Are Human Embryos All Female to Begin With?
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The evidence suggests that human embryos are not all female to begin with. In fact, there appears to be a higher proportion of male embryos at the earliest stages of development. However, a female bias in derived embryonic stem cell lines is observed, likely due to culture conditions rather than an inherent gender imbalance in the embryos themselves.
The question of whether human embryos are all female to begin with is a topic of interest in developmental biology. This inquiry delves into the sex determination and differentiation processes during early embryonic development.
Key Insights
Sex Ratio in Early Embryos
A study analyzing 1,452 human embryonal gonads found that the ratio of male to female embryos was fairly constant at later stages of development but showed a higher male ratio at earlier stages2.
This suggests that the primary sex ratio may be higher than the secondary sex ratio observed at birth, indicating a higher proportion of male embryos in the earliest stages of development2.
Female Bias in Embryonic Stem Cell Lines
Research on preimplantation genetic diagnosis (PGD)-derived human embryonic stem cell (HESC) lines showed a significant increase in female cell lines (76%)1.
This female preponderance was attributed to suboptimal culture conditions rather than an actual gender imbalance in the embryos used for derivation1.
Are human embryos all female to begin with?
Taylor Pini has answered Extremely Unlikely
An expert from University of Sydney in Reproductive Biology
No, the sex of the embryo is determined at fertilization and depends on whether the sperm that fertilizes the egg carries either an X or a Y chromosome (sperm only carry one or the other, because they are “haploid” cells). All eggs carry an X, because females are XX. If the fertilizing sperm carries an X, the resulting embryo will be XX (female) and if the fertilizing sperm carries a Y, the resulting embryo will be male (XY).
What this video talks about is the activation or “switching on” of a special gene (called SRY), carried only on the Y chromosome. XX and XY embryos essentially undergo the same development up until the SRY gene is turned on. Once this gene is activated, development of XY embryos is steered towards producing testes rather than ovaries (i.e. male reproductive organs), and will not produce a uterus or fallopian tubes (i.e. female reproductive organs).
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