Prenatal Phase

Lesson 4: Page 3 of 26

Developmental Biology : Prenatal Phase

Sex Differentiation and Initial Development

The sex of a fetus is determined solely by genetics. In females, the 23rd pair of chromosomes consists of an XX pair, and in males, an XY pair. The presence of the SRY gene on the Y chromosome is necessary for the testes to develop. View the process of Meiosis for Sperm Cells.

During weeks 8 to 16 of the prenatal stage, the fetus begins to show differentiation of external reproductive organs, and the female subsequently develops primitive egg cells. Androgens play an important role in this early differentiation and in the womb environment throughout the pregnancy.

Through week 24, there are significant sex differences in fetal pituitary gonadotropin levels, and the circulating testosterone levels in the male fetus are high.³ There is no difference in the concentrations of estradiol or other estrogens in serum in female and male fetuses.
At about week 9, the testes produce testosterone, which is essential to the development of the male sexual tract and genitalia, and is responsible for brain differences that distinguish the normal male from the female. These early developments are important determinants of the individual's biological and behavioral growth. Without the SRY gene, the fetus continues normal development as a female.
During weeks 11 to 12 of fetal development, germ cells in the ovary begin meiosis and the transition of oogonia to oocytes. Fetal ovarian secretions are not required for female sex differentiation.⁴


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Table of Contents Introduction Lessons: Sex and Gender in Biomedical Research Legislative Process Framework Cell Physiology Developmental Biology Pharmacodynamics and Pharmacokinetics Clinical Applications of Genomics Course Resources	Prenatal Phase Lesson 4: Page 3 of 26 Developmental Biology : Prenatal Phase Sex Differentiation and Initial Development The sex of a fetus is determined solely by genetics. In females, the 23rd pair of chromosomes consists of an XX pair, and in males, an XY pair. The presence of the SRY gene on the Y chromosome is necessary for the testes to develop. View the process of Meiosis for Sperm Cells. During weeks 8 to 16 of the prenatal stage, the fetus begins to show differentiation of external reproductive organs, and the female subsequently develops primitive egg cells. Androgens play an important role in this early differentiation and in the womb environment throughout the pregnancy. Through week 24, there are significant sex differences in fetal pituitary gonadotropin levels, and the circulating testosterone levels in the male fetus are high.³ There is no difference in the concentrations of estradiol or other estrogens in serum in female and male fetuses. At about week 9, the testes produce testosterone, which is essential to the development of the male sexual tract and genitalia, and is responsible for brain differences that distinguish the normal male from the female. These early developments are important determinants of the individual's biological and behavioral growth. Without the SRY gene, the fetus continues normal development as a female. During weeks 11 to 12 of fetal development, germ cells in the ovary begin meiosis and the transition of oogonia to oocytes. Fetal ovarian secretions are not required for female sex differentiation.⁴ << Previous Page Next Page >>