Early in life, embryos of male and female sex are indistinguishable from one another (Table 18-1). At critical stages of embryonic development, insults can lead to congenital anatomic disorders of the reproductive tract. Influences include genetic mutation, epigenetic factors, developmental arrest, or abnormal hormonal exposures. Disorders range from congenital absence of the vagina and uterus, to lateral or vertical fusion defects of the müllerian ducts, to external genitalia that are ambiguous. Sexual differentiation is complex and requires both hormonal pathways and morphologic development to be normal and correctly integrated. Thus, it is not surprising that neonates with genital anomalies often have multiple other malformations. Associated urinary tract defects are especially frequent and are linked to the concurrent embryonic development of both reproductive and urinary tracts (Hutson, 2014).
TABLE 18-1Embryonic Urogenital Structures and Their Adult Homologues |Favorite Table|Download (.pdf) TABLE 18-1 Embryonic Urogenital Structures and Their Adult Homologues
|Indifferent Structure ||Female ||Male |
|Genital ridge ||Ovary ||Testis |
|Primordial germ cells ||Ova ||Spermatozoa |
|Sex cords ||Granulosa cells ||Seminiferous tubules, Sertoli cells |
|Gubernaculum ||Uteroovarian and round ligaments ||Gubernaculum testis |
|Mesonephric tubules ||Epoophoron, paroophoron ||Efferent ductules, paradidymis |
|Mesonephric ducts ||Gartner duct ||Epididymis, ductus deferens, ejaculatory duct |
|Paramesonephric ducts ||Uterus, fallopian tubes, upper vagina ||Prostatic utricle, appendix of testis |
|Urogenital sinus ||Bladder, urethra |
Greater (Bartholin) and lesser vestibular glands
|Bladder, urethra |
|Genital tubercle ||Clitoris ||Glans penis |
|Urogenital folds ||Labia minora ||Floor of penile urethra |
|Labioscrotal swellings ||Labia majora ||Scrotum |
The urogenital tract is functionally divided into the urinary system and genital system. The urinary organs include the kidney, ureters, bladder, and urethra. The reproductive organs are the gonads, ductal system, and external genitalia. Like most organ systems, the female urogenital tract develops from multiple cell types that undergo important spatial growth and differentiation. These develop during relatively narrow time windows and are governed by time-linked patterns of gene expression (Park, 2005).
Both the urinary and genital systems develop from intermediate mesoderm, which extends along the entire embryo length. During initial embryo folding, a longitudinal ridge of this intermediate mesoderm develops along each side of the primitive abdominal aorta and is called the urogenital ridge. Subsequently, the urogenital ridge divides into the nephrogenic ridge and the genital ridge, also called the gonadal ridge (Fig. 18-1).
Early development of the embryonic genitourinary tract. A. In the developing embryo, the urogenital ridge forms from intermediate mesoderm lateral to the primitive aorta. The dotted line reflects the level from which part B is taken. B. Cross section through the embryo shows division of the urogenital ridges into the genital ridge (future gonad) and nephrogenic ridge, which contains the mesonephros and mesonephric (wolffian) ducts. The mesonephros is the primitive kidney and is connected by the mesonephric ducts to ...