I. Vaginogenesis: The embryonic origin of the human vagina is subject to different academic opinions, the first being that the uterine duct (utero-vaginal duct) forms the uterus and the entire vagina. Later, it was observed that the solid protrusion of the posterior wall of the urogenital sinus evolved into a hollow tube that formed part of the vagina. Most scholars now believe that the upper (upper 4/5) vagina derives from the parameconium (i.e., Mullerian duct) and the lower (lower 1/5) vagina from the urogenital sinus of the endoderm. During embryonic development, the bilateral paramedian ducts develop into the female reproductive tract. The middle and caudal segments of the paramedian ducts go inward and downward, meeting and fusing with the opposite side in the midline, with the caudal end reaching the dorsal aspect of the urogenital sinus, and the unfused head segment on both sides developing into the fallopian tubes. When the embryo reaches 5-6 weeks of development, a concave groove, the paramedian canal, appears on the lateral side of the mid-renal canal (i.e., Wufei’s canal) of the urogenital crest. When the embryo reaches 6-8 weeks of development, the paramedian canal elongates caudally and crosses the ventral side of the mid-renal canal when it reaches the caudal part, and at the midline of the embryo, the lower ends of the two paramedian canals fuse to form a Y shape, forming a tubular uterovaginal primordium. At 9 weeks of development, the tubular structure of this utero-vaginal primordium protrudes into the ridge of the urogenital sinus, forming a protrusion known as Muller’s node. The cells of the endoderm of the urogenital sinus together with the cells of the lower end of the paramedian tubule form the sinovaginal bulb. The bulb proliferates and lengthens upward to form a solid vaginal cord that lies between the urogenital sinus and the paramedian nephron. The cells of the vaginal cord proliferate together with the cells of the lower end of the paramedian nephron tube to form the vaginal plate, which gradually increases in size and enlarges the distance between the uterus and the urogenital sinus. When the embryo reaches 11 weeks of development, the lower end of the vaginal plate is gradually ducted from bottom to top, forming the vaginal cavity by the 20th week. The upper end of the vaginal epithelial plate is at the lower end of the uterine cervix, forming the vaginal vault. At 12 weeks of embryonic development, when the vaginal cavity is formed, the endoderm of the urogenital cavity or the ectodermal cells of the ectodermal membrane of the urogenital cavity pass through the urogenital cavity and enter the lower part of the vagina, gradually replacing the original vaginal epithelium from the bottom up to become the permanent vaginal epithelium, which is composed of multiple layers of squamous cells, while the cells of the endocervix are a single layer of columnar cells. It has also been suggested that the permanent vaginal epithelium is formed by the mesodermal cells of the mesonephric duct. At 16 weeks of embryonic age, the septum between the two paramedian ducts has dissolved and the two paramedian ducts are completely connected. The vagina is fully formed at 20-22 weeks of embryonic life. Factors affecting the development of the vagina: 1. The guiding role of the middle renal duct: the development of the paramedian duct must be guided by the middle renal duct. The fusion of the two collateral mesonephric ducts must also be guided by the mesonephric duct, and when the mesonephric ducts are abnormal, this results in a double uterus and double vagina. It also affects the formation of the kidneys, leading to absence or ectopic kidneys on one side. 2. Drug effects: The main drugs that can affect the development of reproductive organs are hormonal drugs. If too many androgens are taken during pregnancy, they can lead to vaginal agenesis or atresia of the lower vaginal segment in female fetuses. Benzoylecgonine can lead to congenital absence of the uterus and vagina due to the non-development of the paramedian tube. 3. Genital tract luminalization abnormalities: Due to autosomal recessive genes and polygenic variants, or alternating local tissue abnormalities in the embryo, can lead to luminalization abnormalities in the genital tract, such as producing vaginal diaphragm or lower vaginal atresia. Types of vaginal anomalies 1. Vaginal diaphragm The cause of embryonic development of vaginal diaphragm is not clear, some scholars believe that the fused caudal end of the paramedian nephron is not connected to the urogenital sinus or only partially connected to it. However, this theory cannot explain the formation of the upper vaginal diaphragm, since the upper 4/5 of the vagina is formed by the paramedian tubules. It has also been suggested that the vaginal plate entity is not penetrated or is not fully luminalized due to blocked absorption of some tissues during bottom-up luminalization, resulting in the formation of the diaphragm. It has also been suggested that the vaginal wall tissue or submucosal tissue protrudes into the vaginal cavity and forms a diaphragm. 2. Longitudinal vaginal septum The vagina is divided into two vaginas by a longitudinal septum in the middle of the vagina, which can be either completely or incompletely longitudinal. The reason for this is that when the two paramedian tubes are fused, only the caudal end is incompletely fused, but the uterus and cervix are formed, and the tissue of the upper middle part of the vagina is not fused, then the vagina is completely longitudinal, and if some of the tissue is not fused, then the vagina is incompletely longitudinal. Longitudinal vaginal septum is also often associated with a double uterus, a bicornuate uterus, and a longitudinal uterus. The diaphragm originates between the two cervixes and deviates distally from the midline and slants towards the vaginal wall, making one side of the vagina blind, often with a double uterine body and a double cervical malformation. The reason for this is not known, but it may be due to the fact that the development of the paramedian tubule depends on the development of the mesonephric duct, which may affect the development of the ipsilateral paramedian duct when one side of the mesonephric duct is underdeveloped. 4. Lower vaginal atresia The lower part of the vagina is closed by fibrous connective tissue, and the atretic segment is usually more than 2 cm thick, while the upper part of the vagina is normal. This is due to embryogenesis of the urogenital sinus not participating in the formation of the lower vaginal segment, or to irregular luminalization of the vaginal plate, which is partially absorbed and remains solid. 5. Congenital anovaginal embryos occur because the caudal end of the paramedian tubule is either dysplastic or underdeveloped, or the development is stagnant and does not extend downward, so it is often accompanied by hypoplastic fallopian tubes and uterus. The fallopian tubes are sometimes well developed. A few have normal fallopian tubes and uterus. The ovaries are mostly normal in development and function well.