The epithelium of the urethra in both sexes originates in the endoderm; the surrounding connective and smooth muscle tissue is derived from splanchnic mesoderm. At the end of the third month, epithelium of the prostatic urethra begins to proliferate and forms a number of outgrowths that penetrate the surrounding mesenchyme. In the male, these buds form the prostate gland (Fig. 14.13B). In the female, the cranial part of the urethra gives rise to the urethral and paraurethral glands.
When the lumen of the intraembryonic portion of the allantois persists, a urachal fistula may cause urine to drain from the umbilicus (Fig. 14.15A). If only a local area of the allantois persists, secretory activity of its lining results in a cystic dilation, a urachal cyst (Fig. 14.15B). When the lumen in the upper part persists, it forms a urachal sinus. This sinus is usually continuous with the urinary bladder (Fig. 14.15C).
Exstrophy of the bladder (Fig. 14.16 A) is a ventral body wall defect in which the bladder mucosa is exposed. Epispadias is a constant feature (see Fig. 14.33), and the open urinary tract extends along the dorsal aspect of the penis through the bladder to the umbilicus. Exstrophy of the bladder may be caused by a lack of mesodermal migration into the region between the umbilicus and genital tubercle, followed by rupture of the thin layer of ectoderm. This anomaly is rare, occurring in 2/100,000 live births.
Exstrophy of the cloaca (Fig. 14.16B) is a more severe ventral body wall defect in which migration of mesoderm to the midline is inhibited and the tail
(caudal) fold fails to progress. As a result, there is an extended thin layer of ectoderm that ruptures. The defect includes exstrophy of the bladder, spinal defects with or without meningomyelocele, imperforate anus, and usually omphalocele. Occurrence is rare (1/30,000), and causation has not been defined, although the defect is associated with early amniotic rupture.
Sex differentiation is a complex process that involves many genes, including some that are autosomal. The key to sexual dimorphism is the Y chromosome, which contains the SRY (sex-determining region on Y) gene on its short arm (Yp11). The protein product of this gene is a transcription factor initiating a cascade of downstream genes that determine the fate of rudimentary sexual organs. The SRY protein is the testis-determining factor; under its influence male development occurs; in its absence female development is established.
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