Sex hormones, sexual differentiation, and the menstrual cycle - a summary of chapter 5 of Understanding human sexuality by Hyde and DeLamater

Sexology
Chapter 5
Sex hormones, sexual differentiation, and the menstrual cycle

Many of the structural differences between males and females arise before birth, during the prenatal period, in a process called prenatal sexual differentiation.
Prenatal period: the time from conception to birth.
Further differences develop during puberty.

Sex hormones

Hormones: chemical substances secreted by the endocrine glands into the bloodstream.
Because they go into the blood, their effects are felt rapidly and at places in the body quite distant from where they were manufactured.
The most important sex hormones are

• Testosterone
  A hormone secreted by the testes in males (also present at lower levels in females)
  One of a group of hormones called androgens.
• Estrogens
  The group of sex hormones secreted by the ovaries in females (and present at lower levels in males)
• Progesterone
  A sex hormone secreted by the ovaries

The pituitary gland and the hypothalamus are also important.

• The hypothalamus regulates the pituitary glands
  Also plays a part in regulating many vital behaviours.
• The pituitary gland regulates the other glands
  In particular the testes and ovaries.
  Because of this, the pituitary gland is also called master gland fo the endocrine system.
  • Anterior lobe
    Interacts with the gonads (testes or ovaries)

These three structures function together.
They influence important sexual functions

• Menstrual cycle
• Pregnancy
• The changes of puberty
• Sexual behaviour

Sex hormone systems in males

The pituitary and testes both produce hormones.
The important hormone produced by the testes is testosterone.
Has important functions in:

• Stimulating and maintaining the secondary sex characteristics
• Maintaining the genitals and their sperm-producing capability
• Stimulating the growth of bone and muscle

The pituitary produces several hormones, two of which are important here. These hormones affect the functioning of the testes

• Follicle-stimulating hormone (FSH)
  Stimulates follicle development in females and sperm production in males
• Luteinizing hormone (LH)
  Regulates estrogen secretion and ovum development in females and testosterone production in males

Testosterone levels in males are relatively constant.
The hypothalamus, pituitary, and testes operate in a negative feedback loop that maintains these constant levels.

The levels of LH are regulated GnRH (gonadotropin-releasing hormone), which is secreted by the hypothalamus.
The hypothalamus monitors the levels of testosterone present, and this way testosterone influence the output of GnRH.
This loop is sometimes called the HPG axis, for hypothalamus-pituitary-gonad axis.

The pituitary’s production of LH stimulates the testes to produce testosterone, but when testosterone levels get high, the hypothalamus reduces its production of GnRH, in turn causing the pituitary to reduce production of LH, and consequently decreasing the production of testosterone.
When testosterone levels drop, the hypothalamus increases the production of GnRH.

Inhibin: a hormone secreted by the testes and ovaries that regulates FSH levels in a negative feedback loop.

Sex hormone systems in females

The ovaries produce two important hormones

• Estrogen
  Brings about many of the changes of puberty
  Maintains the mucous membranes of the vagina and stopping the growth of bone and muscle
• Progesterone

In adult women the levels of estrogen and progesterone fluctuate according to the phases of the menstrual cycle and during various other stages such as pregnancy and menopause.

FSH and LH regulate the levels of estrogen en progesterone by a negative feedback loop that is similar to the negative feedback loop in males.

Inhibin is produced by the ovaries and inhibits FSH production. It participates in the feedback loop that controls the menstrual cycle.

The pituitary produces two other hormones

• Prolactin
  Stimulates secretion of milk by the mammary glands after a women has given birth to a baby
• Oxytocin
  Stimulates ejection of that milk from the nipples
  Also stimulates contractions of the uterus during childbirth.
  And it seems to promote affectionate bonding
  • Is produced in both males and females

Prenatal sexual differentiation

Sex chromosomes

The specific sex chromosomes carried in a fertilized egg are the deciding factors in whether it will become a male or female.

