Kidney
Development
,
-
and
Function
in
the
Fetus
Bob
Caruthers,
CST,
PhD
The kidneys produce urine, a blood filtrate, and regulate uri-
nary volume and composition. These regulatory activities
involve balancing water and solute transport, conserving
nutrients, eliminating waste products,
and
regulating acid and
bases. The primary
purpose
of kidney function is to maintain
a
stable environment in which cellular and tissue metabolic
activity can proceed at an optimal level. The kidneys secrete
the hormone renin, erythropoietin and 1.25-dihydroxy vita-
min
D.
Renin helps regulate blood pressure. Erythropoietin
helps regulate erythrocyte production. 1,25-dihydroxy vitamin
D
plays a role in calcium metabolism. This article will discuss
the development of the kidney, and its role in the fetus."'
Since the kidneys are bilateral structures, development
involves both right and left kidneys. During fetal develop-
ment, three separate nephric structures develop in succession;
pronephros, mesonephros and metanephros (Figure
1
).
The
associated urinary bladder and urethra both develop from a
derivative of the embryonic hindgut, the urogenital sinvs,
The
pronephras forms
first,
is not active in urine production, and
rapidly undergoes camplete regression.
The
mesonephros
forms the setond kidney. The body of the mesoneghros
regresses,/but gar& of the nesonephric duct remain to form
the drainage sysrem of the male gonad. While existing, the
mesonephros
is
thaught to produce urine in the human ferns.
The third and definitive kidney, the metanephros, forms two
rudiments that make up the complete adult kidney and ureter.
The metanephras produces urine during fetal devel~pmant,~
PRONEPHROS
The pronephros is a transient structure which originates
in
the intermediate mesoderm asswiated with the cranial and
cervical somitrs. Component parts, cords of tissue or vesicles,
can
be
seen in the fourth week. The pronephros is complete1
regred by the start of the fifth week. The pronephros form
and regresses in a cranial-to-caudal sequence. No pronephric
glomeruli (cluster of capillaries) have been observed, and no
vesicles
are
associated with the pronephric duct. The
pronephros
is,
therefore, not active in urine producti0n.Y
MPSONEPHROS
The mesonephros originates in the nephrogenic cord that
is
part of the intermediate mesoderm. Early mesonephric forma-
tion is evident before the pronephros has completed
its
regre:
sion. The mesonephros also degenerates in a cranial-to-cauda
sequence. Some of the cranial structures are degenerating in
the fifth week of fetal development while the caudal structun
are still differentiating. By the eighth week of fetal develop-
ment, most of the mesonephric structures have disappeared.$
The nephrogenic cord elongates caudally.
As
it grows caudai
ly. the nephrogenic cord differentiates into mesonephric vesicle
These
vesicles
fm
from
the
first
thoracic
to
the third lumbar
wgment and quickly
increase
in number
by
the
means
of bifurc.
tion andlor subdivision. The vesicles elongate and appear
as
either pear-shaped
or
S-shaped vesicles.
These
vesicl form
rud
mentary nephrom.
The
S-shaped
vsicb develap communica-
tion with
the
mesonephric duct (Wolffian), and branches from
the
dorsal aorta
grow
toward
these
$-shaped
vesidq. The
bodit
of
the
shaped
vesicles form
a
mdlmnmry
type
Bowman's
caF
sule.
Blood
vmeb form capillary
tufts
&at push against
the
developing Bowman's capsule and connect
to
vessels
from the
donJ aorta. Portions of the $-shaped vesicle not involved in
tJ
previously described development become more convoluted
an
associate with efferent arterioles that carry blood from
the
glomeruli (capillary
tufts)
to the posterior cardinal vein!
