Embryology and pathophysiology of epispadias and bladder exstrophy

The mesenchyme from the primitive groove, migrating, grows between these two layers, strengthening the subumbilical region during the period of regression of the cloacal membrane. Before the rupture of the cloacal membrane, the urorectal fold joins it, dividing the cloaca into urogenital and anal components. As a result of the rupture of the membrane, the urogenital (urogenital) opening appears at the base of the genital tubercle.

Epispadias is theoretically thought to occur when the genital tubercle migrates caudally to where the urogenital fold divides the cloaca. Later disruption of the cloacal membrane may result in the dorsal urethra becoming exposed. Continued caudal displacement of the tubercle prevents mesoderm migration toward the midline and results in bladder exstrophy. One might assume that if this disorder results from caudal displacement of the genital tubercle, then anomalies associated with minimal displacement (epispadias) would be more common than those resulting from later, further displacement (classic bladder exstrophy), but this is not the case.

If the displacement of the genital tubercle is the cause of the defect, then one would expect that a sharp displacement of the penis away from the pubic tubercle would occur at least occasionally, but this never occurs in bladder exstrophy. Given the above, it has been theoretically suggested that the normal migration of mesenchyme between the layers of the cloacal membrane in bladder exstrophy is impaired by the increased thickness of the membrane. Delayed rupture of the membrane without its reinforcement by the mesodermal layer can lead to the formation of bladder exstrophy.

An interesting theory of the etiology of the exstrophy-epispadias complex and hypospadias was proposed by FD Stephens and JM Hutson (2005), who argue that the tail of the embryo plays a key role in the development of this defect.

During the 13th stage of development (28 days), it is evident that the embryo's tail is directed toward the umbilical cord area, which results in embryonic hernias and other umbilical ring defects. By analogy, it has been suggested that the embryo's tail may exert compression on the area of developing genitals and disrupt the connection of the cloacal tubercles and the cloacal membrane. Different durations and strengths of pressure lead to anomalies of varying degrees. Strong pressure of the tail on the area of the cloacal tubercles and the cloacal membrane throughout the entire pregnancy may lead to the formation of cloacal exstrophy. Less strong pressure throughout the entire pregnancy contributes to the development of classical exstrophy.

Strong pressure of the tail on the genital area in the first half of pregnancy and the elimination of this effect in the second half leads to closed forms of exstrophy or total epispadias with divergence of the pubic bones. Superficial pressure of the embryo's tail on the penis in the second half of pregnancy creates conditions for the development of trunk epispadias without disruption of the pubic symphysis fusion. This theory provides a reasonable explanation for why simpler forms of the defect, such as epispadias, are less common than severe ones (classical exstrophy). Similar embryogenesis disorders lead to the development of another penile defect - hypospadias. According to the author of the hypothesis, superficial pressure of the tail on the genital area at later stages of fetal development prevents the urethral tube from closing and the scrotum from forming. This theory is also suitable for explaining the mechanism of cloacal exstrophy.

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