The development of the external reproductive system (male, female, and variations) explained in 15 minutes.
If you are completely new to embryology and you want to understand it quickly, this should be the first video you watch:
- kzfaq.info/get/bejne/opuXiKSDzb67hI0.html
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SUMMARY OF THE VIDEO FOR YOUR NOTES
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1. The indifferent stage
- In week 3 of development, mesenchyme cells coming from the primitive streak migrate to the region around the cloacal membrane forming cloacal folds
- Superiorly to this membrane, the folds will fuse to form the genital tubercle
- Inferiorly to the folds, they will form the urethral folds and the anal folds
- Genital swellings appear lateral to the urethral folds
- Sex cannot be determined until week 7
2. Male external genitalia development
- Testosterone signals the genital tubercle to grow rapidly becoming the phallus. This elongation will pull the urethral folds with it and by week 12 the folds fuse and contain a lumen called the urethra
- Ectoderm cells invade the urethra keeping it solid at the distal end, but later the urethra will canalise to be a patent lumen
- The genital swellings are the scrotal swellings, they move inferiorly.
- In the male the urethral folds will fuse
- Hypospadias are the result of an incomplete fusion of the urethral folds
3. Female external genitalia development
- Oestrogen enables the development of the external genitalia in females.
- The genital tubercle will become the clitoris
- The urethral folds develop into the labia minora
- The genital swellings will become the labia majora
4. Variations
- Ambiguous genitalia
- ?Hermaphrodites
- Ovotestes - both ovarian and testicular tissue present. 70% of the time the genitalia are ambiguous or mostly female
- Congenital adrenal hyperplasia (CAH): increased adrenocorticotropic hormone (ATCH) and reduced steroid formation
- Androgen insensitivity syndrome (AIS) - males but lack of either androgen receptors, or unable to respond to dihydrotestosterone (DHT)
- Complete androgen insensitivity syndorme (CAIS) - vagina is present but short, testes found in inguinal or labial regions but spermatogenesis does not occur. (increased risk of testicular cancer/malignancy)
- 5-alpha-reductase deficiency - unable to convert testosterone to DHT
- Klinefelter syndrome
- Swyer syndrome (gonadal dysgenesis)
- Turner syndrome