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Development of the Penis and Scrotum
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Development of the Penis and Scrotum

Development of the Testes and Scrotum
Includes the penis, penile urethra, and scrotum (which houses the testes). During gestation, development is heavily influenced by testosterone produced by the Leydig (aka, interstitial) cells of the testes.
Weeks 3-4
  • Period of undifferentiated primordial genitalia
  • The genital tubercle, which is the precursor to the penis, emerges.
  • Cloacal membrane is bordered by the primorida of the genital tubercle, and is surrounded by mesenchymal cells.
  • Anterior fusion of the primordia forms the definitive genital tubercle.
— Posteriorly, the unfused portion gives rise to the urogenital (aka, genital) folds, which enclose the cloacal membrane. — Lateral to the folds the genital swellings (aka, labioscrotal swellings) arise.
Weeks 7-12
The urorectal septum has reached the cloacal membrane and now forms the perineal body; male differentiation begins.
  • The perineal body divides the cloacal membrane into urogenital and anal membranes, which then rupture, forming the urogenital sinus and anus.
  • The genital tubercle enlarges and is now referred to as the phallus.
  • The urogenital folds are now the urethral folds, as they line the urethral groove.
— The roof of the groove comprises an endodermal expansion.
  • Distally is the glans, and the glans plate, which is a solid endodermal remnant of the cloacal membrane.
  • The genital swellings are now the scrotal swellings.
  • Posteriorly, the perineum separates the urethral folds and groove from the anus.
Week 12
The urethral folds and scrotal swellings fuse at the midline.
  • Fusion of the urethral folds forms the floor of the spongy urethra within the shaft of the penis.
  • Midline raphes reflect fusion of the tissues.
  • The surrounding mesenchyme will give rise to the erectile tissues and vasculature of the penis.
  • Formation of the glans portion of the urethra occurs separately, via canalization of the glans plate (the exact mechanism is uncertain).
  • At the midline, the raphe reflects the fusion of the urethral folds.
9 Months
Folds are fully fused, and urethral opening is at the tip of the penis. The scrotal sac houses the testes.

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