Sexual Differentiation in Mammals
Role of Sex
Chromosomes
§
Genetic
sex (XX or XY) is determined by the type of sperm (X-bearing or Y-bearing) that
fertilizes the egg.
§
Early
gonads have potential to be either ovaries and testes for ~6 weeks
§
Sex-determining
region of the Y chromosome (SRY) is a gene producing a protein which causes the
middle of the neuter gonads to become testes.
§
If
testes develop, they begin to produce androgens like testosterone.
§
If
this gene is not present, the outside of the neuter gonads turn into ovaries.
Evidence
§
Removal
of SRY gene from Yà XY mouse develops as a
female
§
Add
SRY gene to X à XX mouse develops as a male
§After Gonad Development, the
remainder of sexual development depends on hormone environment.
§Normal sources of sex
hormones:
l Testes and ovaries
l Adrenal cortex
Organizational
Effects of Hormones
§
We
all begin with ducts, genitals and brains which can go either way (male or
female)
§
Differentiation
of male ducts, genitals, & brain depends on action of androgens
(testosterone & dihydrotestosterone (DHT)).
§
Female
differentiation requires no hormones.
§
In
fact, in males, development of female ducts must be actively inhibited by
release of another hormone: Mullerian
inhibiting hormone (MIH), usually in 2nd & 3rd month of
gestation.
Timecourse of
Differentiation
§
Differentiation
of gonads, genitals, ducts, and brain occurs at different (but overlapping)
time periods during gestation.
§
Example:
newly formed testes begin producing androgens about week 7-8 beginning the
masculinization of genitals but penis may not be visibly distinct until 3rd or
4th month & not complete until ~5th month.
Differentiation of
the Brain
§
Androgens
also have defeminizing and masculizing effects on developing brain.
§
Example:
Exposure to androgens “program” the hypothalamus for the fairly constant sex
hormone secretions seen in males vs the cyclic hormone secretion of females
§
There
is some evidence that masculinization of brain occurs later than
masculinization of body.
Sexually Dimorphic
Nucleus (SDN) of Preoptic Region of Hypothalamus
Structural
Differences in the Brain
§
Rats
- part of medial preoptic hypothalamus shows testosterone-related size
differences; ventromedial hypothalamus
important in females
§
“Sexually
dimorphic nucleus” is larger in males
and testosterone-treated females and smaller in females and castrated
males.
§
Simon
LeVay- 3rd interstitial nucleus of the anterior hypothalamus (INAH3) in humans
is larger in heterosexual males and smaller in females and gay males
§
Many
other regions show sex differences – e.g. females have more neurons in
Wernicke’s area and more axons in corpus callosum
Sex-Typical
Behaviors
Another of Mother
Nature’s Jokes
§
In
rats testosterone entering the brain is turned into estradiol by the process of
“aromatization” and estradiol, in turn, triggers the “masculinization” of the
brain (e.g. SDN and medial preoptic hypothalamus).
§
Maternal
estrogens present during pregnancy do not masculinize the brain because they
bind to alpha-fetoprotein and can’t get past the BBB.
§
However,
if excess estrogens are present, some do get into brain and do bias behavior in
a male direction.
Other Species
§
Decreased
perinatal testosterone in males or testosterone treatment of females has
induced same sex preferences and changes in sex-typical patterns of sexual
behavior in a wide range in species (rats, hamsters, ferrets, pigs, finches,
dogs, sheep etc.).
§
There
is correlational data from both men and women suggesting that early hormone
environment may also affect sexual orientation in humans.
Other Evidence for
a Biological Basis of Sex Orientation
§
Brothers
both homosexual?
l identical twins - 52% of
time
l fraternal twins - 22% of
time
l adopted brothers - 11% of
time
§
Sisters
both lesbian?
l Identical twins - 48%
l fraternal twins - 16%
l adopted sisters - 6%
Special Cases
§
XY
baby whose body can’t respond to androgens due to lack of androgen receptors:
“androgen-insensitivity” - baby has female genitalia, feels female
§
XX
baby exposed to more androgens than usual - body & brain masculinized;
“androgenization of females”, e.g. congenital adrenal hyperplasia or
adrenogenital syndrome
§
Lack
of DHT – female genitalia until puberty, then penis & scrotum develops.
Assumes male identity despite 10+ years of “female experience”
§
Homosexuality
also may be due to unusual prenatal hormone exposure (decreased masculinization
of brain), as may cases of gender identity disorder
§
Estimated
that 1 out of 100 has some degree of genital ambiguity.
Role of
Environment?
§
The
Case of John/Joan – unsuccessful rearing as female
§
DHT
deficiency – despite years as a female, easy switch to male identity at puberty.
§
In
both cases brain would be masculinized by early testosterone.
Summary
§
Early
in life hormones have an organizational
effect on body and brain.
§
The
amount and timing of hormones will have a life-long effect on physiology and
behavior.
§
Alterations
in hormones during critical sensitive periods of prenatal development are the
likely cause of variations in sex preference.
Puberty and Beyond
§
At
puberty both males & females need hormones to develop secondary sexual
characteristics.
§
From
that point on, hormones have an activational rather than organizational effect,
influencing sexual motivation rather than sex preference.
§
Sex
drive in females seems more closely related to androstendione than to estrogen-
ovariectomy does not decrease drive
Hormonal Drugs
§
Because
of the feedback mechanisms (body-> brain), taking hormone supplements
commonly leads to decreased production of natural hormones.
§
E.g.:Taking
testosterone like drugs (anabolic steroids) can cause atrophy of testes and
breast development due to conversion to estrogen