Changes in gender characteristics during fetal development
The reproductive distinction of the fetus is determined after the fertilized egg is formed, while the gender characteristics are gradually formed during the development of the fetus. During the four months of fetal development, the gender characteristics of male and female fetuses change differently.
Formation of gonads
During the 4th week of embryonic development, the reproductive ectoderm begins to differentiate into the reproductive venation. In the 6th week, the reproductive venation further differentiates into gonad primordia. The gonad primordia of male and female fetuses are similar in morphology, but in the 7th week, the gonad primordia of male fetuses begin to differentiate into testes, while the gonad primordia of female fetuses develop into ovaries. This is also the earliest manifestation of the reproductive differences between male and female fetuses.
Secretion of gender hormones
Both the testicles and ovaries secrete gender hormones. During the 8th to 14th week of the male fetus, the testicles secrete a large amount of testosterone, which promotes the development of the reproductive tract, urethra and external genitalia through endocrine effects. The ovaries of female fetuses also secrete a small amount of estrogen a few weeks before birth, which is helpful for the normal development of the female fetus\’s reproductive system and the proliferation of mesenchymal cells.
Formation of external genitalia
There are also great differences in the development process of the external genitalia of male and female fetuses. In the 12th week, the penis and scrotum of male fetuses begin to form, and the vagina of female fetuses begins to differentiate. At 16 weeks, the male fetus\’s penis begins to erect, and the female fetus\’s vagina begins to grow forward. Around the 20th week, the external genitalia of male and female fetuses have developed into obvious male or female characteristics.
Determination of genetic sex
During the division of germ cells, sperm and eggs each contain a set of chromosomes. Among them, males are composed of XY sex chromosomes and females are composed of XX sex chromosomes. During the formation of a fertilized egg, if the sperm carries a Y chromosome, the fertilized egg will develop into a male fetus; if the sperm carries an X chromosome, the fertilized egg will develop into a female fetus. Therefore, genetic sex is also an important factor in determining the reproductive differences between male and female fetuses.
Summary
Within four months of the fetus, the reproductive differences between male and female fetuses are mainly reflected in the formation of gonads, secretion of gender hormones, formation of external genitalia and determination of genetic sex. These changes are complex and precise and require the synergy of multiple factors to form the unique gender characteristics of male and female fetuses.
