Understanding human sexuality by Hyde and DeLamater - a summary
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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.
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
The pituitary gland and the hypothalamus are also important.
These three structures function together.
They influence important sexual functions
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:
The pituitary produces several hormones, two of which are important here. These hormones affect the functioning of the testes
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
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
Sex chromosomes
The specific sex chromosomes carried in a fertilized egg are the deciding factors in whether it will become a male or female.
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 7th 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 7th week after conception, the sex chromosomes direct the gonads to begin differentiation.
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.
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.
The testes go down into the scrotum via the inguinal canal.
Normally, this occurs around the 7th month after conception.
The inguinal canal closes off after the testes descend.
Two problems may occur in this process
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.
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.
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
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
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.
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
Other body changes in girls during puberty include
The changes are produced by the endocrine system and its upsurge in sex hormone production during puberty.
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.
Problems by puberty
Changes in behaviour
Puberty increases
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
The phases of the menstrual cycle
The menstrual cycle has four phases
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
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.
This is a summary of the book Understanding human sexuality by Hyde and DeLamater. The book is about topics ranging from sex is different cultures to sexual disfunctions. The book is used in the course 'Sexology' at the university of Amsterdam. Because of this only the
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