SRY gene and testis development
The SRY gene, which sits on the Y chromosome, sends the indifferent gonad (one that looks the same in male and female embryos) towards a testis.
Remember
The gonad follows the SRY gene: SRY on the Y chromosome turns the indifferent gonad into a testis, and the hormones that shape the ducts and genitals act afterwards.
Every early embryo starts with an indifferent gonad, an early gonad that looks the same in male and female embryos. It is made of two ingredients: germ cells (the future sperm or eggs) and primitive sex cords (columns of cells in the early gonad).
Whether this gonad becomes a testis or an ovary depends on the SRY gene, which is found only on the Y chromosome, so normally only XY embryos switch it on. SRY makes the primitive sex cords develop into testis cords. These later become the seminiferous tubules (coiled tubes in which sperm are made after puberty). Without SRY, the cords degenerate and the gonad becomes an ovary.
Hormones come second. Once the testis exists, Sertoli cells (which line the tubules) make anti-Müllerian hormone (a hormone that makes the female duct system regress), and Leydig cells (which lie between the tubules) make testosterone from about 8 weeks. These hormones then shape the ducts and the external genitals, so they are results of SRY, not the decision itself.
The gonad follows the SRY gene, not the karyotype (the chromosome pattern) alone. An XY embryo whose SRY gene does not work forms no testes, and its gonad takes the ovarian route, but it becomes an underdeveloped 'streak' gonad rather than a functional ovary.
Key facts
- The switch
- SRY, a gene found only on the Y chromosome, sends the indifferent gonad towards a testis, and without it an ovary forms.
- Shared steps
- Germ cell migration and primitive sex cord formation happen in both sexes, so neither decides between a testis and an ovary.
- Order of events
- The order of events is SRY, then testis cords, then Sertoli and Leydig cells, then the hormones that shape the genitals.
- Leydig cells
- They lie between the tubules and begin making testosterone at about 8 weeks.
- Swyer syndrome
- An XY person with a non-functional SRY gene has a female appearance and underdeveloped 'streak' gonads instead of testes, and typically presents with primary amenorrhoea (periods that never start).
Exam traps
- Germ cell migration into the gonad does not decide between a testis and an ovary, because the future sperm or eggs arrive in both sexes.
- Anti-Müllerian hormone does not start testis formation, because it is made by Sertoli cells, which appear after the testis has begun to form, so the hormone follows testis formation rather than causing it.
- SOX9 is a downstream gene (it is activated by SRY), so without SRY at the critical point in normal development it is not expressed (switched on) and cannot initiate testis formation.
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