Are all embryos female at first
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Are All Embryos Female at First?
Embryonic Sex Differentiation
The question of whether all embryos start as female has been a topic of scientific inquiry for decades. Research by the French endocrinologist Jost in the 1950s concluded that early embryos are not undifferentiated but are initially female. Genetic sex is determined at fertilization, but the influence of sex genes becomes apparent only around the fifth to sixth week of fetal life. If the genetic sex is female, the germ cells migrate to the gonadal cortex, forming ovaries and producing fetal estrogen. If the genetic sex is male, the germ cells migrate to the gonadal medulla, where they stimulate the production of androgens, leading to the development of male genitalia.
Role of Maternal and Paternal Contributions
The development of embryos involves contributions from both maternal and paternal factors. Studies using time-lapse systems to analyze embryo development from donor oocytes and sperm have shown that while female components significantly influence early embryo kinetics, male contributions are also present but less pronounced. This indicates that both maternal and paternal factors play roles in early embryonic development, although the initial stages are heavily influenced by maternal physiology.
Evolutionary Perspective on Embryo Implantation
From an evolutionary standpoint, the initial stages of embryo implantation in eutherian (placental) mammals required changes in maternal physiology. This "female-first" scenario suggests that maternal innovations were crucial for the evolution of sustained fetal-maternal interactions. In contrast, marsupials exhibit a more mutual relationship between the fetus and the mother, characterized by cooperative inflammatory signaling pathways.
Gender-Specific Embryo Development
Research using time-lapse technology to compare male and female embryo development has found that while there are some differences in growth dynamics, these differences are not statistically significant. Female embryos tend to show earlier cavitation than male embryos, but overall, the cell division kinetics from early cleavage to the blastocyst stage are similar for both genders. Another study found that specific kinetic variables, such as second synchrony and timing of morula formation, could predict the likelihood of an embryo being female, suggesting that embryo development is indeed affected by sex.
Conclusion
In summary, while genetic sex is established at fertilization, early embryos exhibit female characteristics until the influence of sex genes becomes apparent around the fifth to sixth week of fetal life. Both maternal and paternal factors contribute to early embryonic development, with maternal physiology playing a significant role in the initial stages. Although there are some gender-specific differences in embryo development, these differences are generally not significant. Thus, it can be concluded that embryos start with female characteristics before differentiating into their respective sexes.
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