What are sex hormones, sexual differentiation and the menstrual cycle? - Chapter 5

During the prenatal period most differences between men and women arise. This complex process is named: parental sexual differentiation. In this chapter the process of the menstrual cycle and sexual differentiation is discussed.

What are sex hormones?

The chemical substances that are created by the endocrine glands and secreted into the bloodstream are called hormones. The effects of hormones are felt rapidly through its secretion into the bloodstream. The hormones that are most important for sex are the following:

• Testosterone, this hormone belongs to the group called androgens.

• Oestrogens

• Progesterone

The endocrine glands that are of interest are those for sex, these are the testes in males and the ovaries in females. The pituitary gland is related to the hypothalamus. The hypothalamus regulates the pituitary, which regulates the ovaries and the testes. Therefore, it has the nickname of master gland. The hypothalamus plays a role in vital behaviours such as drinking, easting and having sex. The pituitary, hypothalamus and gonads (which are the ovaries and testes) work together to regulate the functions needed for sex.

What are sex hormone systems in males?

Both the testes and the pituitary produce hormones, the most important one in men is testosterone. It has a function in maintaining and stimulating the secondary characteristics of sex, such as beard growth. It also is responsible for maintaining genitals and their production of sperm and for stimulation of the bones and muscles. The pituitary produces two important hormones, both hormones influence the functioning of the testes.

1. Follicle stimulating hormone (FSH): controls sperm production.

2. Luteinizing hormone (LH): controls testosterone production.

Because of the negative feedback loop between the pituitary, the testes and the hypothalamus, the testosterone levels in males remain relatively constant. The level of the LH and FSH hormone is regulated by gonadotropin-releasing hormone or GnRH. This hormone is secreted by the hypothalamus. The feedback loop, where the hypothalamus monitors the testosterone levels and influences the level of GnRH, is called the HPG-axis. When testosterone levels are high, the hypothalamus produces GnRH to make sure the pituitary stops producing as much LH. This negative feedback loop is important for keeping the testosterone level constant. The hormone inhibin is produced by the Sertoli cells in the testes. It regulates FSH in a negative feedback loop, inhibin suppresses FSH, which suppresses sperm production. The hormone shows great potential to serve as a male anticonception method.

What are sex hormone systems in females?

Oestrogen and progesterone are the hormones produced by the ovaries. Oestrogen is responsible for the changes in puberty. It stimulates the growth of the vagina, uterus, pelvis and breasts. It also regulates membranes of the vagina and stops muscle and bone growth in females. The levels of these hormones fluctuate during pregnancy, menopause and the menstrual cycle. The two hormones produced by the pituitary, LH and FSH regulate the levels of progesterone and oestrogen. They are also controlled by the negative feedback loop of the hypothalamus, like the negative feedback loop for testosterone in men. Inhibin is produced by the ovaries instead of the testes. Besides this, the pituitary produces two other hormones: prolactin and oxytocin. The first is responsible for the production of milk after the pregnancy and the second stimulates the contractions of the uterus during childbirth. After birth, oxytocin seems to promote bonding with the child.

What are prenatal sexual differences?

Sex chromosomes

A new human starts from a single cell and develops into a male or female. The specific chromosomes within that cell determine its gender. Two X chromosomes is typically a woman, an X and Y chromosome becomes a man. These chromosomes give instructions on which organs to develop. Some children are born with a different combination. This often results in the Klinefelter syndrome. A genetic male has the XXY chromosomes. Testosterone levels are low and there is no sperm being produced. The single cell duplicates many times during growth and turns into an embryo. By the seventh week you can tell whether it’s a male or female.

Gonads

After the seventh week, differentiation of the sex chromosomes begins. One important gene involved in differentiation is the SRY-gene, or sex-determining region, Y chromosome. This gene is responsible for the testes-determining factor (TDF). The X chromosome consists of genes that control ovary functions.

Prenatal hormones and the genitals

After differentiation, the sex hormones are produced and cause changes in the internal and external genitals. In females, the Mullerian ducts become fallopian tubes, the vagina and the uterus. The tubercle changes into the clitoris, inner -and outer lips. In men, the Wolffian ducts turn into the ejaculatory duct, the vas deferens and the epididymis. The tubercle becomes the glans of the penis and the scrotum.

Descent of the testes and ovaries

While these differentiations take place, the testes and ovaries change position and shape. Before shifting to the final position in the pelvis they remain at the upper end. The testes undergo a similar change going through the inguinal canal. There are two issues that can occur during this process:

1. Cryptorchidism meaning undescended testes

2. Inguinal hernia meaning an unfinished inguinal canal

Brain differentiation

Aside from differentiation in the genitals, there is also brain differentiation. Epigenetics refers to changes in DNA that does not alter the DNA code, but rather leads to changes in the expression of specific genes. Genes responsible for brain differentiation can be found in several genes. New studies using MRI reveal important insights in the androgen and oestrogen receptors responsible for gender differences. Studying the brain using fMRI revealed that there is an activation of a region in the hypothalamus in men when they were sexually aroused. Neuroscientists emphasise the brain’s plasticity and argue that it is continuously changing in response to certain experiences.

Homologous organs

Men and women have the same origins and develop the same embryonic tissue. The organs are called homologous. When two organs have a similar function, they are referred to as analogous.

What are atypical differences between genders?

There are several variables of gender (Money, 1987) they are the following:

1. Chromosomal gender. This is XY in males and XX in females.

2. Gonadal gender. These are testes in men and ovaries in women.

3. Prenatal hormonal gender and prenatal and neonatal brain differentiation. The existence of MIS and testosterone in men, but not in women before birth. Testosterone is present for masculinization.

