984
UNIT 4
Maintenance of the Body
25
so gradually that they produce no symptoms until about 40
years of age. Ten both kidneys begin to enlarge as blister-
like cysts containing fluid accumulate. Te damage caused by
these cysts progresses slowly, and many victims live without
problems until their 60s. Ultimately, however, the kidneys
become “knobby” and grossly enlarged, reaching a mass of
up to 14 kg (30 lb) each.
Autosomal recessive PKD
is more severe and much less com-
mon, affecting 1 in 20,000 people. Almost half of newborns
with recessive PKD die just aFer birth, and survivors gener-
ally develop renal failure in early childhood.
Mutations associated with PKD have been identified in two
gene families that code for signaling proteins. Tese signaling
proteins form a mechanoreceptor complex that is part of the pri-
mary cilium (see p. 91) that detects the flow of fluid past kidney
cells. It is not yet clear how defects in these proteins lead to cyst
formation. As yet, the only treatments are the usual treatments
for kidney failure—renal dialysis or a kidney transplant.
Because its bladder is very small and its kidneys are less able
to concentrate urine for the first two months, a newborn baby
Homeostatic Imbalance
25.8
Tree of the most common congenital abnormalities of the uri-
nary system are horseshoe kidney, hypospadias, and polycystic
kidney.
When ascending from the pelvis the kidneys are very close
together, and in 1 out of 600 people they fuse across the midline,
forming a single, U-shaped
horseshoe kidney
. Tis condition is
usually asymptomatic, but it may be associated with other kid-
ney abnormalities, such as obstructed drainage, that increase
the risk of frequent kidney infections.
Hypospadias
(hi
0
po-spa
9
de-as), found in male infants only,
is the most common congenital abnormality of the urethra. It
occurs when the urethral orifice is located on the ventral surface
of the penis. Tis problem is corrected surgically when the child
is around 12 months old.
Polycystic kidney disease
(
PKD
) is a group of disorders charac-
terized by the presence of many fluid-filled cysts in the kidneys.
Tese interfere with renal function, ultimately leading to renal
failure. Tese disorders can be grouped into two general forms:
Autosomal dominant PKD
, the less severe form, is much
more common, affecting 1 in 500 people. Te cysts develop
Degenerating
pronephros
Degenerating
pronephros
Urogenital
ridge
Developing
digestive tract
Mesonephros
Duct to
yolk sac
Allantois
Hindgut
Ureteric bud
Cloaca
Urogenital
sinus
(developing
urinary
bladder)
Metanephros
(kidney)
Gonad
Rectum
Mesonephric duct
(initially, pronephric duct)
(a) Week 5
(c) Week 7
(b) Week 6
(d) Week 8
Mesonephros
Mesonephric
duct
Gonad
Kidney
Urinary bladder
Urethra
Anus
Ureter
Rectum
Duct to yolk sac
Allantois
Body stalk
Urogenital sinus
Rectum
Ureteric bud
Metanephros
Figure 25.22
Development of the urinary system in the embryo.
Red arrows indicate
the direction of metanephros migration as it develops.
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