Human Sexual Differentiation

BrainGonads and

Internal Duct Systems

External Genitalia

All mammals including humans begin life as embryos that have the potential to be

either male or female in three ways

Gonads and Internal Duct Systems

A male has one X and one Y chromosome. The Y chromosome has an SRY gene, that produces a substance called H-Y antigen. H-Y antigen causes the undifferentiated gonads to become testes.

Male DifferentiationTestes and Ducts

SRY Gene

H-Y antigen

Testes

A female has two X chromosomes. There is no SRY gene, because that gene only exists on a Y chromosome. In the absence of the SRY gene, no H-Y antigen is produced. In the absence of H-Y antigen, the undifferentiated gonads become ovaries.

Female DifferentiationOvaries and Ducts

No

Antigens

produced

Development of External GenitaliaDevelopment of External Genitalia

Male Differentiation External Genitalia

The testes produce testosterone. The testosterone circulates in the bloodstream and is converted to dihydrotestosterone by a substance called 5 alpha reductase. Dihydrotestosterone causes the undifferentiated external genital tissue to develop into the penis and scrotum.

Adult Male External Genitalia

Male Adult Gonads and Ducts

Female Differentiation External Genitalia

The Ovaries produce no testosterone. In the absence of testosterone, the undifferentiated external genital tissue develops into the clitoris, labia minora and labia majora.

Adult Female Anatomy

Adult Female Gonads and Ducts

Male DifferentiationBrain and Behavior

Estradiol

4. The Presence of estradiol in the brain causes it to become masculinized.Blood-Brain Barrier

1. The testes produce testosterone.

Testosterone

Aromatase

2. Testosterone crosses the blood brain barrier into the brain.

3. A substance present in the brain called aromatase converts the testosterone to estradiol, a type of estrogen.

Female DifferentiationBrain and Behavior

Estradiol

2. Estradiol from the Mother crosses the Placenta into the baby’s blood stream

3. Alpha-fetoproteins present in the baby’s system cannot cross the blood-brain barrier. They bind with the estradiol and prevent it from crossing into the brain.

4. The absence of estradiol in the brain causes it to become feminized.

Blood-Brain Barrier

1. There are no testes to produce testosterone that can be converted to estradiol in the brain Alpha-fetoproteins

Four Examples of Ways Sexual Differentition Can Go Wrong

1. A genetically male embryo lacks the ability to sense the presence of testosterone.

2. The mother of a genetically male embryo is producing large amounts of adrenaline, which floods the embryo’s body. The adrenaline binds to the receptors that usually detect testosterone, preventing it from being recognized by the body, even if it is present in large amounts.

3. Lack of 5 alpha reductase that converts testosterone to dihydrotestosterone.

4. A genetically female embryo is exposed to external sources of testosterone, or substances that are mistaken for testosterone in the body. Eg. Diethylstilbestrol

X & Y Chromosomes

DNA

SRY gene

H-Y antigen

Gonad

Mesonephros

Mullerian ducts

Wolffian ducts

Testosterone

Key Terms5 alpha reductase

Dihydrotestosterone

Aromatase

Alpha-fetoprotein

Estradiol

Ovary

Fallopian tubes

Uterus

Vagina

Sexual Differentiation

Testis

Seminal vesicle

Prostate

Vas deferens

Epididymis

Penis

Clitoris

Scrotum

Vulva (Labia)

Ambiguous Genitalia

Hermaphrodite Genitalia

Surgical Reconstruction to Female

Surgical Reconstruction to Male