CHAPTER 12
Urinary System
233
Figure 12-10A.
Distal tubules.
H&E,
3
739
The
distal tubules
are lined by small cuboidal cells
with faintly eosinophilic or clear cytoplasm. The lat-
eral boundaries between cells are not as distinguishable
as those of the collecting duct. The distal convoluted
and straight tubules are similar in structure. The distal
straight tubule exits the medullary ray and approaches
the vascular pole of the renal corpuscle of the same
nephron to which the distal tubule belongs. As the distal
tubule passes adjacent to the afferent and efferent arteri-
oles, part of its wall becomes modiF ed as a sensory struc-
ture,
macula densa
, which monitors ionic content and
water volume of the F ltrate (±ig. 12-5A,B). The macula
densa is considered to mark the transition from the dis-
tal straight to the distal convoluted tubule. This F gure
shows the distal convoluted tubules in the renal cortex.
Distal tubules function mainly to remove sodium and
add potassium ions to the F ltrate when stimulated by
aldosterone
, a hormone produced by the adrenal gland.
Proximal
Proximal
tubule
tubule
Proximal
tubule
Proximal
Proximal
tubule
tubule
Proximal
tubule
Lumen of
Lumen of
distal tubule
distal tubule
Lumen of
distal tubule
Lumen of
Lumen of
collecting duct
collecting duct
Lumen of
collecting duct
A
Figure 12-10B.
Distal tubule.
EM,
3
4,441; insets
3
7,377
A cross section of a
distal tubule
, which is lined by cuboidal and columnar cells, is shown. In contrast to the extravagant brush
border of cells lining the proximal tubule, these cells have just a few, short
microvilli
. These cells have basally located nuclei and
tightly interdigitated lateral walls. In distal tubules, there are many
mitochondria
in the cytoplasm as there are in the proximal
tubules. The
upper left inset
shows short and irregular microvilli bulging into the
lumen
and many mitochondria beneath them. The
lower left inset
shows a
nucleus
and
basal enfolding
(basal plasma membrane enfolding) of the cell. The basal enfolding is due to
corrugation of the cell membrane in the basal region of the cell. This increases the surface area of the cell and is closely associated
with mitochondria, which produce adenosine triphosphate for active transport of ions.
Nucleus
Nucleus
Nucleus
Blood vessel
Blood vessel
Blood vessel
Microvilli
Microvilli
Microvilli
Basement
membrane
Basal enfolding
Basal enfolding
Basal enfolding
Lumen
Lumen
Lumen
Microvilli
Microvilli
Microvilli
Mitochondria
Mitochondria
Mitochondria
Basal enfolding
Basal enfolding
Basal enfolding
B
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