COGNITIVE SCIENCE 17

Why Sex is Necessary

Part 1 Jaime A. Pineda, Ph.D.

Why is Sex Necessary?

• Sexually dimorphic behaviors– Anatomical,

physiological, and behavioral differences between males and females of the same species.

• Courting

• Mating

• Parental behavior

• Aggressive behavior

Without these behaviors most species would not survive

Is Sexual Identity Learned?• The case of Bruce who

became Brenda who became David

– Socialization over biology?

• XX (female) and XY (male) genes– A person’s genetic sex is

determined by the father– SRY gene on short arm of Y

chromosome- dictates the development of the fetal testis (testis-determining factor) and triggers action of receptors to receive chemical message hormone brings

Is Sexual Identity Learned?• The Y chromosome controls

the development of the glands that produce the male sex hormones

• The default sex (if the Y chromosome isn’t present or doesn’t work) is female

Initially, there are more males than females

Sexual Development

• Three categories of sex organs (primary sex characteristics)

– Gonads (testes or ovaries)• Develop first (Sry testes;

otherwise ovaries)• Produce ova/sperm and

secret hormones• Critical Period of sexual

development: Gestational weeks 7-12

– Internal sex organs– External sex organs

Sexual Development

Gonads can become either Testes or Ovaries

Develop as a function of the presence or absence of SRY gene

In males, at 7th week, outer portions of gonads degenerate and inner portions develop into testes

In females, at 11th week, inner portions of gonads degenerate and outer portions develop into ovaries

Sexual Development

Gonads become testes in males

Testes - endocrine glands that produce

• male hormones (androgens)

• Mullerian Inhibiting Substance

Most changes reflect the presence or absence of androgen

Sexual Development

Gonads become ovaries in females

Ovaries- endocrine glands that produce

• female hormones (e.g.estrogen)

Sexual Development

Internal sex organ precursors• Mullerian system female

– Develops into fimbriae, fallopian tubes, uterus, and vagina

• Wolffian system male– Develops into epididymis, vas deferens, seminal vesicles

• Develop as a function of hormones released by testes

– Mullerian inhibiting hormone or anti Mullerian hormone(defeminizing effect)

– Androgens (masculinizing effect): testosterone/dihydrotestosterone

Sexual Development

Wolffian Ducts - later become

• epididymis• vas deferens• seminal vesicles

Mullerian Ducts - later become

• uterus• fallopian tubes• upper part of vagina

Sexual Development

• External sex organs

– Penis and scrotum in males– Labia, clitoris, and outer vagina in females

– Female development is the default • “Nature’s impulse is to create a female”

– Male development requires androgens

Undifferentiated External Genitalia Undifferentiated External Genitalia (<7 weeks)(<7 weeks)

Undifferentiated External Genitalia Undifferentiated External Genitalia (<7 weeks)(<7 weeks)

Partially differentiated external Partially differentiated external male & female genitaliamale & female genitalia

Partially differentiated external Partially differentiated external male & female genitaliamale & female genitalia

Fully developed male & female external genitalia

Fully developed male & female external genitalia

Fetal External Genitals: FemaleFetal External Genitals: Female

11-12 weeks 19 weeks

Fetal External Genitals: FemaleFetal External Genitals: Female

5 months

Fetal External Genitals: MaleFetal External Genitals: Male

15 weeks 16 weeks

Sexual Development Dysfunctions

• Androgen insensitivity syndrome– Genetic mutation that prevents the formation of

androgen receptors• Gonads become testes (normal process)• Defeminization (normal process)• Lack of masculinization (abnormal process)

An XY male with female external genitalia;

have a woman’s body but not internal female sex organs

Sexual Development Dysfunctions

• Persistent Mullerian duct syndrome– Failure to produce anti-Mullerian hormone– Absence of receptors for this hormone

• In an XY male, defeminization does not occur but masculanization does

• Person is born with both sets of internal sex organs

• Turner’s syndrome– Individuals have only one sex hormone: an X

chromosome• Essentially develop into females (normal internal/external sex

organs)• However, no ovaries since two XX chromosomes needed

PUBERTY - development of secondary sex characteristics

• Puberty occurs when hypothalamus begins to secrete gonadotropin-releasing hormones (GnRH) causing pituitary to release

– Follicle-stimulating hormone (FSH)– Luteinizing hormone (LH)

In males, these hormones stimulate testes to produce sperms and secrete testosterone (androgens)

In females, they stimulate the ovaries to produce estradiol (estrogens)

Puberty – cont.

• Nutrition affects age of puberty– Reduced in developing countries– Thin girls reach puberty later

• Due to the presence of leptin

HypothalamusHypothalamus

Anterior Pituitary

FSH-RF

LH-RF

HormonalChanges in Puberty

HypothalamusHypothalamus

OVARIES TESTES

Anterior Pituitary

FSH-RF

LH-RF

FSH FSHLH ICSH(LH)

HormonalChanges inPuberty

HypothalamusHypothalamusHypothalamusHypothalamus

OVARIES TESTES

Anterior Pituitary

FSH-RF

LH-RF

FSH FSHLH ICSH*(LH)

OvulationCorpus Luteum

Progesterone

OvumGrowth

Estrogen (estradiol)

Testosterone

Spermatogenesis

HormonalChanges inPuberty

* Interstitial-cell stimulating hormone

Sexual Maturation

• Secondary sex characteristics (onset of puberty)– Females (estradiol)

• Enlarged breasts• Growth of the lining of the uterus• Widened hips• Maturation of genitalia

– Females (androgens)• Underarm and pubic hair (face as well)

– Males (androgens)• Facial, underarm, and pubic hair • Deep voice• Alter hairline (baldness)• Muscle development• Maturation of genitalia

– Males (estradiol)• Enlarged breasts

Hormonal Control of Reproductive Cycle

• Menstrual cycle– Begins with secretion of FSH to stimulate growth of ovarian

follicles (epithelial cells surrounding each ovum)– As ovarian follicles mature they secrete estradiol causing the

growth of the lining of the uterus (preparation for fertilization)– Increasing levels of estradiol triggers the release of LH causing

ovulation (release of ovum)– Ovum enters a Fallopian tube and starts migrating towards

uterus.– If it meets sperm and becomes fertilized it begins to divide and

then attaches itself to uterus wall– If it is not fertilized, the ruptured ovarian follicle (corpus luteum)

and the lining of the uterine wall will be expelled – menstruation commences