VOLGOGRAD STATE MEDICAL UNIVERSITY
Department of histology, embryology, cytology
for the 2nd course
English medium students
Volgograd, 2015
Objectives:
1. To appraise the kidneys as exquisite filters,
designed to eliminate from the blood largely
nitrogenous wastes, excess electrolytes, and water.
2. Document the role of the kidney in the preservation
of homeostasis in the body
3. Extrapolate from its unique vasculature, how kidney
is able to concentrate the urine 100-fold.
4. Conceptualize in a sketch the nephron as the
functional and anatomical unit of the kidney.
5. Evaluate the function and structure of the ureter,
urinary bladder and urethra.
6. Assess the kidneys as endocrine organs and how
they are affected by certain hormones produced
elsewhere in the body.
General Provisions:
The urinary system functions in the formation of
urine, regulation of blood pressure and fluid volume
of the body, acid-base balance, and formation and
release of certain hormones.
The components of the urinary system are the
kidneys, ureters, urinary bladder and urethra.
Excretory
Homeostatic
Endocrine
Secretion of
renin (increases
blood pressure)
Served by 1 000 000 nephrons
Eliminate nitrogenous
blood impurities,
electrolytes and water
Preserves homeostasis through:
1)
ultrafiltration
of plasma
2)
selective resorption
of filtrate
Essential to life
FUNCTIONS OF THE KIDNEY
Adult Human Kidney
Each kidney has two distinct
zones: an outer cortex and inner
medulla. The cortex forms an outer shell
and also forms columns (of Bertin)
which lie between the individual units of
the medulla.
The medulla is composed of a
series of conical structures (medullary
pyramids), the base of each cone being
continuous with the inner limit of the
cortex and the pointed peak of the
pyramid protruding into part of the urine
collecting system (calyceal system)
towards the hilum of the kidney. This
pointed tip is known as papilla. Each
human kidney bears 10-18 medullary
pyramids thus 10-18 papillae protrude
into the collecting calyces.
Each medullary pyramid, with its associated shell of cortex,
comprises a functional and structural lobe of kidney. The lobar architecture is less obvious as kidney increases in size with increasing age.
In fetal kidney this lobar
architecture is clearly visible.
Within the kidney in the hilum
where the ureter
in cross
section and vessels entering
and leaving the kidney are seen.
The cortex is located
peripherally,
the
cortical
columns dip between the
medullary pyramids, with the
base of the pyramid resting
against the peripheral cortex
and the apex toward the hilum.
Between the pyramids
are the interlobar vessels: the
artery is a branch of the renal
artery, the veins form the renal
vein in the marginal zone,
between the cortex and base of
the pyramids. The arcuate
arteries and veins are tributaries
of the interlobar vessels.
Kidney and Adrenal, Fetal,
Rhesus monkey,
H. & E., x 6.
Renal Cortex and Medulla
The cortical region is
subdivided into cortical labirynth
with renal corpuscles (RC),
convoluted renal tubules (T),
blood vessels (V), and the
medullary rays (MR).
NB! The medullary rays
though
structurally similar to
medulla, are considered part of
cortex. They are finely striated
extensions of base of a pyramid
reaching into cortex and almost
to renal capsule and representing
an axis of the renal lobe.
In the medulla are many
tubules of the loops of Henle and
collecting ducts (CD), sectioned
in different planes. Islands of
medullary capillaries, the vasa
recta (VR) are shown.
DEFINITIONS:
Renal lobe – is a part of the kidney parenchyma
comprises of a pyramid and overlying renal cortex.
Renal lobule – is a small portion of the renal cortex
composed of a medullary ray with its immediately
associated cortical tissue.
Lobule
Cortex
Lobe
Medullary ray
Pyramid
Medulla
The
medullary
rays
contain the straight portions of
proximal and distal tubules
(medullary segments), the thick
segments of ascending arms of
Henle's* loops, and the straight
collecting tubules.
The cortex is composed
of radiating medullary rays,
alternating
with
regions
containing
glomeruli
and
convoluted
renal
tubules
(cortical labyrinth). Other names
for these two divisions are pars
radiata for the medullary rays,
and the pars convoluta for the
cortical labyrinth. The cortical
labyrinths contain glomeruli,
proximal and distal convoluted
tubules,
and
the
arched
collecting tubules.
Henle was a nineteenth-century German
anatomist.
Kidney, Cortex
H & E, x50.
GENERAL PROVISIONS REGARDING
ORGAN BLOOD SUPPLY
In every organ the capillary network lies between the terminal part
of the arteriolar and proximal part of the venular system and it is
the major site of O2/CO2 exchange.
In contrast renal vascular system has a highly speciailized
preliminary capillary network – the glomerular tuft (glomerulus).
Glomerulus is the site of filtration of the waste products from
plasma.
Glomerulus does not transfer oxygen to the tissues, nor does it
take up significant amount of carbon dioxide.
The major gas exchange takes place in the secondary capillary
system.
Blood Supply of the Kidney
To understand the
histophysiology of kidney its
vascular supply should be
appreciated.
