Chapter 100. The Genital or Reproductive System: Primive Genital Tracts and Sex Determination - Review of Medical Embryology Book

THE GENITAL TRACTS have the same appearance in both male and female embryos until week 7 of development, consisting of the 2 paramesonephric or mullerian ducts and the 2 mesonephric or wolffian ducts

The mesonephric ducts drain the mesonephric kidneys, but persist as the male genital ducts when the mesonephric system undergoes degeneration
The paramesonephric ducts develop bilaterally from invaginations of the coelomic epithelium, on the lateral aspects of the mesonephroi

In the 10 mm embryo, the paramesonephric (mullerian) ducts induce an invagination of the coelomic epithelium opposite the cranial end of each mesonephric duct which creates an epithelial bud that penetrates the mesenchyme and progresses caudally along the mesonephric
The bud hollows out at the same time it grows and thus becomes an open paramesonephric duct in the coelomic or peritoneal cavity

The paramesonephric ducts cross in front of the mesonephric (wolffian) ducts at the lower pole of the mesonephric body and then run alongside it

The terminal parts of the paramesonephric ducts fuse to form a small, single median duct (uterovaginal primordium or canal) which ends blindly at the posterior surface of the urogenital sinus
The blind ending projects into the dorsal wall of the sinus to create an elevation, the sinus or mullerian tubercle which is located between the openings of the mesonephric ducts into the urogenital sinus

The mesonephric and paramesonephric ducts are located in the urogenital cord, which is attached to the anterior external edge of the mesonephric (wolffian) body by the urogenital mesentery. The latter attaches the urogenital cord to the body wall, below the mesonephric body. Furthermore, the 2 urogenital mesenteries (right and left) join below the median line
The mesonephric body is attached to the abdominal wall by the mesonephric mesentery throughout its entire length. Above the body, the urogenital mesentery and mesonephric mesentery extend upward and form the diaphragmatic ligament

The lower pole of the mesonephric body is attached at the inguinal region by the inguinal ligament

Sex determination: before week 7 of embryonic life, the gonads of both sexes are identical in appearance (undifferentiated). Genetic sex is determined by fertilization. Gonadal sex is determined by the sex chromosome complex: XX in the female embryo; XY in the male embryo

THE Y CHROMOSOME has a testis-determining effect on the medulla of the undifferentiated gonad

WITHOUT THE Y CHROMOSOME, an ovary forms
THE TYPE OF SEX CHROMOSOME COMPLEX AT FERTILIZATION determines gonad type, and the latter determines the type of sexual differentiation seen in the genital ducts and external genitalia
WITH ABNORMAL SEX CHROMOSOME COMPLEXES

Loss of a sex chromosome (XO females) causes ovarian dysgenesis
Loss of an X chromosome does not interfere with migration of the primordial germ cells to the gonadal ridges because the germ cells are seen in the XO females
Two X chromosomes, however, are needed for complete ovarian development