The urogenital sinus develops into the urinary bladder, the urethra, and, in females, the vestibule of the vagina
Early in embryological development, a tubular structure called the cloaca is the common termination of the urogenital and gastrointestinal systems (Fig. 53-7A). The cloacal membrane separates the cloaca from the amniotic fluid. Eventually, a wedge of mesenchymal tissue separates the cloaca into a dorsal and a ventral cavity (see Fig. 53-7B). The dorsal cavity is the rectum. The ventral compartment is the urogenital sinus. Both the wolffian and the müllerian ducts empty into this urogenital sinus (see Fig. 53-5A).
FIGURE 53-7 Differentiation of the urogenital sinus. A, The urorectal septum begins to separate the rectum (dorsal) from the urogenital sinus (ventral). The urogenital sinus is divided into a vesicle (i.e., urinary bladder) part, a pelvic part, and a phallic part. The common space into which the rectum and urogenital sinus empty, the cloaca, is closed by the cloacal membrane. B, At this stage, the rectum and the urogenital sinus are fully separated. The urogenital membrane separates the urogenital sinus from the outside of the embryo. C, The male has a common opening for the reproductive and urinary tracts. The prostatic utricle, which is the male homolog of the vagina, empties into the prostatic urethra. D, A solid core of tissue called the vaginal plate grows caudally from the posterior wall of the urogenital sinus. The lumen of the vagina forms as the center of this plate resorbs. Thus, the female has separate openings for the urinary and reproductive systems.
The urogenital sinus can be divided into three regions: vesicle, pelvic, and phallic. In males (see Fig. 53-7C), the vesicle region becomes the urinary bladder, the pelvic region becomes the prostatic part of the urethra, and the phallic region becomes the initial portion of the penile urethra.
In females (see Fig. 53-7D), the vesicle region of the urogenital sinus also develops into the urinary bladder. The pelvic region becomes the entire female urethra. The phallic region of the urogenital sinus develops into the vestibule of the vagina; into this vestibule empty the urethra, the vagina, and the ducts of the greater vestibular glands of Bartholin.
As noted above, fusion of the caudal portion of the müllerian ducts produces the uterovaginal primordium. Contact of this primordium with the dorsal wall of the urogenital sinus induces the development of paired sinovaginal bulbs, which grow into the urogenital sinus and then fuse to form a solid core of tissue called the vaginal plate. This plate grows caudally to the phallic portion of the urogenital sinus. Resorption of the center of the vaginal plate creates the vaginal lumen. The remaining cells of the vaginal plate form the vaginal epithelium. During early fetal development, a thin membrane, the hymen, separates the lumen of the vagina from the cavity of the urogenital sinus. Usually, the hymen partially opens during the prenatal period. Occasionally, the hymen persists completely, does not allow escape of the menstrual effluvium at menarche, and gives rise to a condition known clinically as hematocolpos.
In males, the vagina disappears when the müllerian ducts are resorbed. However, remnants of the vagina sometimes persist as a prostatic utricle.
The external genitalia of both sexes develop from common anlagen
Although anatomically separate precursors give rise to the internal sex organs, common anlagen give rise to the external genitalia of the two sexes (Fig. 53-8A). Knowledge of the common origins of the external genitalia during normal development facilitates understanding of the ambiguities of abnormal sexual development.
FIGURE 53-8 Development of the external genitalia. A, Genital folds and genital swellings surround the cloacal membrane. B, Early in the fourth week of development—in both sexes—the genital tubercle begins to enlarge to form the phallus. C, In males, the genital tubercle becomes the glans penis. The urogenital folds fuse to form the shaft of the penis. The labioscrotal swellings become the scrotum. D, In females, the genital tubercle becomes the clitoris. The urogenital folds remain separate as the labia minora. The labioscrotal swellings become the labia majora where they remain unfused. Ventrally, the labioscrotal swellings fuse to form the mons pubis. Dorsally they fuse to form the posterior labial commissure.
The genital tubercle (see Fig. 53-8B) develops during the fourth week on the ventral side of the cloacal membrane. As a result of elongation of the genital tubercle, a phallus develops in both sexes. The genital tubercle of the primitive embryo develops into the glans penis in males (see Fig. 53-8C) and the clitoris in females (see Fig. 53-8D). Until about the end of the first trimester of pregnancy, the external genitalia of males and females are anatomically indistinguishable. The phallus undergoes rapid growth in females initially, but its growth slows, and in the absence of androgens, the phallus becomes the relatively small clitoris in females.
The paired urogenital folds give rise to the ventral aspect of the penis in males (see Fig. 53-8C) and the labia minora in females (see Fig. 53-8D). After formation of the urogenital opening, a groove—the urethral groove—forms on the ventral side of the phallus; this groove is continuous with the urogenital opening. The bilateral urogenital folds fuse over the urethral groove to form an enclosed spongy urethra; the line of fusion is the penile raphé. As the urogenital folds fuse to form the ventral covering of the penis, they do so in a posterior-to-anterior direction, thus displacing the urethral orifice to the tip of the penis. Elongation of the genital tubercle and fusion of the genital folds occur at the 12th to the 14th week of gestation. However, in females, the urogenital folds normally remain separate as the labia minora.
In males, the genital or labioscrotal swellings fuse to give rise to the scrotum. In females, however, the labioscrotal swellings fuse anteriorly to give rise to the mons pubis and posteriorly to form the posterior labial commissure. The unfused labioscrotal swellings give rise to the labia majora.