Chapter 102. Development of The Male External Genital Organs - Review of Medical Embryology Book

THE CLOACAL MEMBRANE, in week 3, is gradually surrounded by mesenchyme from the primitive streak. This mesenchyme forms a pair of elevations, the cloacal folds, which fuse with each other in front of the cloacal membrane to form the cloacal eminence

The cloacal membrane, in week 6, is subdivided by the urorectal septum into the urogenital and anal membranes; the cloacal swellings also are split into the genital or urethral folds. The membranes rupture a week later to form the urogenital and anal openings, respectively
The cloacal eminence elongates, by week 4, to form the genital tubercle

ANOTHER PAIR OF ELEVATIONS, THE LABIOSCROTAL OR GENITAL SWELLINGS, are seen on each side of the genital folds

In the male, the swellings differentiate into the scrotal swellings; in the female, they become the labia majora

THE MALE AND FEMALE EXTERNAL GENITALIA are similar until week 9, even though distinguishing external sexual characteristics are seen during the early fetal period. The final form is usually not seen until week 12

External male genital organs: development and differentiation are evident beginning at month 3 and are related to the action of androgens produced by the testes

THE GENITAL TUBERCLE, initially seen in week 4, elongates by week 7 to form a phallus, which in turn will form the future penis. The phallus is as large in the female as in the male at this stage

The phallus carries with it (pulls forward), the genital or urethral (urogenital) folds surrounding the phallic segment of the urogenital sinus. The folds elongate on the ventral side of the penis to form the lateral walls of the urethral (urogenital) groove

The urethral groove is lined by an extension of the entoderm from the phallic portion of the urogenital sinus and is continuous with the urogenital opening
At the base of the groove, the entoderm thickens into a urethral plate

The posterior portion of the genital swellings thickens to form the scrotal swellings

THE GENITAL FOLDS, which circumscribe the median urethral (urogenital) groove, fuse along the ventral (under) side of the penis, from behind forward, at about 3 months, changing the groove into a duct, the definitive penile urethra

The penile urethra ends blindly just before the end of the penis and is surrounded by a mass of erectile tissue of mesenchymal origin, the corpus cavernosum urethrae or spongiosum. This erectile tissue forms the end of the penis, the glans penis

The paired corpora cavernosa penis, also of mesenchymal origin, complete the erectile tissue in the shaft of the penis

THE LABIOSCROTAL OR GENITAL SWELLINGS grow toward each other and fuse in the midline to form the scrotum

Both the scrotum and penis bear the signs of their early formation through closure of the urogenital groove as is evidenced by the median raph?

At the tip of the glans, an ectodermal ingrowth forms a cellular cord, the glandular epithelial plate. Splitting of this plate forms a groove, the glandular urethra, on the ventral part of the glans that is continuous with the urethral groove in the body of the penis. Closure of the groove in the glans moves the urethral opening to the tip of the glans and joins the 2 urethral parts
The second invagination is circular and is called the preputial epithelial plate. Cleavage of this plate before birth separates the glans penis from the prepuce or foreskin. The latter is a fold of skin at the tip of the penis which, during week 12, grows over the glans and surrounds it by week 1 It is fused to the glans and not retractable at birth, but breakdown of the fused surfaces normally occurs during infancy