Intrauterine Reproductive Development

Physiology of Intrauterine Reproductive Development

The genetic sex of an individual is determined at conception. The sperm contains either an X or Y chromosome while the ovum always carries an X chromosome. Fertilization is the process where the union of egg and sperm cell occur. If a sperm containing an X chromosome fertilizes the ovum (X), the resulting zygote will become a chromosomal female (XX). But if the inherited chromosome from the father is Y combined with X from the mother the result will be a genetic male (XY). Thus, it is the father that determines the sex of the baby.

Gonads are body organs that produce sex cells in an individual. It is ovaries in females and testes in males. Several processes take place in the utero before gonads are formed and fetal external genitalia are differentiated to male and female.

  1. Primitive gonadal tissue is the first component of gonads formed at about 5 weeks of intrauterine life. During this time male and female reproductive system still looks similar or sexually undifferentiated.
  2. In both sexes, still at 5 weeks I.U. life, two indistinguishable ducts are formed and present. The first is called a mesonephric and the other is paramesonephric duct.
  3. Mesonephric or also called a Wolffian duct is a pair of organ found during intrauterine period that connects the primitive kidney to cloaca (opening of reproductive tracts) and will develop into male reproductive organ.
  4. Paramesonephric or mullerian ducts are paired ducts present during the prenatal period that develops into female reproductive organs.
  5. At about 7-8 weeks, internal reproductive structures starts to develop. Primitive testes begin to differentiate in genetic males. Testes (testis is the singular form) are responsible for producing testosterone. In response to the presence of testosterone, the mesonephric or Wolffian duct starts to develop into the male reproductive organs. Paramesonephric or mullerian duct on the other hand, undergoes degeneration. To easily remember because mesonephric duct starts with letter “M,” it is the structure that develops into male reproductive organs.
  6. The external genitals continue to look similar until the ninth week.
  7. Around 10 weeks, if no testosterone would cause the differentiation of mesonephric duct, the primitive gonadal tissues will develop into ovaries. The paramesonephric or mullerian duct then progresses to become the female reproductive organs. It is during this time that all oocytes (immature female reproductive cells) are formed inside the ovaries.
  8. Differentiation of external genitalia in both sexes is complete at approximately 12 weeks.
  9. The noticeable difference is noted because the influence of testosterone in male causes the penile tissue to elongate and the urogenital fold situated at the ventral portion of the penis closes to form the urethra.
  10. If no testosterone is detected in the system, the urogenital folds remains wide and apart (open) which forms the labia minora in females. The scrotal tissue in males is the one that develops as labia majora in females.
  11. Though the external reproductive organs are already different at 12 weeks, to avoid false interpretation during ultrasonography, it is recommended to undergo the test in determining the sex of the fetus at the midpoint of pregnancy where further external sexual development takes place. However, some are assessing the gender of the baby at about 18-22 weeks age of gestation. More inaccurate results are reported with an ultrasound done before 18 weeks.

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Daisy Jane Antipuesto RN MN

Currently a Nursing Local Board Examination Reviewer. Subjects handled are Pediatric, Obstetric and Psychiatric Nursing. Previous work experiences include: Clinical instructor/lecturer, clinical coordinator (Level II), caregiver instructor/lecturer, NC2 examination reviewer and staff/clinic nurse. Areas of specialization: Emergency room, Orthopedic Ward and Delivery Room. Also an IELTS passer.

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