• Two X chromosomes
  The result is typically a female
• X and Y
  The result is typically a male

Occasionally, individuals receive at conception a sex chromosome combination other than XX or XY.
Such atypical sex chromosome complements may lead to a variation of clinical syndromes.

By the 7^th week after conception, some basic structures have been formed that will eventually become either a male or female reproductive system.
At this point, the embryo has a pair of gonads, two sets of ducts, and rudimentary external genitals.

Gonads

In the 7^th week after conception, the sex chromosomes direct the gonads to begin differentiation.

• In males, the undifferentiated gonad develops in a testis about 7 weeks
• In females, the process occurs somewhat later, with the ovaries developing at around 13 to 14 weeks

SRY: sex-determining region, Y chromosome.
SRY is an important gene that directs differentiation of the gonads.
Causes the manufacture of a substance called testis-determining factor (TDF), which makes the gonads differentiate into testis, and male development occurs.

The X chromosome carries genes that control normal functioning of the ovaries.
A number of genes on the X chromosome also affects cells in the testes that manufacture sperm.

Prenatal hormones and the genitals

Once the ovaries and testes have differentiated, they begin to produce different sex hormones, which then direct the differentiation of the rest of the internal and external genital systems.

• In the female, the Wolffian ducts degenerate, dan the Müllerian ducts turn into the fallopian tubes, the uterus, and the upper part of the vagina.
  The tubercle becomes the clitoris, the folds become the inner lips, and the swelling develops into the outer lips
• The testes secrete Müllerian inhibiting substance.
  MIS causes the Müllerian ducts to degenerate, while the Wolffian ducts, supported by testosterone, turn into the epididymis, the vas deferens, and the ejaculatory duct.
  The tubercle becomes the glans of the penis, the folds form the shaft of the penis and the sewlling develops into the scrotum.

By 12 weeks after conception, the gender of the fetus is clear from the appearance of the external genitals.

Descent of the testes and ovaries

As these developmental changes are taking place, the ovaries and testes are changing in shape and position.

• At first, the ovaries and testes lie near the top of the abdominal cavity
• By the 10^th week they have grown and have moved down to the level of the upper edge of the pelvis
  • The ovaries remain there until after birth, and later they shift to their adult position in the pelvis

The testes go down into the scrotum via the inguinal canal.
Normally, this occurs around the 7^th month after conception.
The inguinal canal closes off after the testes descend.
Two problems may occur in this process

• Cryptorchidism
  Undescended testes. The condition in which the testes do not descend to the scrotum as they should during prenatal development.
  • In most of these cases, the testes do descend by the first birthday.
    Otherwise, surgery is needed.
    The optimum time for doing this is soon after the first birthday. Otherwise, if both testes fail to descend, the man will be sterile (because the high temperature of the body inhibits the production of sperm).
    Undescended testes are also more likely to develop cancer.
• Inguinal hernia
  When the inguinal canal does not close off completely
  It may reopen later in life, creating a passageway through which loops of intestine can enter the scrotum.
  Can be remedied by simple surgery

Brain differentiation

During the prenatal period, sex hormones are also acting on the brain.
In certain regions there are differences between male and female brains.

• The hypothalamus
  In the preoptic area
  The determination of the estrogen sensitivity of certain cells in the hypothalamus, cells that have estrogen receptors.
  • If testosterone is present during fetal development, these specialized cells in the hypothalamus become insensitive to estrogen
  • This sensitivity is crucial to the hypothalamic-pituitary-gonad feedback loop.
  • Male hypothalamic cells have more androgen receptors

Epigenetics: a functional change to DNA that does not alter the genetic code itself, but leads to charges in gene expression.
Often an epigenetic change involves methylation, a methyl group that is attached to the base cytosine in the DNA.
Prenatal sexual differentiation of the brain may involve more than anatomical differences in structures and androgen receptors, but may also involve epigenetic factors.

