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Objectives
After studying this chapter, you should be able to:
Trace the development of the male and female reproductive systems from their origins in the tubiform embryo.
Describe the development of the testes and ovaries from the indifferent gonads, including their descent to the scrotum or pelvis, respectively.
Describe the development of the male and female genital ducts from the mesonephric and paramesonephric ducts, respectively.
Describe the development of the male and female external genitalia from the genital tubercle, urogenital folds, and labioscrotal swellings.
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ORIGINS OF THE REPRODUCTIVE SYSTEM
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The components of the reproductive system are derived from primordial germ cells and all three germ layers (Figure 8-1).
Primordial germ cells migrate from the epiblast through the primitive streak during gastrulation to reach the endodermal wall of the yolk sac near the allantois. They will give rise to the germ cells of the gonads (spermatogonia and oogonia).
The intermediate mesoderm gives rise to gonadal support cells and the genital ducts.
The endoderm of the urogenital sinus forms the epithelial lining of the vagina and urethra.
Ectoderm contributes to the surface of the external genitalia and the distal penile urethra.
Although the genetic sex of the embryo is determined at the time of fertilization, the development of the reproductive system proceeds similarly in male and female embryos during weeks 1–6. Phenotypic sexual differentiation begins in week 7.
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During week 5, a thickening forms on the medial aspect of the intermediate mesoderm of the mesonephros, a portion of the developing urinary system (see Chapter 7) (Figure 8-1).
This thickening is the gonadal (genital) ridge.
Together, the gonadal ridge and the mesonephros form the urogenital ridge, which runs craniocaudally along the posterior abdominal wall on each side of the embryo.
Fingerlike primitive sex cords grow inward from the surface epithelium of the gonadal ridge into the underlying mesenchyme (Figure 8-2).
Primordial germ cells migrate from the yolk sac, ...