Male genital tract

Abstract

Until the 12th week of gestation it is difficult to ascertain the sex of a human embryo based on the appearance of the external genitalia and yet the process is complete by 16-17 weeks. Our understanding of the complexity of the genetic and endocrinological interactions controlling this process continues to develop. There is emerging evidence that penile development has much in common with the development of limb buds. The developmental direction the indeterminate external genitalia take is driven by gonadal development which in turn is controlled by genetic sex determination. Though presented as sequential events, much of this happens in parallel. Between the 4th and 6th weeks the cloaca becomes divided into a posterior anorectal canal and an anterior urogenital sinus by the formation of the urorectal septum, the tip of which will eventually form the perineum. Simultaneously the mesoderm antero-lateral to the developing urogenital sinus expands to create the genital tubercle. When the cloacal membrane ruptures it exposes the floor of the urogenital sinus that will form the urethral plate. The mesoderm on either side of the urethral plate expands to form urogenital folds that extend into the genital tubercle. These are flanked by a pair of labioscrotal swellings. During the 6th week the urethral plate develops into a urethral groove which becomes the penile urethra as a result of fusion of the urogenital folds from proximal to distal, and is usually complete by 14 weeks. The formation of the glanular urethra is still under investigation and it is still unclear if it occurs by tubularization of the endoderm as in the penile urethra or through canalization of ectoderm distally. The prepuce itself develops as a result of ectodermal folding and cellular ingrowth resulting in the glans penis and inner prepuce sharing a common mucosal lining which gradually separates over years.