Sex Differences: Hormones and Development

Neurotransmitters vs. Hormones

  • Neurotransmitters: Chemical messengers communicating between neurons across a synaptic cleft.
  • Hormones: Chemical messengers released into the bloodstream to reach their target.
  • Some molecules can act as both, such as noradrenaline.

Sex vs. Gender

  • Sex: Biological aspects.
  • Gender: Cultural aspects.
  • Both influence each other.
  • The lecture primarily focuses on biological aspects but acknowledges cultural influences.
  • Even the biological concept of sex has complexities.

Chromosomal Sex Determination

  • Females typically have two X chromosomes (XX).
  • Males typically have one X and one Y chromosome (XY).
  • Gametes (reproductive cells) are haploid, containing one set of chromosomes.
  • Egg cells always have an X chromosome.
  • Sperm cells have either an X or a Y chromosome.

Fertilization

  • Ovum (X) + X-bearing sperm = XX (typically female).
  • Ovum (X) + Y-bearing sperm = XY (typically male).

Role of the Y Chromosome

  • The Y chromosome determines maleness in mammals.
  • The SRY gene on the Y chromosome is crucial.
    • Mutant mice without the SRY gene develop as female, even with a Y chromosome.
    • The SRY gene triggers the development of the early, undifferentiated fetal gonad into a testis.

Gonadal Development

  • Early gonad is undifferentiated; it can become either a testis or an ovary.
  • The SRY gene initiates testis development.
  • Without the SRY gene, the gonad develops into an ovary.

Hormones Produced by Testes

  • Testes produce two types of hormones:
    • Androgens:
      • A group of hormones; testosterone is the most important one.
      • Literally means "man-making" in Greek (Andros = man, Day nine = to make).
    • Anti-Müllerian Hormone: Its function is explained later.
  • Ovaries initially make no hormones during early embryonic development.
  • Female-specific Pattern:
    • If anti-Müllerian hormone and androgens are absent, internal genitalia develop in a female-specific pattern.
  • Male-specific Pattern:
    • If anti-Müllerian hormone and androgens are present, internal genitalia develop in a male-specific pattern.

Müllerian and Wolffian Systems

  • Müllerian System: Precursor to female internal sex organs (fallopian tubes, uterus, upper vagina).
  • Wolffian System: Precursor to male internal sex organs (vas deferens, seminal vesicles, urethra).
Hormonal Influence on These Systems
  • Anti-Müllerian Hormone:
    • Causes the Müllerian system to disintegrate.
  • Androgens:
    • Maintain the Wolffian system, allowing it to develop into male internal anatomy.
  • Female Development (no androgens or anti-Müllerian hormone):
    • The Wolffian system disappears.
    • The Müllerian system stays and develops into female internal sex organs.

External Anatomy

  • Early external anatomy is undifferentiated in both XY and XX embryos.
  • Around 7-8 weeks after gonadal differentiation, external genitalia become sensitive to androgens, specifically Dihydrotestosterone (DHT).
  • Male Development (DHT present):
    • Scrotum, penis, and glans develop.
  • Female Development (DHT absent or ineffective):
    • The scrotum becomes the external labia.
    • The shaft of the penis becomes the internal labia.
    • The glans becomes the clitoris.

Summary of Sex Differentiation

  • XY Embryo:
    • Y chromosome → SRY gene (Testis Determining Factor) → Testis development.
    • Testis produces Anti-Müllerian Hormone → Müllerian system disappears.
    • Testis produces Androgens → Wolffian system and male-specific external genitalia develop.
  • XX Embryo:
    • Ovary develops (no hormones initially).
    • Müllerian system develops.
    • Wolffian system disappears.
    • Female-specific external genitalia develop.

Persistent Müllerian Duct Syndrome

  • Occurs in XY individuals.
  • The anti-Müllerian hormone is not working (either not produced or receptors are absent).
  • Results in both male (testes, vas deferens, etc.) and female (fallopian tubes, uterus) internal anatomy developing.
  • Masculinizing and de-feminizing are separate processes.

Question: Factors Influencing Sex of Baby

  • Currently, there is no reliable way to manipulate the sex of a baby.
  • There is a possibility that early hormones do affect gender identity later in life.

Androgen Insensitivity Syndrome

  • Individuals are insensitive to androgens (testosterone, DHT).
  • Male-specific external genitalia do not develop due to a lack of androgen receptor function.

Intersex

  • Encompasses conditions with non-standard sexual development.
  • Persistent Müllerian duct syndrome and androgen insensitivity syndrome are forms of intersex.

Organizational vs. Activational Effects of Hormones

  • These terms describe hormone effects, not specific molecules.
    *These terms describes the effect the molecule is having.
  • Organizational Effects:
    • Permanent changes to the body or brain.
    • Occur during sensitive periods (e.g., embryonic development or early development).
    • Removing the hormone does not reverse the change.
    • Example: exposure to testosterone in utero
  • Activational Effects:
    • Temporary effects that occur only when the hormone is present.
    • Removing the hormone eliminates the effect.
    • Example: During puberty.

Puberty

  • Childhood: Gonads produce very few hormones.
  • Puberty: Gonads start producing hormones, leading to sexual maturation and development of secondary sexual characteristics.
  • The average age of puberty for girls in the 1840s was 17, and in the 1960s, it was 13.
  • Genetic factors, environmental factors, and social factors play a role.
  • Onset of puberty is related to size and development; being bigger in terms of fat reserve help with earlier puberty.

Secondary Sexual Characteristics

  • Organizational Secondary Sexual Characteristics
    • Pelvis changes
    • Shoulder width
    • Voice drop
  • Activational Secondary Sexual Characteristics
    • Facial hair
    • Muscle mass
    • Breast Development (partly organizational)

Hormonal Control of Puberty Onset

  • Hypothalamus: Contains gonadotropin-releasing hormone (GnRH) neurons.
  • During childhood, GABA/NPY neurons inhibit GnRH neurons, preventing the release of GnRH.
  • At puberty, kisspeptin neurons activate and inhibit the inhibitory neurons, stimulating the GnRH neurons.
Cascade Process
  • Kisspeptin neuron activation → stimulates GnRH neurons.
  • GnRH released in pulses every two hours.
  • GnRH (Gonadotropin-Releasing Hormone) is released into the pituitary portal system, reaching the anterior pituitary gland.
  • Cells in the anterior pituitary respond to GnRH by releasing gonadotropins (hormones that affect the gonads).
  • Two main gonadotropins: Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
  • These hormones then stimulate the testes to produce testosterone and the ovaries to produce oestrogen.