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A baby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female or male. The (XX) chromosome means the baby will be a girl, and (XY) means it will be a boy. Even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until -the fourth month of pregnancy—seven weeks after conception. The fetus appears to be sexually indifferent, looking neither like a male or a female. Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation.

Sex Determination

In both male and females, the sex organs consist of three structures: the gonads, the internal genitalia, and the external genitalia. In males, the gonads are the testes and in females they are the ovaries. These are the organs that produce gametes (egg and sperm), the reproductive cells that will eventually meet to form the fertilized egg (zygote). As the zygote divides, it first becomes the embryo, typically between zero to eight weeks, then from the eighth week until birth, it is considered the fetus. The internal genitalia are all the accessory glands and ducts that connect the gonads to the outside environment. The external genitalia consist of all the external reproductive structures.

Sex Chromosomes

The sex chromosomes that a person inherits will determine the genetic sex of that individual. To be genetically female, one needs to be (XX), whereas to be a genetic male, (XY) is needed. It is the Y chromosome that is essential for the development of the male reproductive organs. In fact, with the absence of a Y chromosome, an embryo will develop into a female. A zygote that only gets one X chromosome (XO) results in Turner’s syndrome and will develop into a female.[1]

Sex Differentiation

Sexual differentiation is defined as the phenotypic development of structures consequent upon the action of hormones produced following gonadal determination.[2] The sex of an early embryo cannot be determined because the reproductive structures do not differentiate until the seventh week. Prior to this, the tissue is considered bipotential because it cannot be identified as male or female. The internal genitalia consist of two accessory ducts: Wolffian (male) and Mullerian (female). As development proceeds, one of the pairs of ducts develops while the other regresses. Sex determination depends on the presence or absence of the sex determining region of the Y chromosome, also known as the SRY gene.[3] In the presence of a functional SRY gene, the bipotential gonads develop into testes.

Male Development

The SRY gene when transcribed and processed produces SRY protein that binds to DNA and directs the development of the gonad into testes. Male development can only occur when the fetal testis secretes key hormones at a critical period in early gestation. The testes begin to secrete three hormones that influence the male internal and external genitalia: they secrete anti-Müllerian hormone (AMH), testosterone, and dihydrotestosterone (DHT). Anti-Müllerian hormone causes the Müllerian ducts to regress. Testosterone converts the Wolffian ducts into male accessory structures, including the epididymis, vas deferens, and seminal vesicle. Testosterone will also control the descending of the testes from the abdomen into the scrotom. Dihydrotestosterone will differentiate the remaining male characteristics of the external genitalia.[4]

References

  • A zygote that only gets one X chromosome (XO) results in Turner’s syndrome and will develop into a female.[5]
  • Sexual differentiation is defined as the phenotypic development of structures consequent upon the action of hormones produced following gonadal determination.[6]
  • Sex determination depends on the presence or absence of the sex determining region of the Y chromosome, also known as the SRY gene.[7]
  • The testes begin to secrete three hormones that influence the male internal and external genitalia. They secrete anti-Müllerian hormone, testosterone, and Dihydrotestosterone. Anti-Müllerian hormone (AMH) causes the Müllerian ducts to regress. Testosterone, which is secreted and converts the Wolffian ducts into male accessory structures, such as epididymis, vas deferens and seminal vesicle. Testosterone will also control the descending of the testes from the abdomen into the scrotom. Dihydrotestosterone, also known as (DHT) will differentiate the remaining male characteristics of the external genitalia.[8]
  • Figure One The Wolffian System Pathway [9]
  1. Rey,Rodolfo, MD, PhD, Josso, Nathalie MD, PhD. (). Chapter 7. Sexual Differentiation. Available: http://www.endotext.org/pediatrics/pediatrics7/pediatrics7.html. Last accessed may 21, 2011.
  2. Hughes, Ieuan A. . (June 12, 2011). Minireview: Sex Differentiation. Available: http://endo.endojournals.org/content/142/8/3281.full. Last accessed May 21, 2011.
  3. Rey,Rodolfo, MD, PhD, Josso, Nathalie MD, PhD. (). Chapter 7. Sexual Differentiation. Available: http://www.endotext.org/pediatrics/pediatrics7/pediatrics7.html. Last accessed may 21, 2011.
  4. Hughes, Ieuan A. . (June 12, 2011). Minireview: Sex Differentiation. Available: http://endo.endojournals.org/content/142/8/3281.full. Last accessed May 21, 2011.
  5. Rey,Rodolfo, MD, PhD, Josso, Nathalie MD, PhD. (). Chapter 7. Sexual Differentiation. Available: http://www.endotext.org/pediatrics/pediatrics7/pediatrics7.html. Last accessed may 21, 2011.
  6. Hughes, Ieuan A. . (June 12, 2011). Minireview: Sex Differentiation. Available: http://endo.endojournals.org/content/142/8/3281.full. Last accessed May 21, 2011.
  7. Rey,Rodolfo, MD, PhD, Josso, Nathalie MD, PhD. (). Chapter 7. Sexual Differentiation. Available: http://www.endotext.org/pediatrics/pediatrics7/pediatrics7.html. Last accessed may 21, 2011.
  8. Hughes, Ieuan A. . (June 12, 2011). Minireview: Sex Differentiation. Available: http://endo.endojournals.org/content/142/8/3281.full. Last accessed May 21, 2011.
  9. Silverthorn, Dee, U.. (2010). Reproduction and Development. In: Human Physiology: an integrated approach. 5th ed. san francisco: Pearson education. p828-831.
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