A basic understanding of genitourinary embryology facilitates
learning many aspects of urology. Embryologically, the genital and
urinary systems are intimately related. Associated anomalies of the
two systems are commonly encountered.
The kidneys pass through three embryonic phases (Figure
38–1): (1) The pronephros is a vestigial
structure without function in human embryos that, except for its
primary duct, disappears completely by the fourth week. (2) The
pronephric duct gains connection to the mesonephric tubules and
becomes the mesonephric duct. While most of the mesonephric tubules
degenerate, the mesonephric duct persists bilaterally; from where
it bends to open into the cloaca, the ureteral bud grows cranially
to interact with the metanephric blastema. (3) This forms the metanephros, which
is the final phase. The metanephros develops into the kidney. During cephalad
migration and rotation, the metanephric tissue progressively enlarges, with
rapid internal differentiation into the nephron and the uriniferous
tubules. Simultaneously, the cephalad end of the ureteral bud expands
and divides within the metanephros to form the renal pelvis, calices,
and collecting tubules.
Schematic of the development of the nephric system. Only
a few of the tubules of the pronephros are seen early in the fourth
week, while the mesonephric tissue differentiates into mesonephric
tubules that progressively join the mesonephric duct. The first
sign of the ureteral bud from the mesonephric duct is seen at 4
weeks. At 6 weeks, the pronephros has completely degenerated and
the mesonephric tubules start to do so. The ureteral bud grows dorsocranially
and has met the metanephric blastema. At the eighth week, there
is cranial migration of the differentiating metanephros. The cranial
end of the ureteral bud expands and starts to show multiple successive
outgrowths (renal calices).
Subdivision of the cloaca (the blind end of the hindgut) into
a ventral (urogenital sinus) and a dorsal (rectum) segment is completed
during the seventh week and initiates early differentiation of the
urinary bladder and urethra. The urogenital sinus receives the mesonephric
duct and absorbs its caudal end, so that by the end of the seventh
week, the ureteral bud and mesonephric duct have independent openings.
The ureteral orifice migrates upward and laterally. The mesonephric
duct orifice moves downward and medially, and the structure in between
(the trigone) is formed by the absorbed mesodermal tissue, which maintains
direct continuity between the two tubes (Figure
The development of the ureteral bud from the mesonephric
duct and their relationship to the urogenital sinus. The ureteral
bud appears at the fourth week. The mesonephric duct distal to this
ureteral bud is gradually absorbed into the urogenital sinus, resulting
in separate endings for the ureter and the mesonephric duct. The ...
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