1. ORGANOGENESIS OF THE KIDNEY
Wolffian duct
In the outer part of the intermediate mesoderm, immediately under the ectoderm, in the region
from the fifth cervical segment to the third thoracic segment, a series of short evaginations from
each segment grows dorsally and extends caudally, fusing successively from before backward to
form the pronephric duct. This continues to grow caudally until it opens into the ventral part of the
cloaca; beyond the pronephros it is termed the Wolffian duct. Thus, the Wolffian duct is what
remains of the pronephric duct after the atrophy of the pronephros.
Pronephros
The original evaginations form
a series of transverse tubules
each of which communicates
by means of a funnel-shaped
ciliated opening with the
abdominal cavity, and in the
course of each duct a
glomerulus also is developed.
A secondary glomerulus is
formed ventral to each of
these, and the complete
group constitutes the
pronephros. In humans, the
pronephros is just
rudimentary, and undergoes
rapid atrophy and disappears.
Mesonephros
On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a
series of tubules, the Wolffian tubules, develops. They increase in number by outgrowths from the
original tubules. They change from solid masses of cells to instead become hollowed in the
center. One end grows toward and finally opens into the Wolffian duct, the other dilates and is
invaginated by a tuft of capillary bloodvessels to form a glomerulus. The tubules collectively
constitute the mesonephros.
The mesonephros persists and form the permanent kidneys in fishes and amphibians, but in
reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the
permanent kidney (metanephros) develops beginning during the sixth or seventh week, so that by
the beginning of the fifth month only the ducts and a few of the tubules of the mesonephros
remain..

2. Metanephros and definitive kidney
The metanephros is the definite, permanent, but yet immature kidney. It arises from two
directions. On one hand, the precursor of the ureter buds from the Wolffian duct, while on the
other hand, the precursor of the renal tubules develop from the metanephrogenic blastema. The
ureteric bud subsequently grows into the latter mass, forming the parts of the nephron. Other
changes include e.g. the translocation of the ureteric opening directly into the cloaca.
Ureteric bud
The rudiments of the permanent kidneys make their appearance about the end of the first or the
beginning of the second month. Each kidney originate as an ureteric bud from the caudal end of
the Wolffian duct, which, in turn, originates from intermediate mesoderm. The ureteric bud starts
close to where the Wolffian duct opens into the cloaca, and grows dorsalward and rostralward
along the posterior abdominal wall, where its blind extremity expands and subsequently divides
into several buds, which form the rudiments of the renal pelvis and renal calyces; by continued
growth and subdivision it gives rise to the collecting duct system of the kidney. The other, more
superficial, portion of the diverticulum, on the other hand, becomes the ureter.

3. Metanephrogenic blastema
The renal corpuscles and renal tubules, in contrast, are developed from the metanephrogenic
blastema instead of from the ureteric bud. The metanephrogenic blastema is moulded over the
growing end of the latter, and becomes a part of the metanephros in this way. The renal tubules
of the metanephros, unlike those of the pronephros and mesonephros, do not open into the
Wolffian duct. Instead, the tubules rapidly elongate to form the parts of the nephron: the proximal
tubules, the loops of Henle and the distal convoluted tubules. These last join and establish
communications with the collecting duct system derived from the ultimate ramifications of the
ureteric bud. In the other end, the renal tubules give rise to Bowman's capsules and glomeruli.