Sexual Development
• Production of Gametes and Fertilization
• A person’s genetic sex is determined at the time of fertilization of the ovum by the
father’s sperm.
• Twenty-two of the twenty-three pairs of chromosomes determine the organism’s physical
development independent of its sex.
• The last pair consists of two sex chromosomes, which contain genes that determine
whether the offspring will be a boy or a girl.
• sex chromosome
• The X and Y chromosomes, which determine an organism’s gender. Normally, XX
individuals are female, and XY individuals are male.
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sexually dimorphic behavior
A behavior that has different forms or that occurs with different probabilities or under
different circumstances in males and females.
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Sexual Development
• Gonads
• Sry
• The gene on the Y chromosome whose product instructs the undifferentiated fetal
gonads to develop into testes.
• Both sexes have a pair of identical undifferentiated gonads, which have the potential of
developing into either testes or ovaries.
• The factor that controls their development appears to be a single gene on the Y
chromosome called Sry (sex-determining region Y).
• This gene produces a protein that binds to the DNA of cells in the undifferentiated gonads
and causes them to become testes.
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Sexual Development
• Internal Sex Organs
• Early in embryonic development the internal sex organs are bisexual; that is, all embryos
contain the precursors for both female and male sex organs.
• However, during the third month of gestation, only one of these precursors develops; the
other withers away.
• The precursor of the internal female sex organs, which develops into the fimbriae and
Fallopian tubes, the uterus, and the inner two-thirds of the vagina, is called the Müllerian
system.
• Müllerian system
• The embryonic precursors of the female internal sex organs.
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Sexual Development
• Internal Sex Organs
• Müllerian system
• The embryonic precursors of the female internal sex organs.
• Wolffian system
• The embryonic precursors of the male internal sex organs
• anti-Müllerian hormone
• A peptide secreted by the fetal testes that inhibits the development of the Müllerian
system, which would otherwise become the female internal sex organs.
• androgen (an dro jen)
• A male sex steroid hormone. Testosterone is the principal mammalian androgen.
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Sexual Development
• Internal Sex Organs
• testosterone (tess tahss ter own)
• The principal androgen found in males.
•
• dihydrotestosterone (dy hy dro tess tahss ter own)
• An androgen, produced from testosterone through the action of the enzyme 5
reductase.
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Sexual Development
• Internal Sex Organs
• androgen insensitivity syndrome
• A condition caused by a congenital lack of functioning androgen receptors; in aperson
with XY sex chromosomes, causes the development of a female with testes but no
internal sex organs.
• “Nature’s impulse is to create a female.” People with Turner’s syndrome have only one
sex chromosome: an X chromosome.
• Turner’s syndrome
• The presence of only one sex chromosome (an X chromosome); characterized by
lack of ovaries but otherwise normal female sex organs and genitalia.
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Sexual Development
• Sexual Maturation
• gonadotropin-releasing hormone (GnRH) (go nad oh trow pin)
• A hypothalamic hormone that stimulates the anterior pituitary gland to secrete
gonadotropic hormone.
• gonadotropic hormone
• A hormone of the anterior pituitary gland that has a stimulating effect on cells of the
gonads.
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Sexual Development
•
Sexual Maturation
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follicle-stimulating hormone (FSH)
• The hormone of the anterior pituitary gland that causes development of an ovarian follicle
and the maturation of an ovum.
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luteinizing hormone (LH) (lew tee a nize ing)
• A hormone of the anterior pituitary gland that causes ovulation and development of the
ovarian follicle into a corpus luteum.
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estradiol (ess tra dye ahl)
• The principal estrogen of many mammals, including humans.
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estrogen (ess trow jen)
• A class of sex hormones that cause maturation of the female genitalia, growth of breast
tissue, and development of other physical features characteristic of females.
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Hormonal Control of Sexual Behavior
• Hormonal Control of Female Reproductive Cycles
• menstrual cycle (men strew al)
• The female reproductive cycle of most primates, including humans; characterized by
growth of the lining of the uterus, ovulation, development of a corpus luteum, and (if
pregnancy does not occur), menstruation.
• estrous cycle
• The female reproductive cycle of mammals other than primates.
• Estrus means “gadfly”; when a female rat is in estrus, her hormonal condition goads
her to act differently than she does at other times.
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Hormonal Control of Sexual Behavior
• Hormonal Control of Sexual Behavior of Laboratory Animals
• Oxytocin is a hormone produced by the posterior pituitary gland that contracts the milk
ducts and thus causes milk ejection in lactating females.
