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Human Sexual Differentiation. 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. Gonads and Internal Duct Systems. Male Differentiation Testes and Ducts. SRY Gene.
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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
Male DifferentiationTestes 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
Female DifferentiationOvaries and Ducts No 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.
Development of External Genitalia Development of External Genitalia
Male DifferentiationExternal 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.
Female DifferentiationExternal 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.
Male DifferentiationBrain 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
Female DifferentiationBrain 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
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
Key Terms Sexual Differentiation • 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)
