An understanding of the embryologic development of the genital tract will enhance a clinicians' ability to diagnose abnormalities and understand their etiology.
Primordial germ cells will eventually develop into either oocytes or sperm. Primordial germ cells are first seen in the caudal portion of the yolk sac of the embryo and they migrate dorsally up the hind gut mesentery to a position ventral and medial to the mesonephros. The primordial germ cells populate the mesoderm forming a structure called the genital ridge which will eventually become the gonad. At this stage the embryo is undifferentiated and has the potential to become either male or female. The coelomic epithelium overlying the genital ridge hypertrophies and results in an enlargement of the gonad. Mesonephric cells invade the genital ridge to form gonadal cords which encompass the primary germ cells.
Cross-section of an embryo at the genital ridge. The genital ridge is medial to the mesonephric duct.
The undifferentiated embryo has the potential to become either male or female. If nothing happens the embryo will develop into a female. If testosterone and Müllerian inhibiting Hormone (MIH/MIF) are present a male fetus will develop. If testosterone and MIH are not present the indifferent gonad will form an ovary and the Müllerian duct, or paramesonephric duct will develop while the Wolffian duct system, or the mesonephric duct system, which has the potential to become the male, will regress.
The Müllerian or paramesonephric duct system will form the oviduct, uterine horns, uterine body, the cervix, and the cranial portion of the vagina. The paramesonephric ducts fuse caudally to form the uterine body, cervix and anterior vagina.
The primordial germ cells migrate to the mesenchyme medial and ventral to the mesonephros and form the genital ridge and the coelomic epithelium overlying the genital ridge hypertrophies. Mesonephric cells invade the genital ridge and form gonadal cords which contain the primary germ cells. These gonadal cord form follicles primarily in the cortical region close to the coelomic epithelium. The gonadal cords in the medullary region degenerate. The nongerminal central region will form the medulla of the ovary. Theca cells and granulosa cells develop. The germ cells in the ovary undergo mitotic division in the fetal and early postnatal period. The end of this period is marked by the formation of primary follicles.
If testosterone and MIH are present the undifferentiated genital tract will develop into a male genital tract. MIH causes regression of the Mullerian or paramesonephric duct system. The mesonephric duct system will form the epididymis, the ductus deferens, the cranial portion of the pelvic urethra and the seminal vesicles. The urogenital sinus or the urogenital tubercle will eventually become the penis and there are swellings that will eventually form the scrotum.
The primordial germ cells migrate to the mesenchyme medial and ventral to the mesonephros and form the genital ridge. The coelomic epithelium overlying the genital ridge hypertrophies. Mesonephric cells invade the genital ridge and form gonadal cords which contain the primary germ cells. A testis-determining gene (Sry) on the Y chromosome encodes a protein that initiates testis differentiation. The gonadal cords form seminiferous tubules and connect to the mesonephric ducts and will form the rete testis and efferent ducts. Early in testis development the gonadal cords are separated from the coelomic epithelium by the tunica albugineas and differentiation of the seminiferous tubules occurs in the medullary area. Sox9 plays a role in Sertoli cell differentiation. Sertoli cells secrete Mullerian Inhibiting Hormone which causes regression of the paramesonephric ducts. Leydig cells develop between the seminiferous tubules and secrete testosterone which stimulates Wolffian duct differentiation into the epididymis and ductus deferens.