• oxytocin (ox ee tow sin)
• A hormone secreted by the posterior pituitary gland; causes contraction of the smooth
muscle of the milk ducts, the uterus, and the male ejaculatory system; also serves as
a neurotransmitter in the brain.
• lordosis
• A spinal sexual reflex seen in many four-legged female mammals; arching of the back
in response to approach of a male or to touching the flanks, which elevates the
hindquarters
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Hormonal Control of Sexual Behavior
• Effects of Pheromones
• A study by Russell (1976) found that people were able to distinguish by odor between Tshirts that they had worn and those previously worn by other people.
• They could also tell whether the unknown owner of a T-shirt was male or female.
• Thus, it is likely that men and women can learn to be attracted by their partners’
characteristic odors, just as they can learn to be attracted by the sound of their voice.
• In an instance like this, the odors are serving simply as sensory cues, not as
pheromones.
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Hormonal Control of Sexual Behavior
Activational Effects of Sex Hormones in Women
• Men initiated sexual activity at about the same rate throughout the woman’s cycle,
whereas sexual activity initiated by women showed a distinct peak around the time of
ovulation, when estradiol levels are highest. (See Figure 10.10.)
• Bullivant et al. (2004) found that women were more likely to initiate sexual activity and
were more likely to engage in sexual fantasies just before and during the surge in
luteinizing hormone that stimulates ovulation.
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Hormonal Control of Sexual Behavior
•
Sexual Orientation
•
Prenatal Androgenization of Genetic Females
•
The secretion of androgens begins prenatally; thus, the syndrome causes prenatal
masculinization. Boys born with CAH develop normally; the extra androgen does not seem to
have significant effects.
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congenital adrenal hyperplasia (CAH) (hy per play zha)
• A condition characterized by hypersecretion of androgens by the adrenal cortex; in
females, causes masculinization of the external genitalia.
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As we saw, genetic males with androgen insensitivity syndrome develop as females, with
female external genitalia—but also with testes and without uterus or Fallopian tubes
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If an individual with this syndrome is raised as a girl, all is well.
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Hormonal Control of Sexual Behavior
Sexual Orientation and the Brain
• A study by Roselli et al. (2004) discovered a sexually dimorphic nucleus in the medial
preoptic/anterior hypothalamic area that was significantly larger in males than in females.
• They also found that this nucleus was twice as large in female-oriented (heterosexual)
rams as in male-oriented (homosexual) rams. (See Figure 10.12.)
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Hormonal Control of Sexual Behavior
• Sexual Orientation
• Sexual Orientation and the Brain
• Although this section has been considering sexual orientation—the sex of those to whom
an individual is sexually and romantically attracted—another sexual characteristic is
related to structural differences in the brain. In a study of postmortem brains, Zhou et al.
(1995) found that the size of a particular region of the forebrain, the central subdivision of
the bed nucleus of the stria terminalis (BNST), is larger in males than in females.
• They also found that in male-to-female transsexuals this nucleus is as small as it is in
normal females.
• The size of this nucleus was as large in male homosexuals as in male heterosexuals.
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Hormonal Control of Sexual Behavior
Sexual Orientation and the Brain
• Thus, its size was related to gender identity, not to sexual orientation. (Most male
homosexuals have male sexual identities; although they are romantically and sexually
oriented toward other men, they do not regard themselves as women, nor do they wish to
be.)
• Kruijver et al. (2000) replicated these results and found that the size of this region in
female-to-male transsexuals was within the range of sizes seen in normal males.
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Hormonal Control of Sexual Behavior
• Sexual Orientation
• Heredity and Sexual Orientation
• If both twins are homosexual, they are said to be concordant for this trait. If only one is
homosexual, the twins are said to be discordant.
• Thus, if homosexuality has a genetic basis, the percentage of monozygotic twins who are
concordant for homosexuality should be higher than that for dizygotic twins.
• This is exactly what Bailey and Pillard found: The concordance rate was 52 percent for
identical twins and only 22 percent for fraternal twins—a difference of 30 percent. Other
studies have shown differences of up to 60 percent (Gooren, 2006).
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Hormonal Control of Sexual Behavior
• Sexual Orientation
• Heredity and Sexual Orientation
• Genetic factors also appear to affect female homosexuality.
• Bailey et al. (1993) found that the concordance of female monozygotic twins for
homosexuality was 48 percent, while that of dizygotic twins was 16 percent.
• Another study, by Pattatucci and Hamer (1995), found an increased incidence of
homosexuality and bisexuality in sisters, daughters, nieces, and female cousins (through
a paternal uncle) of homosexual women.
