Overview
This lecture explains the embryological development of the female reproductive system, focusing on the origin and differentiation of tissues, especially from the intermediate mesoderm.
Early Embryonic Development
- Fertilization forms a blastula, which implants into the uterine wall.
- Gastrulation creates three germ layers: ectoderm (skin/nervous system), mesoderm (muscles, organs), and endoderm (gut tube).
Mesoderm and Reproductive Tract Origins
- The female reproductive system develops from intermediate mesoderm, also the source of kidneys.
- The intermediate mesoderm grows from cranial to caudal and forms structures called nephrogenic cords.
Kidney Development Stages
- Three kidney stages: pronephros (rudimentary, disappears by week 4), mesonephros (contains mesonephric/Wolffian duct), and metanephros (permanent kidney).
- Female reproductive tract mainly relates to the mesonephros and its ducts.
Gonadal (Ovary) Development
- Gonads begin as genital ridges on nephrogenic cords formed by epithelial and mesenchymal proliferation.
- Primordial germ cells migrate through the dorsal mesentery to invade genital ridges.
- Germ cells group with gonadal cells to form primitive sex cords in the medullary region, which later degenerate.
- Cortical sex cords remain and develop into follicular cells and oocytes.
Differentiation and Duct Development
- Sex is determined at fertilization (XX = female); ovary development proceeds without a Y chromosome.
- Mesonephric (Wolffian) duct degenerates in females, sometimes leaving remnants (epoophoron, paroophoron) with no function.
- Paramesonephric (Müllerian) duct forms from mesoderm; cranial end forms fimbriae, middle becomes the uterine tube, caudal end fuses to form the uterus.
Uterus and Vagina Development
- By week 9, paramesonephric ducts fuse and the uterine septum degenerates, creating a uterine cavity.
- At week 12, the duct invades the urogenital sinus to form the vaginal plate.
- The vagina canalizes by week 20, with origins from both paramesonephric duct (upper) and urogenital sinus (lower).
- The hymen separates the vagina from the urogenital sinus and contains both epithelial types.
Key Terms & Definitions
- Blastula — early ball of cells after fertilization.
- Gastrulation — process forming ectoderm, mesoderm, endoderm.
- Intermediate Mesoderm — middle germ layer forming kidneys and genital tracts.
- Nephrogenic Cord — tissue strip giving rise to kidneys and gonads.
- Mesonephric (Wolffian) Duct — embryonic duct, degenerates in females.
- Paramesonephric (Müllerian) Duct — embryonic duct developing into uterus, uterine tubes, upper vagina.
- Genital Ridge — precursor tissue for gonads.
- Cortical Sex Cords — ovary structures forming follicular cells.
- Canalization — formation of a hollow tube (e.g., vagina).
- Hymen — membrane at vaginal entrance, from urogenital sinus and vaginal epithelium.
Action Items / Next Steps
- Review diagrams of embryonic cross-sections and duct development.
- Prepare for external genitalia development in next lecture.