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Development of the Penis and Scrotum
The development of the penis and scrotum is androgen-dependent (specifically, DHT).
Undifferentiated primordial genitalia during weeks 4-5.
The cloacal membrane, which covers the opening of the cloaca prior to separation of the distal urinary and digestive tracts. The paired mesenchymal primordia of the genital tubercle develop cranial to the cloacal membrane; notice the surrounding mesenchymal cells.
By the end of approximately week 5, the primordia coalesce cranially to form the definitive genital tubercle. Posteriorly, the unfused cloacal folds become the urogenital folds, surrounding the cloacal membrane.
Posteriorly, the unfused portion of the cloacal folds become the genital folds, with lateral swellings called genital swellings (aka labioscrotal swellings).
Approximately weeks 9-12.
The perineal body forms between the urogenital and anal regions.
The genital tubercle is enlarged, and we now refer to it as the phallus.
At the tip of the phallus is the glans, and glans plate, which is a solid epithelial component involved in the formation of the urethra within the glans penis.
The urethral folds are derived from the undifferentiated genital folds; the urethral groove lies between the folds.
The urethral plate is an endodermally derived epithelial plate along the ventral surface of the penis; its canalization produces the urethral groove.
The lateral genital swellings become the scrotal swellings.
Posteriorly, the perineum separates the urethral folds and groove from the anus.
Near the end of this period, the urethral folds fuse to form the penile urethra, and the scrotal swellings fuse to form the scrotal sac. The surrounding mesenchyme will give rise to the erectile tissues and vasculature of the penis.
The glanular portion of the urethra forms distally as the epithelial plate canalizes and becomes continuous with the penile urethra.
Weeks 12-14
The urethral folds have fused and the the urethral raphe lies at the midline of the shaft; the external urethral opening is at the tip of the glans.
The scrotum is fully fused; it will later receive the descending testes.
Hypospadias
A spectrum of defects in which incomplete fusion of the urethral folds leads to displacement of the external urethral opening. See our notes for more information.
Summary:
The genital tubercle gives rise to the shaft and glans of the penis.
The genital folds give rise to the Ventral penile tissues surrounding the penile urethra.
And, the genital swellings give rise to the scrotum.