There are general trends of sexual differentiation that are true for most people, but biology is far from straight forward.
Gonads, meaning testes, ovaries, and ovotestes, are initially the same tissue in the embryo and serve the role of producing gametes (sperm or eggs) and gonadal steroids. By the fifth week, this bipotential gonad begins to develop (Atasha & Kewal, 2022). Generally, gonadal differentiation follows one of two paths: development into testes and development into ovaries. In some cases, this process leads to the development of ovotestes: a gonad with both ovarian and testicular tissues (Ortenberg et al., 2002).
Ovary development is driven by several genes, including Wnt4. This process begins after the 7th week of gestation and ends around the 7th month (Atasha & Kewal, 2022).
Testes development is triggered by the testis-determining factor of the SRY gene and the action of the SOX9 gene (Atasha & Kewal, 2022).
We’ll start by taking a look at internal genitalia.
Just like for gonads, the organs that will become the internal genital tract are found in all human embryos. These organs are the Wolffian and Mullerian ducts. They are important to the fetal kidney and are connected to the urogenital sinus. Generally, only one of these ducts is maintained during fetal development.
Around the 9th week of pregnancy, testicular tissue begins to produce testosterone which leads to the maintenance of the Wolffian duct and its development into the epididymis, vas deferens, and seminal vesicle (Atasha & Kewal, 2022). The testes also produce another hormone known as the anti-Mullerian hormone which causes the Mullerian duct to atrophy (Atasha & Kewal, 2022; Sajjad, 2010).
In the absence of testosterone, the Wolffian duct atrophies instead and the Mullerian duct forms the fallopian tubes, uterus, and upper part of the vagina (Atasha & Kewal, 2022; Sajjad, 2010).
Source: SimpleMed original by Maddie Swannack
Now onto external genitalia.
Up until the 9th week of gestation, external genitalia is indifferent and is made up from the following structures: the genital tubercle, the urogenital folds, of which a portion will become the urethra, and the labioscrotal swelling (Atasha & Kewal, 2022; Sajjad, 2010).
Again, androgens play a major role in the development of differentiated external genitals. Their presence causes the genital tubercle to elongate into the penis, connecting it to the urogenital sinus through the urethra. The urogenital sinus also develops into the prostate. The labioscrotal swelling fuses, creating the scrotum (Atasha & Kewal, 2022; Sajjad, 2010). This process runs its course by week 14 of gestation.
When levels of androgens are low and levels of estrogen are high in the placental environment, these external tissues develop differently. The genital tubercle also elongates, but less, forming the clitoris. The labias majora and minora develop from the unfused urogenital folds and labioscrotal swelling. The Mullerian duct joins with the urogenital sinus to create the lower part of the vagina. This happens between the 11th and 20th weeks of gestation (Atasha & Kewal, 2022; Sajjad, 2010).
These processes can also lead to ambiguous genitalia (Mehmood & Rentea, 2023). This can include a larger than expected clitoris, or a penile urethra that is not at the tip of the penis. While some of these genital variations can have health implications, intersex advocates state that most of them are not harmful and do not require surgery. Genital surgery on intersex infants is a frequent practice that seeks to “normalize” the genitalia and may do more harm than good (interACT, n.d.).
Source: Sciencia58, CC0, via Wikimedia Commons
Many structures are homologous, which means they came from the same tissue and serve similar purposes, in human genitalia. For example, Bartholin's glands, which are located near the vaginal opening and secrete mucus, are homologous to the bulbourethral glands, which are found near the prostate and also secrete mucus (Chughtai et al., 2005; Nguyen & Duong, 2022). Another example is Skene’s glands which are homologous to the prostate. Both have important roles in urination and ejaculation (Nguyen & Duong, 2022).