The Biology of the Development of the Genital Organs. A Multimedia Teaching Program
Authors: Sótonyi P. 
Published: November 2005

In my presentation, I review the sexual differentiation from the genetic sex until the appearance of the external genitalia and the developmental anomalies to use an animated cartoon. The first critical stage of sexual differentiation occurs at the moment of fertilization, when the genetic sex of the zygote is determined by the nature of the sex chromosome contributed by the sperm. Although an XY zygote is destined to become a male, no distinctive differences between the early development of male and female embryos have been noted. This is accomplished after migration of the primordial germ cell into the early gonad. Because of the early commonality of genital structures, anomalies are the result of abnormal retention or loss of appropriate genital structures. Therefore, most genital anomalies are some form of intersex. During the early differentiation of the gonads, while the mesonephros is still the dominant excretory organ, the gonads arise as ridge like thickenings (gonadal ridge) on its ventromedial face. Differentiation of the indifferent gonads into ovaries or testes occurs after the arrival of the primordial germ cells. The primordial germ cells arise from the endodermal cells of the yolk. The principal function of the Y chromosome is to direct the differentiation of the presented indifferent gonad into a testis from the sixth week, while two X chromosome are presented the ovaries start to develop, from the 12th week. The next and most obvious phase in sexual differentiation of the embryo is the differentiation of the somatic sex. The early embryo develops a dual set of potential genital ducts, one is the original mesonephric (Wolff ) ducts, which persists after degeneration of the mesonephros as an excretory organ, and the another is newly formed pair of ducts called the paramesonephric (Müllerian) ducts. Under the influence of testosterone secreted by the testes, the mesonephric ducts develop into the duct system through which the spermatozoa are conveyed from the testes to the urethra. The potentially female paramesonephric ducts regress under the influence of another product of the embryonic testes, the Müllerian inhibitory factor, a glycoprotein secreted by the Sertoli cells. In genetically female embryos, neither testosterone nor Müllerian inhibitory factor are secreted by the gonads. In the absence of testosterone the mesonephric ducts regress and lack of Müllerian inhibitory factor permits the paramesonephric ducts to develop into oviducts, the uterus and part of the vagina. The next stage is the development of the external genitalia. In very young embryos, a vaguely outlined elevation known as the genital eminence can be seen in the midline, just cephalic to the proctodeal depression. This is soon differentiated into a central prominence (genital tubercle) closely flanked by a pair of folds (genital folds) extending toward the proctodeum. Somewhat farther to either side are rounded elevation known as the genital swellings. From this common starting point the external genitalia of both sex differentiate. If the individual is to develop into a male the genital tubercle, under the influence of dihydrotestosterone, becomes greatly elongated to form the penis and the genital swellings become enlarged to form the scrotal pouches. During the growth of the penis a groove develops along the entire length of its caudal face and is continuous with the slit‐like opening of the urogenital sinus. This groove later becomes closed over by a ventral fusion of the genital folds, establishing the penile portion of the urethra. The portion of the urogenital sinus between the neck of the bladder and the original opening of the urogenital sinus becomes the prostetic urethra. In the female, the genital tubercle becomes the clitoris, the genital folds become the labia minora, and the genital swellings become the labia majora. The urethra in the female is derived from the urogenital sinus, being homologous with the prostatic portion of the male urethra.