tailieunhanh - Ebook High-Yield embryology (5/E): Part 2

Part 2 book “High-Yield Embryology” has contents: Female reproductive system, male reproductive system, respiratory system, head and neck, nervous system, ear, eye, body cavities, pregnancy, teratology. | Chapter 9 Female Reproductive System I The Indifferent Embryo A. The genotype of the embryo (46,XX or 46,XY) is established at fertilization. B. DURING WEEKS 1–6, the embryo remains in a sexually indifferent or undifferentiated stage. This means that genetically female embryos and genetically male embryos are phenotypically indistinguishable. C. DURING WEEK 7, the indifferent embryo begins phenotypic sexual differentiation. D. BY WEEK 12, female or male characteristics of the external genitalia can be recognized. E. BY WEEK 20, phenotypic differentiation is complete. F. Phenotypic sexual differentiation is determined by the Sry gene located on the short arm of the Y chromosome and may result in individuals with a female phenotype, an intersex phenotype, or a male phenotype. The Sry gene encodes for a protein called testes-determining factor (TDF). G. As the indifferent gonad develops into the testes, Leydig cells and Sertoli cells differentiate to produce testosterone and Müllerian inhibiting factor (MIF), respectively. In the presence of TDF, testosterone, and MIF, the indifferent embryo will be directed to the male phenotype. In the absence of TDF, testosterone, and MIF, the indifferent embryo will be directed to the female phenotype. II Development of the Gonads A. THE OVARY 1. The intermediate mesoderm forms a longitudinal elevation along the dorsal body wall called the urogenital ridge, which later forms the gonadal ridge. 2. Primary sex cords develop from the gonadal ridge and incorporate primordial germ cells (XX genotype), which migrate into the gonad from the wall of the yolk sac. Primary sex cords extend into the medulla and develop into the rete ovarii, which eventually degenerates. 3. Secondary sex cords develop and incorporate primordial germ cells as a thin tunica albuginea forms. 4. The secondary sex cords break apart and form isolated cell clusters called primordial follicles, which contain primary oocytes surrounded by a layer of .