CHAPTER 12
Urinary System
229
Figure 12-6A.
Glomerulus, renal cortex.
H&E,
3
310; insets
3
841
Each
glomerulus
is formed by a tuft of capillaries that is
fed by the
afferent
arteriole
and drains into the
efferent
arteriole
. Pressure in the glomerulus due to resistance
of the efferent artery provides the force for F
ltration
into the
Bowman space
. The glomerular capillaries are
fenestrated capillaries lined by endothelial cells with
gaps (
fenestrae
) that lack the usual diaphragms (see
±ig. 9-13A). These capillaries are covered by processes
of
podocytes
. The endothelial cells, basal lamina, and
podocytes combine to form a
glomerular ±
ltration bar-
rier
. Intraglomerular mesangial cells within the glom-
erulus provide structural support as well as phagocy-
tosis of debris and large molecules, thereby preventing
material from accumulating on the F
ltration barrier.
They also have a contractile capability, which may
function in regulating glomerular blood fl
ow.
Figure 12-6B.
Glomerulus and ± ltration barrier.
EM,
3
16,667; insets,
lower left
3
29,206;
upper left
3
41,818;
upper right
(color),
H&E
3
219
The central part of a renal corpuscle is composed of a bed of capillaries, the
glomerulus
, and the cells and structures associated with the
glomerulus. In addition to the endothelial cells of the capillaries are two other cell types,
podocytes
and the
intraglomerular mesangial
cells
. The endothelial cells of the fenestrated glomerular capillaries coproduce and share a common basal lamina with the terminal
podocyte processes (pedicles
,
foot processes)
that cover them. As blood fl ows through the capillaries, a F
ltrate of plasma is formed
as it passes through several layers (fenestrations of the capillary, trilayered basal lamina, and F ltration slits between podocytes) to
enter the
Bowman space
(
urinary space
).
Intraglomerular mesangial cells
, lodged among the podocytes and endothelial cells, serve an
incompletely understood maintenance function. The
upper left inset
shows the
basement membrane
of the glomerulus,
pedicles
(
small
podocyte processes
), and cytoplasm of the
endothelial cell
, which together form a
± ltration barrier
that selectively allows water, ions,
and small molecules to pass through but not large molecules and blood cells. The
lower left inset
shows foot processes of the
podocyte
resting on the
basement membrane
of the glomerulus. The
upper right color inset
indicates the afferent and efferent arterioles.
Macula densa
Macula densa
of distal tubule
of distal tubule
Macula densa
of distal tubule
Fenestrated
Fenestrated
capillary wall
capillary wall
Fenestrated
capillary wall
Filtration slit
Filtration slit
Filtration slit
Lamina
Lamina
rara
rara
interna
interna
Lamina
rara
interna
Lamina
Lamina
rara
rara
externa
externa
Lamina
rara
externa
Lamina
Lamina
densa
densa
Lamina
densa
Endothelial
Endothelial
cell
cell
Endothelial
cell
Basement
Basement
membrane
membrane
Basement
membrane
Podocyte
Podocyte
Pedicles
Pedicles
Pedicles
Podocyte
Basement
Basement
membrane
membrane
Bowman
Bowman
space
space
Bowman
space
Basement
membrane
Erythrocyte
Erythrocyte
Erythrocyte
Erythrocyte
Erythrocyte
Erythrocyte
Luman of
Luman of
Glomerular
Glomerular
capillary
capillary
Lumen of
Glomerular
capillary
Basement
Basement
membrane
membrane
Lumen of
glomerular
capillary
Basement
membrane
Bowman space
Podocytes
Intraglomerular
mesangial cell
Capillaries
Capillaries
Capillaries
Arterioles
Arterioles
Arterioles
B
Macula densa
Macula densa
Macula densa
Arteriole
Arteriole
Arteriole
Glomerulus
Glomerulus
Bowman space
Bowman space
Glomerulus
Bowman space
Bowman
Bowman
capsule
capsule
Distal
Distal
convoluted tubules
convoluted tubules
Distal
convoluted tubules
Bowman
capsule
Mesangial
Mesangial
cell
cell
Mesangial
cell
Endothelial cell
Endothelial cell
Endothelial cell
Podocytes
Podocytes
Podocytes
B
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