Answer from top 2 papers
The notion that every embryo starts as female is not entirely accurate based on the provided research papers.
Recent molecular studies suggest that sex determination may begin shortly after conception, rather than at the time of gonadal ridge formation (). Evidence indicates that male and female preimplantation embryos are antigenically distinguishable, suggesting sex differences in gene expression very early in development (). Additionally, the SRY gene, responsible for testis determination, is transcribed in preimplantation embryos ().
Interestingly, some studies have found that male embryos develop faster than female embryos in certain species. For instance, in bovine in vitro-fertilized embryos, male blastocysts developed faster than female blastocysts (). Similarly, in mice, male embryos were found to be heavier and contain more protein than female embryos as early as day 12, before gonadal differentiation is apparent ((Scott and Holson, 1977)).
In conclusion, while the default developmental pathway may appear female in the absence of male-specific genes and hormones, the embryo is not truly "female" from the start. Sex-specific differences are present from very early stages of development, and the process of sex determination is complex, involving multiple genes and signaling pathways ( and ). The idea that every embryo starts as female is an oversimplification of the intricate process of sexual differentiation in mammals.
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