Embryological Development of Female Reproductive System

Overview

This lecture explains the embryological development of the female reproductive system, focusing on key steps from germ layer formation to the differentiation of internal reproductive organs.

Germ Layer Formation and Mesoderm Specialization

• Fertilization produces a blastula, which implants in the uterine wall.
• Gastrulation forms three germ layers: ectoderm (skin/nervous tissue), mesoderm (muscles, kidneys, reproductive organs), and endoderm (gut).
• The intermediate mesoderm, located between paraxial and lateral plate mesoderm, gives rise to the female reproductive system and kidneys.

Development of Gonads and Germ Cell Migration

• The intermediate mesoderm forms the nephrogenic cord, which becomes the genital ridges (potential gonads).
• Germ cells migrate via amoebic movement from the yolk sac into the dorsal mesentery and invade the genital ridge.
• The genital ridge proliferation is necessary to receive and support invading germ cells.

Differentiation of Ovaries and Sex Cords

• Initially, primitive sex cords form in the medullary region but later degenerate in females.
• Cortical sex cords persist and develop into follicular cells, surrounding developing oocytes.
• By week 20, the ovary contains primary oocytes in follicular cells.

Duct System Development (Mesonephric and Paramesonephric Ducts)

• Mesonephric (Wolffian) duct degenerates in females; remnants may persist without function.
• Paramesonephric (Müllerian) duct forms the uterine tubes (fimbriae and tubal segments), uterus, cervix, and upper vagina.
• Fusion of paramesonephric ducts forms the uterine cavity; the lower part connects with the urogenital sinus to create the vaginal plate.

Formation of the Vagina and Associated Structures

• The solid tip of the paramesonephric duct invades the urogenital sinus, forming the vaginal plate (sinovaginal bulb).
• Canalization transforms the vaginal plate into the vaginal lumen by week 20.
• The hymen, separating vaginal lumen from urogenital sinus, arises from both the paramesonephric duct and urogenital sinus.

Key Terms & Definitions

• Blastula — ball of cells formed after fertilization.
• Gastrulation — process generating three germ layers.
• Ectoderm — germ layer forming skin and nervous system.
• Mesoderm — germ layer forming muscle, kidneys, reproductive organs.
• Endoderm — germ layer forming gut tube.
• Intermediate Mesoderm — part of mesoderm that forms kidneys and reproductive tract.
• Nephrogenic Cord — ridge of intermediate mesoderm producing kidneys and gonads.
• Genital Ridge — precursor to gonads (ovaries or testes).
• Germ Cells — precursors of eggs or sperm.
• Paramesonephric (Müllerian) Duct — forms female reproductive tract.
• Mesonephric (Wolffian) Duct — forms male reproductive tract (degenerates in females).
• Vaginal Plate — solid tissue precursor of the vaginal lumen.

Action Items / Next Steps

• Review diagrams of embryonic cross-sections to visualize structures.
• Study the timeline of reproductive tract development, including key weeks (7, 9, 12, 20, birth).
• Prepare for upcoming material on external genitalia development.