The brains of men and women are actually quite similar in most regions, but a few brain structures show gender differentiation.

• Hypothalamus
• The amygdala
  Important in emotion

Neuroscientists emphasize the plasticity of the brain instead of wardwiring present at birth.

Homologous organs

Homologous organs: organs in the male and female that develop from the same embryonic tissue.
Analogous organs: organs in the male and female that have similar functions.

Atypical prenatal gender differentiation

We can distinguish among the following eight variables of gender

• Chromosomal gender
  XX in females, XY in males
• Gonadal gender
  Ovaries in females, testes in males
• Prenatal hormonal gender
  Testosterone and MIS in males, but not in females before birth
• Prenatal and neonatal brain differentiation
  Testosterone present for masculinization, absent for feminization
• Internal organs
  Fallopian tubes, uterus, and upper vagina in females
  Prostate, vas, and seminal vesicles in males
• External genital appearance
  Clitoris, inner and outer lips, and vaginal opening in females
  Penis and scrotum in males
• Pubertal hormonal gender
  At puberty, estrogen and progesterone in females
  Testosterone in males
• Assigned gender
  The announcement at birth based on the appearance of the external genitals
• Gender identity
  The person’s private, internal sense of maleness or femaleness

These variables might be subdivided into biological and psychological variables.
In most cases, all the variables are in agreement in an individual.

As a result of any one of a number of factors during the course of prenatal development, the gender indicated by one or more of these variables may disagree with the gender indicated by others.
Intersex: a condition in which the individual has a mixture of male and female reproductive structures, so that it is not clear at birth whether the individual is a male or female.
Disorders of sex development (DSD): another term for intersex conditions.
A number of syndromes can cause an intersex condition

• Congenital adrenal hyperplasia (CAH)
  A condition in which a genetic female produces excess levels of the androgens prenatally and therefore has male-appearing genitals at birth.
• Androgen-insensitivity syndrome (AIS)
  A genetic condition in which the body is unresponsive to androgens so that a genetic male may be born with a female-appearing body

Sexual differentiation during puberty

Puberty: the time during which there is sudden enlargement and maturation of the gonads, other genitalia, and secondary sex characteristics, sot that the individual becomes capable of reproduction.

The physiological process that underlies puberty in both genders is a marked increase in levels of sex hormones.

Adolescence is a socially defined period of development that bears some relationship to puberty.
It represents a psychological transition from the behaviour and attitudes of a child to the behaviour, attitudes and responsibilities of an adult.
In some cultures, adolescence does not exist.

• The timing of the pubertal process differs considerably for males and females
  Girls begin to change around 8 to 12 years of age
  Boys about 2 years later
• There are large individual differences in age at which the process of puberty takes place

Changes in girls

The first sign of puberty in girls is the beginning of breast development, on average around 8 to 9 years of age.
The ducts in the nipple area swell, and there is growth of fatty and connective tissue, causing the small, conical buds to increase in size.
These changes are produced by increases in the levels of sex hormones.
A similar increase takes place at the hips and buttocks, leading to the rounded contours.

Another visible sign of puberty is the growth of public hair, which occurs shortly after breast development begins.
About two years later, underarm hear appears.

Body growth increases sharply during puberty.
Estrogen eventually stops the growth spurt in girls.

At about 12 years of age, the menarche (first menstruation) occurs.
But, the girl is not capable of becoming pregnant until ovulation begins, typically about two years after the menarche.

The percent body fat hypothesis
During puberty, deposits of body fat increase in females
According to the hypothesis, the percentage of body weight that is fat must rise to a certain level for menstruation to occur for the first time and for it to be maintained

• Leptin
  A hormone related to the onset of puberty in boys and girls
  In prepuberal girls and boys, leptin levels rise as body fat increases.
• Kisspeptin
  A hormone involved in the initiation of pubertal development
  Kisspeptin stimulates the hypothalamus to produce more GnRH and to produce it in pulses.
  This initiates a cascade of secretion of hormones, including LH and FSH, which stimulates the ovaries to produce estrogen, and puberty begins

Other body changes in girls during puberty include

• A development of the blood supply to the clitoris
• A thickening of the walls of the vagina
• A rapid growth of the uterus
• The pelvic bone structure grows and widens

The changes are produced by the endocrine system and its upsurge in sex hormone production during puberty.

