Urethral folds and genital swellings: labia and scrotum
In typical development, urethral folds (paired ridges) form labia minora (inner lips) in females; genital swellings (paired bulges) form labia majora (outer lips) in females or the scrotum in males.
Remember
In typical development, urethral folds left unfused become the labia minora and genital swellings left in place become the labia majora; in males the folds fuse into the penile urethra and the swellings fuse into the scrotum.
Until about 9 weeks, male and female embryos have identical external genitalia. Each has a genital tubercle (a small bud at the front), a pair of urethral folds (ridges of tissue beside the midline) and a pair of genital swellings (bulges lying outside the folds).
In a male, testosterone is converted to dihydrotestosterone (DHT), a more potent form, and this makes the tubercle grow into the penis. The tubercle pulls the urethral folds along, and they then fuse to enclose the urethra. In males, the genital swellings move backwards (caudally, towards the tail end), fusing in the midline to form the scrotum.
A female has no Leydig cells (cells in the testis that produce testosterone), so the tubercle grows only slightly, forming the clitoris. The urethral folds are not stretched and they do not fuse, so they remain separate and form the labia minora, the smaller inner lips. The genital swellings have no need to move caudally, so they remain where they are and become the labia majora, the larger outer lips.
In a female, folds give the inner lips, and swellings give the outer lips.
Key facts
- Urethral folds
- they form the labia minora in females, and in males they are pulled along by the elongating tubercle to form the penile urethra.
- Genital swellings
- they stay in place to form the labia majora in females, and in males they move caudally (towards the tail end) and fuse to form the scrotum.
- Genital tubercle
- it elongates rapidly into the penis in males, but grows only slightly into the clitoris in females.
- Homologous structures
- the scrotum and the labia majora are homologous, meaning both arise from the same embryonic tissue, the genital swellings.
- Clinical link
- failure of the urethral folds to fuse in a male causes hypospadias, in which the urethra then opens on the underside of the penis.
Exam traps
- Genital swellings that remain in place in a female form the outer lips, the labia majora, and do not give rise to the labia minora, which come from the urethral folds.
- The clitoris does not come from the urethral folds: it develops from the genital tubercle, which grows only slightly in a female.
- The genital tubercle does not form the scrotum: it becomes the clitoris in females and the penis in males, and the swellings beside it give the scrotum instead.
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