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Embryo Development

Internal and external genital organs

Internal and external genitals



  

Details of genital development

 
At conception, gender is determined by chromosome characteristics -

The genetic sex of a child is established at conception based on the 23rd pair of chromosomes it inherits. The mother's egg contains an X chromosome, and the father's sperm will contain either an X or Y chromosome. It is the male (or rather the male's sperm) that dictates whether the baby will be a boy or a girl.
X chromosome
X chromosome
 
  • A baby who inherits the X chromosome from the father is a genetic female (a pair of X chromosomes).
  • If the embryo is female (XX), then no testosterone is made. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs. The female clitoris is the remnants of the Wolffian duct.
  •    
    Y chromosome
    Y chromosome
     
  • A baby who inherits the Y chromosome from the father is a genetic male (one X and one Y chromosome).
  • If the embryo is a male (XY chromosomes), then testosterone will stimulate the Wolffian duct to develop male sex organs, and the Mullerian duct will degrade.
  •    
    gonads and ductsDuring the first few weeks of fetus development, your baby's internal and external genital structures are the same, regardless of whether you are ultimately going to have a boy or a girl.

    They have two sets of organs: one that can develop into the female sex organs (Mullerian duct) and one that can develop into the male sex organs (Wolffian ducts).

    The gonads will become ovaries or testicles, the phallus will become a clitoris or a penis, and the genital folds will become labia or scrotum. Which sex organs develop depends on the presence of the male hormone testosterone (in humans, the default sex is female).

    male_internalThe SRY gene, on the short arm of the Y chromosome, initiates male sexual differentiation. The SRY influences the undifferentiated gonad to form a testes, which produces the hormonal milieu that results in male sexual differentiation. Testosterone stimulates the Wolffian structures (epididymis, vas deferens, and seminal vesicles), and anti-Mullerian hormone (AMH) suppresses the development of the Mullerian structures (fallopian tubes, uterus, and upper vagina).

    Testosterone converts to dihydrotestosterone in the skin of the external genitalia and masculinizes the external genital structures. By 12 weeks most of this male differentiation has occurred, but they are still not completely formed. On ultrasound, if your baby is cooperating, the sex can be identified as early as the 16th to 18th week of your pregnancy. The testicles remain inside the abdomen until late in the third trimester, when they usually descend into the scrotum. Sperm is not produced until puberty.


    female_internalFemale development will occur unless maleness is actively induced by the Y chromosome. If the embryo is female (XX), then no testosterone is made. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs. In females, the gonads become ovaries; the uterus, cervix, fallopian tubes, and vagina form; the labia develop; and the phallus becomes a clitoris.

    In girls, the ovaries contain over six million eggs, this decreases to approximately one million by birth and will be reduced to about 400 by the time of puberty.
     
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    External genital changes in Embryo development

    42 days (6 weeks) after fertilization, around week 8 of pregnancy
     
    6 week old embryo (Embryo size = 0.5 inch, 12 mm) 

    1. Buds of the arm
    2. Arc branchial
    3. Placenta membrane
    4. Eye
    5. Genital tuber
    6. Site of the heart
    7. Bud of the leg
    8. Tail
    9. Umbilical cord
     week5 embryoAt the sixth week the site of the genitals is a small bud, called the genital tuber.

    Until the ninth week of embryo development, the embryonic reproductive apparatus is the same one for the two sexes.

    By the end of 20th week, the external genitalia changes for boys and girls are almost done.

    On ultrasound, if your baby is cooperating and is positioned in a favorable way, his or her sex can be identified as early as the 16th to 18th week of your pregnancy.
    More: external genitals >>   
     
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