The metanephros (hind kidney) or permanent kidney: the metanephric blastema, a nonsegmented homogenous
mesodermal mass, enters its active phase of differentiation at the beginning of month 2 (week 5) and
begins to function about 3 weeks later
URINE FORMATION continues actively throughout the fetal life
Urine combines with amniotic fluid, which the fetus drinks, and is absorbed by the intestin The fetal
kidneys regulate amniotic volume, and if the kidneys are absent, amniotic fluid volume is abnormally
small (oligohydramnios)
THERE IS NO VITAL NEED FOR PRENATAL FUNCTION of the kidneys because metabolic wastes are transferred
via the placenta to the mother, but the fetus would die perinatally without the kidneys
Metanephric differentiation begins when the metanephric blastema (mass of mesoderm) is penetrated by
the ureteric bud (metanephric diverticulum) which arises from the lower portion of the mesonephric (wolffian)
duct
THE URETERIC BUD arises as a dorsal bud, at the beginning of month 2, from the lower end of the mesonephric
duct, and rapidly reaches and penetrates the metanephric blastema which, in turn, forms a "cap" over
the bud
The bud then enlarges and forms the beginning of the renal pelvis in week It then develops two
diverticula: a cranial and caudal portion, the future major calyces. The stalk itself becomes
the ureter
The renal pelvis divides into the major and then minor calyces, the latter giving rise to the collecting
ducts or tubules, which penetrate farther into the metanephric mass
The collecting tubules, in turn, further subdivide and, in the process, compress the blastema eccentrically.
They subdivide until about 13 or more generations of tubules (ducts) are forme Differentiation of the
collecting tubules depends on an induction stimulus from the ureteric bud and its derivatives
A complete system of branching tubules (ducts), forming a renal lobe, results from each tubule of the
first generation
While more and more new tubules arise on the periphery of the blastema, the major calyces absorb the
ducts of the third and fourth generations, which are transformed into the minor calyces of the
renal pelvis. The tubules of the fifth and successive generations form the definitive collecting tubules
of the adult kidney
The minor calyces receive the papillary ducts of the tubular system, the terminal end of the
collecting system opening at the apex of the pyramid
The ureteric bud gives rise to the entire collecting system of the kidney, namely, the ureter,
the renal pelois, the calyces, the papillary ducts, and the collecting ducts
THE METANEPHRIC BLASTEMA develops from the intermediate mesoderm, which forms a solid mass of tissue.
The development of its collecting ducts differs from that of the pronephros and mesonephros systems
in that it is formed by the developing metanephric (ureteric) bud. The tubular branching from the bud
compresses the blastema eccentrically and fragments it into tissue caps or clusters of mesenchymal cells
At each end of the cap, a metanephric spheroid arises which undergoes the same transformations described
for the nephrotomes of the pronephros and mesonephros; namely, there is a spheroid stage, vesicle stage,
stage where the vesicle elongates in an S-shape, and finally the vesicle opens into the collecting tubul
Here differentiation is elaborate and leads to the formation of functional units or nephrons
(the glomerulus plus the tubular system). The latter include Bowman's capsule, the proximal and distal
convoluted tubules, and the nephronic (Henle's) loop. The glomerulus and Bowman's capsule make up the
renal corpuscle
Distal convoluted tubules join arching collecting tubules and become confluent
The metanephric blastema thus gives rise to the entire excretory system
Each uriniferous tubule has 2 parts: a nephron from the metanephric mass of mesoderm and a collecting
tubular system from the ureteric bud