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   Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:35:29 JST Vagina Museum

   Is it true that "all foetuses begin as female"? The answer is mostly no... it's a little more complicated than that. Let's look at sex differentiation in foetuses, and the genitals we all start life with...

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:35:46 JST Vagina Museum

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     Sex differentiation isn't actually a singular thing. There's several mechanisms involved, and they don't always entirely map on to each other. It's not as simple as male or female. On a biological level, there's several different types of sex: chromosomal sex (XX, XY or other combination); gonadal sex (ovaries, testes, organs which have characteristics of both, "streak gonads", none); and phenotypic sex (vulva/vagina, penis, other appearance).

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:36:09 JST Vagina Museum

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     Chromosomal sex is determined at conception. Most of the time, in typical development, chromosomal sex will influence gonad development and phenotype. Most of the time (but not all the time), an XX embryo will develop ovaries, a uterus, a vagina and a vulva.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:36:37 JST Vagina Museum

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     However, in early foetal development, neither the gonads nor the internal reproductive organs nor external genitals have developed. At around four weeks of development, the mesonephric ducts (Wolffian ducts) form. This pair of organs will either develop into structures such as the epididymis and vas deferens, or will regress, depending on phenotypic sex.

     At around six weeks, the gonads begin to differentiate.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:36:50 JST Vagina Museum

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     Also at around six weeks the paramesonephric (Mullerian) ducts start to develop. This pair of organs will either develop into the uterus, tubes and upper vagina, or regress, depending on phenotypic sex. An embryo at this stage of development will have both types of duct.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:37:07 JST Vagina Museum

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     In this early stage of development, the external genitals are entirely undifferentiated. To the eye, they look more similar to a vulva than they do to a penis and scrotum.

     Image credit: Sciencia58

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:37:21 JST Vagina Museum

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     These primordial, undifferentiated parts develop differently depending on expression of certain genes and certain hormones. There's lots of genes and hormones involved in this process, and they determine whether the paramesonephric ducts or mesonephric ducts will regress, and whether the external genitals will develop into a penis and scrotum, or a vulva and lower vagina.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:37:51 JST Vagina Museum

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     One of the key genes involved in determining which pathway a foetus develops is Sex-determining region Y (SRY) which is located on the Y chromosome. It was discovered in the 1990s. It produces a protein of the same name which causes the gonads to become testes, triggering a chain reaction which affects the appearance of the genitalia and hormones which cause the paramesonephric ducts to regress.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:38:05 JST Vagina Museum

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     With so many moving parts, not every foetus will develop along one of two binary pathways. Chromosomal sex, gonadal sex, and phenotypic sex can be different, and science isn't entirely sure how many people have these variations.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:38:25 JST Vagina Museum

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     Until recently, it was assumed that without active exposure to proteins such as SRY, a foetus would develop with ovaries, a vulva and a uterus - that "female" is the default, and is passive. However, this might not be the case.

     An experiment by Zhao and colleagues, published in 2017, found that in mice, the process of eliminating the mesonephric ducts was an active process requiring a specific protein. Without this protein, the ducts would persist.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:38:42 JST Vagina Museum

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     So, it's not true to say that all of us start out as female. It also might not be true to say that female is the default developmental pathway in absence of certain genes and hormones expressing.

     So why do so many people believe that we all start out life as female? There's two possible explanations: a sexist one, and a not sexist one!

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:38:52 JST Vagina Museum

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     Under patriarchy, there's a few beliefs that might contribute to the "female as default" view. The phallus is seen as masculine, and absence of a phallus (as seen in early development) is feminine. Along with this is a patriarchal belief of femininity as passive, which is how development has conventionally been seen.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:39:01 JST Vagina Museum

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     On the other hand, others see the "female as default" perspective as challenging sexism. Patriarchal norms set the "male" as default, but "female as default" directly challenges this.

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     Vagina Museum (vagina_museum@masto.ai)'s status on Friday, 24-Jan-2025 01:39:21 JST Vagina Museum

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     The science of genes, and the science of early development is still relatively young. There's a lot we don't know yet.