• The hypothalamus releases pulses of GnRH
• This triggers and increase in secretion of FSH by the pituitary gland
• FSH stimulates the ovaries to produce estrogen
• Estrogen is responsible for many of the changes that occur

Adrenal glands; endocrine glands located just above the kidneys, in females they are major producers of androgens
Androgens stimulate the growth of public hair and axillary hair and are related to the female sex drive.
Adrenarche: in childhood, the maturation of the adrenal glands, resulting in increased secretion of androgens. Begins slightly before the age 8.

Changes in boys

Puberty begins at about 10 or 11 years of age in boys.
The physical changes of puberty in boys parallel those in girls.
They are initiated by increased production of FSH and LH by the pituitary.
At the beginning of puberty, the increase in LH stimulates the testes to produce testosterone, which is responsible for most of the changes of puberty in males.

• The growth of the testes and scrotal sac
  Begins on average at around 9 to 10 years of age as a result of testosterone stimulation
  • The growth of public hair begins at about the same time
• Enlargement of the penis
  About a year later
  Results from testosterone stimulation
  • As the testes enlarge, their production of testosterone increases, leading to rapid growth of the penis, testes, and public hair
• Growth of facial and axillary hair
  Two years after the beginning of public hair growth
  Adult beards do not appear until two or three years later
  Due to testosterone stimulation
• Erections increase in frequency
  • The organs that produce the fluid of semen, particularly the prostate, enlarge
  • By age of 13 or 14, the body is capable of ejaculation
  • About age 15, the boy is fertile
    FSH is responsible for initiating and maintaining the production of mature sperm
  • About a year after the first ejaculation, many boys begin having wet dreams.
• About the same time the penis growth occurs, the larynx (voice box) grows
• Growth spurt
  At around 11 to 16 years of age
  Increase of muscle mass
  Testosterone brings the growth process to an end, but permits the growth longer than in girls

Problems by puberty

• Acne
  A distressing skin condition that stimulated by androgens.
• Gynecomastia
  Breast enlargement.
  May occur temporarily in boys
  Obesity may also be a temporary problem

Changes in behaviour

Puberty increases

• Sensation seeking behaviours
  Sex is one sensation that might be sought
• Reorients social behaviour
  Adolescents are motivated to seek social experiences with their peers and with potential romantic partners

The menstrual cycle

Biology of the menstrual cycle

The menstrual cycle is regulated by fluctuating levels of sex hormones, which produce certain changes in the ovaries and uterus.
The hormone cycles are regulated by the HPG axis and by means of the negative feedback loops.

Other species have estrous cycles, not menstrual.
Differences

• In animals that have estrous cycles there is no menstruation
  There is either no bleeding, or only a slight spotting of blood
• The timing of ovulation in relation to bleeding is different
  For estrous animals, ovulation occurs while the animal is ‘in heat’, or estrus, which is also the time of slight spotting
  In the menstrual cycle, ovulation occurs about midway between the periods of menstruation
• Female animals with estrous cycles engage in sexual behaviour only when they are in heat

