DEVELOPMENT OF THE URINARY SYSTEMEpithelial structures of the urinary system are derived from two sources:
intermediate mesodermandurogenital sinus endoderm.
Three pairsof kidneys develop in cranio-caudal sequence in the uro-
genital ridge of intermediate mesoderm: pronephros, mesonephros,and
metanephros.
The caudal end of the mesonephric duct gives rise to the ureteric bud.
Theureteric budinduces surrounding intermediate mesoderm to form the
metanephric cap,which forms the excretory units of the kidney. The
ureteric bud will form the collecting ducts.
During kidney development, epithelial-mesenchymal interactions
occur reciprocally between the epithelium of the ureteric bud and the mes-
enchyme of the metanephric cap (blastema) to convert the mesenchyme
of the metanephric cap into an epithelium.Those complex inductions are
regulated by a cascade of growth factors that allow a dialogue between
the epithelium and mesenchymeand the eventual formation of urine-
producing (nephron) and collecting portions (i.e., collecting ducts, calyces,
and pelves) of the developing kidney.
The epithelial lining (transitional epithelium) of the ureters,as well as
their muscular and connective tissue components, are derived from inter-
mediate mesoderm.
The transitional epithelium of the bladderand most of the urethra are
derived from hindgut endoderm of the urogenital sinus.Connective tissue
and muscle are derived from splanchnic lateral plate mesoderm.
DEVELOPMENT OF THE REPRODUCTIVE SYSTEMSIntermediate mesodermforms the epithelia, connective tissues, and
smooth muscle of the indifferent sex cordsand their ducts.
Theendoderm of the urogenital sinusgives rise to the epithelia of
distal organs of the reproductive system and the external genitalia. As in
the urinary system, connective tissue and smooth muscle of these terminal
elements are provided by splanchnic lateral plate mesoderm.
Germ cellsmigrate from their origins in yolk sac endoderm into the
indifferent sex cords of the urogenital ridgeby week 6. Further differenti-
ation of both the immature sex cords and the germ cells occurs.
TheSry geneon the Y chromosomedirects the differentiation of the
medullary sex cords into testes.If this gene is notpresent, the cortical sex
cords will develop as ovaries.
High-Yield Facts 11