781
CHAPTER TWENTY
Urinary System
or
pedicels.
The pedicels of each cell interdigitate with those
of adjacent podocytes, and the clefts between them form a
complicated system of
slit pores
(
f
gs.
20.8 and
20.9
).
A cascade of cell-to-cell communication forms glomeruli in the
embryo. Podocyte precursor cells give rise to podocytes, but also
secrete vascular endothelial growth factor (VEGF), which attracts the
squamous epithelium that forms the parietal layer. These epithelial
cells then produce platelet-derived growth factor (PDGF) and extra-
cellular matrix proteins, which in turn signal certain cells to specialize
as mesangial cells. These cells are closely associated with the capillary
tuft, and can contract, which decreases the rate at which the glom-
erulus ±
lters plasma. Mesangial cells also provide structural support
to the glomerulus and phagocytize debris.
The renal tubule leads away from the glomerular capsule
and becomes highly coiled. This coiled portion is the
proximal
convoluted tubule.
Following it is the
nephron loop
(loop of
Henle). The proximal convoluted tubule dips toward the renal
pelvis to become the
descending limb
of the nephron loop. The
tubule then curves back toward its renal corpuscle and forms
the
ascending limb
of the nephron loop. The ascending limb
returns to its renal corpuscle of origin, where it becomes the
distal convoluted tubule
and tightly coils again
.
This distal por-
tion is shorter and straighter than the proximal tubule.
Several distal convoluted tubules merge in the renal cor-
tex to form a
collecting duct
(collecting tubule), technically
not part of any one nephron. The collecting duct passes into
the renal medulla, widening as it joins other collecting ducts.
The resulting tube empties into a minor calyx through an
called a
glomerular
(Bowman’s)
capsule.
Afferent arterioles
give rise to these capillaries, which lead to
efferent arterioles
(ef
er-ent ar-te
re-o
¯lz) (± g. 20.7). Filtration of fl uid from the
glomerular capillaries is the ± rst step in urine formation.
The glomerular capsule is an expansion at the end of a
renal tubule that receives the fl
uid ±
ltered at the glomerulus.
The capsule is composed of two layers of squamous epithe-
lial cells: a visceral layer that closely covers the glomerulus
and an outer parietal layer continuous with the visceral layer
and with the wall of the renal tubule
(f
g. 20.8)
.
The cells of the parietal layer are typical squamous epi-
thelial cells; however, those of the visceral layer are highly
modi± ed epithelial cells called
podocytes.
Each podocyte has
several primary processes extending from its cell body, and
these processes, in turn, bear numerous secondary processes,
Efferent
arteriole
Glomerulus
Afferent
arteriole
Peritubular
capillary
(a)
Renal tubules
Glomerulus
Glomerular
capsule
(b)
FIGURE 20.7
Blood vessels associated wtih nephrons. (
a
) A
scanning electron micrograph of a cast of the renal blood vessels
associated with the glomeruli (200×). (
b
) A scanning electron
micrograph of a glomerular capsule surrounding a glomerulus (480×).
Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy,
by R. G.
Kessel and R. H. Kardon. ©1979 W. H. Freeman and Company.
Afferent
arteriole
Efferent
arteriole
Blood
flow
Blood
flow
Visceral layer of
glomerular capsule
Glomerular
capsule
Glomerulus
Parietal layer
of glomerular
capsule
Proximal
convoluted
tubule
FIGURE 20.8
The glomerular capsule has a visceral layer and a
parietal layer.
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