The phases of the menstrual cycle

The menstrual cycle has four phases

• Follicular phase
  Beginning just after menstruation, during which an egg matures in preparation for ovulation
  At the beginning of this phase, the pituitary secretes relatively high levels of FSH, this signals the follicle to begin to bring an egg to the final stage of maturity.
  At the same time, the follicle secretes estrogen
  • The high levels of estrogen stimulate the endometrium of the uterus to grow, thicken and form glands that will eventually secrete substances to nourish the embryo
• Ovulation
  Release of an egg from the ovaries
  Estrogen has rising to a high level, which inhibits FSH production, and so FSH has fallen back to a low level.
  The high levels of estrogen also stimulate the hypothalamus to produce GnRH, which causes the pituitary to begin production of LH.
  A surge of LH triggers ovulation
• Luteal phase
  After releasing an egg, the follicle, under stimulation of LH, turns into a glandular mass of cells called the corpus luteum.
  This manufactures progesterone.
  High levels of progesterone inhibit the pituitary’s secretion of LH, and as LH levels decline, the corpus luteum degenerates.
  With this degeneration comes a sharp decline in estrogen and progesterone levels at the end of the luteal phase.
  The falling levels of estrogen stimulate the pituitary to begin production of FSH, and the cycle begins again.
  • The progesterone secreted by the corpus luteum stimulates the glands of the endometrium to start secreting the nourishing substances
    It continues to produce estrogen and progesterone for about 10 to 12 days
• Menstruation
  The endometrium of the uterus is sloughed off in the menstrual discharge.
  Estrogen and progesterone levels are low and FSH levels are rising.
  Menstruation is triggered by the sharp decline in estrogen and progesterone levels.

Length and timing of the cycle

Generally, anywhere from 20 to 36 days is considered within the normal range of the menstrual cycle.
There is enormous variation from one women to the next and for one women it can vary.

At cycles that are longer or shorter than 28 days, the principle is that the length of the luteal phase is relatively constant.
Always around 14 days.

Mittelschmerz: women who can feel ovulation.

Ovulation does not occur in every menstrual cycle.
Anovulatory cycle: menstruation without ovulation.

Other cyclic changes

Two other biological processes fluctuate with the menstrual cycle

• The cervical mucus cycle
  Involves glands in the cervix that secrete mucus throughout the menstrual cycle
  One function of the mucus is to protect the entrance to the cervix, helping to keep bacteria out
  These glands respond to the changing levels of estrogen during the cycle.
  • As estrogen increases at the start of a new cycle, the muscus is alkaline, thick, and viscous
  • When LH production begins, the cervical mucus changes
    It becomes alkaline, thin and watery
  • After ovulatoin, the mucus returns to its former viscous, less alkaline state
• The basal body temperature cycle
  The temperature is low during the follicular phase and takes a dip on the day of ovulation
  One the day after ovulation, it rises and then continues at the higher level for the rest of the cycle
  • Progesterone raises body temperature

Menstrual problems

Dysmenorrhea: painful menstruation.
Dysmenorrhea is caused by prostaglandins.
Prostaglandins: chemicals secreted by the uterus that cause the uterine muscles to contract.

Endometriosis: a condition in which the endometrium grows abnormally outside the uterus; the symptoms in unusually painful periods with excessive bleeding.

Amenorrhea: the absence of menstruation.

Psychological aspects of the menstrual cycle

Fluctuations in mood:do women become extra emotional?

Premenstrual syndrome (PMS): a combination of severe physical and psychological symptoms, such as depression and irritability, occurring just before menstruation.
According to some studies, there is no scientific evidence of PMS.
The best conclusion seems that the great majority of women do not experience menstrual fluctuations in mood, but a small percentage may.

Premenstrual dysphoric disorder (PMDD): a diagnostic category in the DSM, characterized by symptoms such as sadness, anxiety, and irritability in the week before menstruation.
Very controversial.

Fluctuations in performance: can a women be president?

Research has found no fluctuations in academic performance, problem solving, memory, or creative thinking.

Fluctuations in sex drive

Maximum sexual arousability does occur at the time of peak fertility.

Why do we believe in PMS?

We believe in PMS because of a long tradition of many cultural forces, such as menstrual taboos, that create negative attitudes toward menstruation.
In addition, women’s expectations may play a role.

Cycles in men

Men’s testosterone levels displayed weekly fluctuations, peaking on weekends.