In general the renal
venous drainage mirrors the
arterial
supply
(except
afferent -efferent arterioles).
Each kidney is supplied by a renal artery, a
direct
branch
of
the
abdominal aorta. It subdivides into the dorsal and
ventral branches as it enters
the hilum of the kidney which
give off interlobar arteries
situated
between
the
pyramids. At the level of the
pyramid bases interlobar
arteries divide into arcuate
arteries.
Blood Supply of Kidney
Interlobular
arteries
derived from the arcuate arteries
enter cortical labyrinth to reach
the renal capsule supplying a
stellate subcapsular arteriolar
and capillary plexus. Finally they
form superficial cortical veins
which unite to form stellate veins
draining into interlobular veins.
Some
interlobular
arteries perforate capsule and,
as capsular arteries vascularize
capsule. There are anastomoses
between capsular arteries and
interlobular
veins.
Understanding of the renal
vasculature is essential for
comprehension of filtration in
the renal corpuscle.
Blood Supply of the Kidney
From the arcuate arteries
interlobular arteries emerge radially
and at fairly regular intervals. From
them, at right angles, arise afferent
arterioles which break up into
capillaries of the renal glomerulus.
Glomerular capillaries unite to
form efferent arterioles which (if
associated with the cortical nephrons)
break up into a peritubular capillary
network which drains both into
interlobular
veins
and
radially
oriented deep cortical veins emptying
into arcuate veins, tributaries of the
interlobar veins from where blood
enters renal vein and finally inferior
vena cava.
The Two Type of the
Nephrons.
The
described
course is true for the
cortical nephrons making
up about 80% of all
nephrons and with renal
corpuscles located in the
peripheral region of the
cortex.
The remaining 20%
of
the
nephrons
are
juxtamedullar with the renal
corpuscles located in the
cortex adjacent to the
medulla. Their blood supply
bears special features.
The
efferent
arterioles of the medullary nephrons pass into
the
medulla
as
descending
thin-walled
nonbranched
arteriolae
rectae spuriae – a part of
the vasa recta. These
form
a hairpin loop,
turning upward toward
cortex to empty as
venous vasa recta into
interlobular veins.
Compare
peritubular plexus of the
cortical nephrons and
vasa
rectae
of
the
juxtamedullary nephrons.
Blood Supply of Cortical and Medullary Nephrons
Blood Supply of Kidney
Interlobular
arteries
derived from the arcuate arteries
enter cortical labyrinth to reach
the renal capsule supplying a
stellate subcapsular arteriolar
and capillary plexus. Finally they
form superficial cortical veins
which unite to form stellate veins
draining into interlobular veins.
Some interlobular arteries
perforate
capsule
and,
as
capsular arteries, vascularize
capsule. There are anastomoses
between capsular arteries and
interlobular veins. Understanding
of the renal vasculature is
essential for comprehension of
filtration in the renal corpuscle.
Renal Cortex
Renal corpuscles (RC) are scattered in the renal cortex
surrounded by tightly packed renal tubules,
mostly proximal
convoluted tubules (PCT) and fewer distal convoluted tubules (DCT)
which usually have a wider lumen, but are smaller in the overall
diameter. Part of a medullary ray (MR) is visible and contains straight
segments of proximal and distal renal tubules, and collecting ducts.
All of these components are supported by the interstitial tissue.
Renal Corpuscle
Collectively
glomerulus
and
the
Bowman’s
capsule
surrounding it are referred
to as the renal corpuscle. It
is roughly spherical and
measures 150 to 250 mcm.
It is entered by the afferent
arteriole and drained by the
efferent arteriole.
The capsule is the a
dilated
blind-ending
proximal part of the renal
(uriniferous)
tubule,
it
contains parietal (outer) in
visceral (inner) layers. The
modified cells of the inner
layer
are
known
as
podocytes.
Kidney
Long thin artery leading to glomerulus (look in lower mid-picture). Note
long, thin endothelial nuclei lining the lumen. Circular muscle fibers have
been cross-cut and look almost like a simple cuboidal epithelium outside
the endothelium.)
Renal Corpuscle
The capillary loops are
seen supported by the podocytes
on one side and by mesangial cells
in the regions where the podocytes
may not come into contact with a
capillary. This is a pericyte-like
intraglomerular
mesangial
cell
which is phagocytic, with a function
of resorbtion of the basal lamina, it
may be contractile as it has
receptors
for
vasoconstrictors
(angiotensin- II) thus reducing
blood flow through glomerulus. It
may be responsible for many
pathological processes in the
kidney.
Another
type
of
the
mesangial
cells
–
the
extraglomerular one – may be seen
at the vascular pole of the
corpuscle.
Renal Corpuscle
Renal
corpuscle
displays polarity.
Afferent
arteriole
(AA) enters vascular
pole
of
a
renal
corpuscle,
and
branches
forming
capillaries
of
the
glomerulus (G).
Bowman’s
or
urinary
space
(S),
separates glomerulus
from the parietal layer
of Bowmen’s capsule
(arrow).
Renal Corpuscles
Continuity of the renal corpuscle (RC) with
proximal convoluted tubule (PCT) marks the urinary pole.
