1. Human Sexual Differentiation

2. Gonads and Internal Duct Systems
All mammals including humans begin life as embryos that have the potential to be either male or female in three ways
Brain
Gonads and Internal Duct Systems
External Genitalia

3. Gonads and Internal Duct Systems

4. Male Differentiation Testes and Ducts
SRY Gene
H-Y antigen
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.
Testes

5. Female Differentiation Ovaries and Ducts
Antigens
produced
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.

6. Development of External Genitalia

7. 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.

8. Adult Male External Genitalia

9. Male Adult Gonads and Ducts

10. 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.

11. Adult Female Anatomy

12. Adult Female Gonads and Ducts

13. Male Differentiation Brain and Behavior
1. The testes produce testosterone.
Testosterone crosses the blood brain barrier into the brain.
4. The Presence of estradiol in the brain causes it to become masculinized.
A substance present in the brain called aromatase converts the testosterone to estradiol, a type of estrogen.
Blood-Brain Barrier
Aromatase
Estradiol
Testosterone

14. Female Differentiation Brain and Behavior
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.
2. Estradiol from the Mother crosses the Placenta into the baby’s blood stream
4. The absence of estradiol in the brain causes it to become feminized.
1. There are no testes to produce testosterone that can be converted to estradiol in the brain
Blood-Brain Barrier
Estradiol
Alpha-fetoproteins

15. Four Examples of Ways Sexual Differentition Can Go Wrong
A genetically male embryo lacks the ability to sense the presence of testosterone.
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.
Lack of 5 alpha reductase that converts testosterone to dihydrotestosterone.
A genetically female embryo is exposed to external sources of testosterone, or substances that are mistaken for testosterone in the body. Eg. Diethylstilbestrol

16. Sexual Differentiation
Key Terms
X & Y Chromosomes
DNA
SRY gene
H-Y antigen
Gonad
Mesonephros
Mullerian ducts
Wolffian ducts
Testosterone
5 alpha reductase
Dihydrotestosterone
Aromatase
Alpha-fetoprotein
Estradiol
Ovary
Fallopian tubes
Uterus
Vagina
Testis
Seminal vesicle
Prostate
Vas deferens
Epididymis
Penis
Clitoris
Scrotum
Vulva (Labia)

17. Ambiguous Genitalia

18. Hermaphrodite Genitalia

19. Surgical Reconstruction to Female

20. Surgical Reconstruction to Male