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Neural Control of Sexual Behavior
• Brain Mechanisms
• The medial preoptic area (MPA), located just rostral to the hypothalamus, is the forebrain
region most critical for male sexual behavior.
• medial preoptic area (MPA)
• An area of cell bodies just rostral to the hypothalamus; plays an essential role in male
sexual behavior.
• Domingues, Gil, and Hull (2006) found that mating increased the release of glutamate in
the MPA and that infusion of glutamate into the MPA increased the frequency of
ejaculation.
• Finally, destruction of the MPA abolishes male sexual behavior.
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Neural Control of Sexual Behavior
• Brain Mechanisms
• Gorski et al. (1978) discovered a nucleus within the MPA of the rat that is three to seven
times larger in males than in females.
• This area is called (appropriately enough) the sexually dimorphic nucleus (SDN) of the
preoptic area. (As we saw in the previous discussion of brain mechanisms of gender
identity, this brain region, which is called the uncinate nucleus in humans, is sexually
dimorphic in our species as well.)
• sexually dimorphic nucleus
• A nucleus in the preoptic area that is much larger in males than in females; first
observed in rats; plays a role in male sexual behavior.
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Neural Control of Sexual Behavior
• Brain Mechanisms
• The size of this nucleus is controlled by the amount of androgens present during fetal
development.
• According to Rhees, Shryne, and Gorski (1990a, 1990b), the critical period for
masculinization of the SDN appears to start on the eighteenth day of gestation and end
once the animals are five days old. (Normally, rats are born on the twenty-second day of
gestation.)
• De Jonge et al. (1989) found that lesions of the SDN decrease masculine sexual
behavior.
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Neural Control of Sexual Behavior
•
Brain Mechanisms
•
periaqueductal gray matter (PAG)
• The region of the midbrain that surrounds the cerebral aqueduct; plays an essential role in
various species-typical behaviors, including female sexual behavior.
•
•
nucleus paragigantocellularis (nPGi)
• A nucleus of the medulla that receives input from the medial preoptic area and contains
neurons whose axons form synapses with motor neurons in the spinal cord that
participate in sexual reflexes in males.
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The nPGi normally inhibits spinal cord sexual reflexes, so one of the tasks of the pathway
originating in the MPA is to suppress this inhibition.
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The MPA suppresses the nPGi directly through an inhibitory pathway and does so indirectly by
inhibiting the activity of the PAG, which normally excites the nPGi .
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Neural Control of Sexual Behavior
• Females
• Just as the MPA plays an essential role in male sex behavior, another region in the ventral
forebrain plays a similar role in female sexual behavior: the ventromedial nucleus of the
hypothalamus (VMH).
• ventromedial nucleus of the hypothalamus (VMH)
• A large nucleus of the hypothalamus located near the walls of the third ventricle;
plays an essential role in female sexual behavior.
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Neural Control of Sexual Behavior
• Females
• The mechanism by which estradiol primes a female’s sensitivity to progesterone appears
to be simple: Estradiol increases the production of progesterone receptors, which greatly
increases the effectiveness of progesterone.
• Blaustein and Feder (1979) administered estradiol to ovariectomized guinea pigs and
found a 150 percent increase in the number of progesterone receptors in the
hypothalamus.
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Neural Control of Sexual Behavior
• Formation of Pair Bonds
• Several studies have revealed a relationship between monogamy and the levels of two
peptides in the brain: vasopressin and oxytocin.
• These compounds are both released as hormones by the posterior pituitary gland and as
neurotransmitters by neurons in the brain. In males, vasopressin appears to play the
more important role.
• This difference appears to be responsible for the presence or absence of monogamy.
• For example, after intercourse, at a time when blood levels of oxytocin are increased,
people report feelings of calmness and well-being, which are certainly compatible with the
formation of bonds with one’s partner.
•
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•Prolactin
•A hormone of the anterior pituitary gland, necessary for production of milk; also
facilitates maternal behavior.
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Parental Behavior
Neural Control of Maternal Behavior
• The medial preoptic area, the region of the forebrain that plays the most critical role in
male sexual behavior, appears to play a similar role in maternal behavior.
• Numan (1974) found that lesions of the MPA disrupted both nest building and pup care.
The mothers simply ignored their offspring.
•Prolactin appears to exert its stimulating effect on maternal behavior by acting on receptors
in the medial preoptic area.
•Bridges et al. (1997, 2001) found that an infusion of prolactin into the MPA of virgin female
rats that has been primed with estradiol and progesterone stimulated maternal behavior, and
an infusion of a prolactin antagonist into the MPA delayed the onset of maternal behavior.
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