
Mammalian sexual determination
If the sperm that enters the egg has a Y chromosome, the offspring is male; if an X, the offspring is female
“Male” and “Female” hormones
No steroid is found exclusively in either males or females; rather, the two sexes differ in the proportion of these steroids
Levels of sex determination/differentiation
When the fetus is first made
6 weeks of gestation- gonads first made
They’re bipotential: the same gonad can become either testes or ovaries
SRY gene
The Y chromosome has the SRY gene (sex-determining region of the Y chromosome) which codes for testes
Without SRY gene, an ovary forms
Sex differentiation driven by hormones
Hormones secreted by gonads, mainly from the testes, direct sexual differentiation of internal and external sex organs
Hormonal sex differentiation in males
In males, the Wolffian ducts develop into the epididymis, vas deferens, and seminal vesicles, and the müllerian system shrinks
Hormonal sex differentiation in females
In females, the müllerian ducts develop into the oviducts, uterus, and vagina– the Wolffian system degenerates
2 hormones produced and released early on in males:
Anti-müllerian hormone (AMH) causes regression of the müllerian system
Testosterone masculinizes the internal organs by promoting development of the Wolffian system
Testosterone gets converted into…
Dihydrotestosterone (DHT) via the enzyme 5a-reductase masculinizes external structures
Testosterone aromatized to ________ via aromatase– masculinizes the brain
Estradiol
Chromosomal anomalies
Turner’s syndrome
Klinefelter’s syndrome
Turner’s syndrome
A person only has one sex chromosome, a single X
The individual develops as a female, due to lack of SRY gene, with abnormal ovaries (no steroid hormone production or eggs)
Estrogen treatment induces breast growth and a female-typical puberty
Klinefelter’s syndrome
A person has an extra chromosome, XXY
A masculine body develops because of the Y chromosome
Testes and penis are abnormally small
Low testosterone production; little to no sperm are produced
Hormonal anomalies
Congenital adrenal hyperplasia (CAH)
Androgen insensitivity syndrome
Congenital adrenal hyperplasia (CAH)
Can happen in XX or XY individuals
Lack enzyme (21-hydroxylase) that produces cortisol, results in increased amount of androgens output from adrenal gland
External genitalia is partially masculinized because of exposure to increased DHT during development
Internal organs are mostly normal
The females exhibit more masculine behaviors
Affected individuals need to keep taking cortisol throughout their lives
Androgen insensitivity syndrome
Occurs in XY people with defective androgen receptors (so, androgens cannot have action)
Will people with androgen insensitivity syndrome develop testes or ovaries?
Testes, because of the SRY gene on the Y receptor
What hormones will be produced in people with androgen insensitivity syndrome? Which will act?
WILL act: AMH and estradiol (because they’re not androgens)
WON’T act: testosterone and DHT (because they’re androgens)
What will be the phenotype of an individual with androgen insensitivity syndrome?
Female, because the testes stay internal and DHT cannot create male genitalia
Internalized testes, no eggs, shallow vagina
T is low, so estradiol stays low and brain is not masculinized
The women use hormone therapy
Sexual dimorphism
The condition in which males and females exhibit marked sex differences in appearance
POA in hypothalamus is important for reproductive behavior in males
masculinization or feminization of the brain
Estradiol masculinizes the brain during early development
This occurs (primarily) in males, since ovaries don’t produce hormones until puberty
activational effect of hormones
Temporary
Occurs in adulthood
organizational effect of hormones
Structural changes in brain
Permanent
Occurs before brain matures
Critical period
Females do not become masculinized during fetal development because…
Ovaries are quiescent until puberty
Maternal estrogen in developing females does not enter the brain
α-fetoprotein binds the estrogens, keeping them from crossing into the brain, thus preventing aromatization and masculinization