4. Internal organs. These include the uterus, the upper vagina, the fallopian tubes, the prostate, vas and seminal vesicles.

5. External genital appearance. This are the inner and outer lips, the clitoris, the vaginal opening, the scrotum and the penis.

6. Pubertal hormonal gender. Testosterone, oestrogen and progesterone.

7. Assigned gender. The announcement whether it is a boy or a girl.

8. Gender identity. The internal sense of female -or maleness.

A person is said to have an intersex condition or disorder of sex development when there are contradictions between several of the above biological factors (1 to 6). There are several syndromes that can cause an intersex condition. One is congenital adrenal hyperplasia, this is when a genetic female produces an excess number of androgens during prenatal sexual differentiation. The external genitals will then appear men. Another condition is androgen-insensitivity syndrome where a male develops normally including its testosterone levels, however the body is insensitive to them. This results in a feminized sexual development. The person is born with the external organs of a female.

What sexual differentiation happens during puberty?

The scientific definition of puberty is the period of sudden maturation and enlargement of the gonads, secondary sex characteristics and other genitalia leading to the capacity to reproduce. The psychological process that people endure during puberty is a result of increased hormone activity. Adolescence is defined as the period of transformation of behaviour, responsibilities and attitudes to become an adult. The timing for pubertal processes is very different between males and females. Boys begin about two years later than girls, who start around the twelfth year of age. Also, there are large individual differences in the age when puberty begins.

Changes in girls

The first sign of puberty in girls is breast development. This starts when a girl is about eight or nine years old. The changes are produced by increases in sex hormone levels. Besides the growth of the breast, another visible sign is the growth of pubic -and underarm (armpit) hair. Oestrogen is responsible for stopping the growth spurt in girls and causes the growth period to be shorter than in boys. The menarche happens around the age of twelve and is the first time of menstruation. There are some factors that contribute to the age of the menarche. One is body fat, leptin is the hormone that determines the onset of puberty and rises with body fat. Kisspeptin is another hormone to stimulate the onset of puberty. It does so by stimulating the hypothalamus to secrete is sex-hormone GnRH and later LH and FSH. The influence of body fat is important in describing two phenomena:

1. The cessation of menstruation in girls with anorexia nervosa.

2. The cessation of menstruation in runners or extreme athletes.

The adrenal glands are also important in puberty. They produce androgens that stimulate pubic -and axillary hair growth. It is also related to female sex drive. The adrenarche is the onset of increasing secretion of the adrenal androgens. The adrenarche is mostly around eight years.

Changes in boys

Puberty starts later in boys and begins with heightened levels of the hormones: LH and FSH. The first sign is growth of the scrotum and testes. Pubic hair growth starts about the same age, around nine. One year later the penis starts to grow, which is caused by testosterone stimulation. Erections increase in frequency and when a boy is around thirteen or fourteen years old, he can ejaculate. One year later, boys start having nocturnal emissions and for a boy that has never masturbated, that can be his first ejaculation. In boys, acne is quite normal during puberty cause by androgens. It is a shame that there is so little interest in the rites of passage for both boys and girls when they enter the stage of puberty. When there is more recognition, boys and girls might better understand the stage they are in.

How does the menstrual cycle work?

Fluctuating hormones in the body regulate the menstrual cycle. There are several phases involved in the menstrual cycle which are all characterised by different hormones.

1. The follicular phase is when high levels of FSH are secreted. This causes an egg to get to the state of final maturity. Oestrogen is secreted at the same time.

2. The ovulation is when the mature egg is released by the follicle. Oestrogen is risen to a level where it inhibits the secretion of FSH and increase produce of GnRH, which causes beginning of production of LH.

3. The luteal phase is when the follicle (caused by LH) turns into a corpus luteum. It manufactures progesterone and inhibits LH secretion causing the corpus luteum to degenerate. After this a sharp decrease of oestrogen and progesterone causes the stimulation of FSH and the cycle can be repeated.

4. The menstruation is when the inner lining of the uterus is passed out through the vagina. During this phase, FSH levels are rising, oestrogen and progesterone levels are low. These low levels of oestrogen and progesterone trigger the menstruation and end the luteal phase.

The menstrual cycle lasts from twenty to thirty-six days with an average of twenty-eight days. There can be menstrual problems, for example dysmenorrhea is the most common and is the experience of a painful menstruation. It is caused by prostaflandis, a hormone causing muscle contraction. The best treatment is NSAIDs or nonsteroidal anti-inflammatory drugs. Another proposed cure is masturbation because the discomfort is caused by the pelvic edema, which increases during an orgasm. The second problem is endometrosis, caused by growth of the endometrium. It should be treated by a physician, otherwise it can lead to sterility. The last menstrual problem is amenorrhea, which is the absence of menstruation.

What are the psychological aspects of the menstrual cycle?

• Fluctuation in mood is related to the premenstrual syndrome (PMS). It is the occurrence of psychological and physical symptoms that occur prior to menstruation. Symptoms are breast pain, water retention and depression and irritability. Researchers showed that only sadness and irritability showed significant changes throughout the menstrual cycle. It was not just prior to menstruation, which is the main concept of PMS. There is no scientific evidence of PMS. There is a premenstrual dysphoric disorder (PMDD) which is found in a small percentage of women. Symptoms are feelings of hopelessness, sadness, anxiety, fearfulness and tension in the luteal phase.

• Fluctuations in performance can be a practical indicator of mood changes during the menstrual cycle. Researchers did not find any fluctuations in performance, there is no reliable evidence of performance decrease caused by the menstrual cycle.

• Fluctuations in sex drive, there is a link between sex drive and menstruation and is associated with the testosterone levels. Therefore, women have a peak in sexuality while ovulating.