The cuboidal epithelium of the tubule changes to
squamous epithelium (arrows) of Bowman’s capsule.
FILTER
BARRIER
The capillaries constituting the glomerulus are similar to the fenestrated
type of capillaries. Their endothelial cells are highly attenuated, except
for the region containing nuclei, but the pores are usually not covered by
a diaphragm. The pores are large, ranging between 70 and 90 nm in
diameter; hence these capillaries act as a barrier only to formed elements
of the blood and to macromolecules whose effective diameter exceeds
the size of the fenestrae (albumin – 69,000 dalton).
Investing the glomerulus is a basal lamina 300 nm thick, consisting of
the 3 layers. The middle dense layer, the lamina densa, is about 100
nm in thickness and consists of collagen of the IV type. Less
electron-dense layers, the laminae rarae, which contain laminin,
fibronectin, and a polyanionic proteoglycan rich in heparan sulfate,
are located on the either side of the lamina densa.
Some refer to a
lamina rara interna
and lamina
rara
externa accordingly.
Fibronectin
and
laminin assist the
pedicels and endothelial
cells
to
maintain their attachment to the
lamina densa.
FILTER BARRIER
The visceral layer of the Bowman’s capsule is composed of
epithelial cells that are highly modified to perform a filtering function.
These large cells called podocytes, bear numerous long, tentacle-like
cytoplasmic extensions, primary (major) processes, which follow but
usually do not come in close contact with the longitudinal axes of the
glomerular capillaries. Each primary process bears many secondary
processes, also known as pedicles, arranged in an orderly fashion.
Pedicles completely envelope most of the glomerular capillaries by
interdigitating with pedicles from neighboring major processes of
different podocytes.
Endothelium
RENAL FILTER BARRIER, TEM, x 50,000
EM of triangular shaped podocyte with its many terminal end feet (foot
processes) touching the basement membrane (dark) which is shared on
its other surface by endothelium of a capillary.
Interdigitation
between
podocytes
occurs in such a fashion
that narrow cleft, 20 to 40
nm in width, known as
filtration
slits
remain
between
adjacent
pedicles. Filtration slits
are not completely open;
instead, they are bridged
by a thin (6 nm thick) slit
diaphragm, which extends
between
neighboring
pedicels and acts as a
part of a filtration barrier.
Filter Barrier, TEM
Filter Barrier, TEM
Detail of end feet of podocyte on the basement membrane. The basement
membrane (basal lamina) is continuous, but the fenestrated capillary
endothelium has pores. Glomerular filtrate passes from the capillary
lumen, through the layers seen here, into the lumen of Bowman's
capsule. Between the foot processes are thin slit membranes.
FILTRATION BARRIER:
1. Filtration slits between adjoining pedicels bridged
by thin diaphragms,
in association with
2. capillary endothelium and
3. the basal laminae of the capillary endothelium and
podocyte,
contribute to the formation of the filtration barrier.
Filtration Process:
fluid leaving the glomerular capillaries through the fenestrae is
filtered by the basal lamina. The lamina densa traps larger
molecules (>69,000 Da), whereas the polyanions of the laminae
rarae impede the passage of negatively charged molecules and
molecules that are incapable of deformation. The fluid that
penetrates the lamina densa, passing through the pores in the
diaphragm of the filtration slits and entering Bowman’s space,
is called the glomerular ultrafiltrate,
becuse the BL traps larger macromolecules, it would become
clogged were it not continuously phagocytosed by
intraglomerular mesangial cells and replenished by both the
visceral layer of Bowman’s capsule (podocytes) and
glomerular endothelial cells.
BASEMENT MEMBRANE ABNORMALITIES
IN GLOMERULAR DISEASE
Abnormalities in the structure of the glomerular BM are
responsible for some important kidney diseases which are
characterized by an excessive loss of protein in urine
(proteinuria). Sometimes so much is lost in the urine that the
capacity of the liver to synthesize fresh protein (particularly
albumin) is outstripped. The patient then develops a low blood
albumin (hypoalbuminemia), and edema due the low oncotic
pressure of the blood.
The combination of proteinuria, hypoalbumonemia and
edema is called nephrotic syndrome.
There are many causes of the nephrotic syndrome, but
all appear to be related to a structural or functional abnormality
of the glomerular BM.
Diseases in which the abnormality is structural include
diabetes mellitus and membranous nephropathy.
In nephrotic syndrome associated with
diabetes mellitus the
glomerular basement
is thickened 3-5 fold
and the demarcation
into the three laminae is lost.23
EM of the filter barrier. A uniformly thickened basement
membrane from a patient with diabetes mellitus who
presented with the nephrotic syndrome.
PODOCYTE ABNORMALITIES IN
GLOMERULAR DISEASE
In children, the most common cause of the nephrotic
syndrome is the so-called minimal change nephropathy.
By light microscope the glomerulus appears normal but EM
reveals loss of the foot process pattern, with the outer
surface of the glomerular capillaries being covered by an
almost continuous sheet of podocyte cytoplasm, probably
representing process remnants.
The abnormality is usually only temporary, structure and
function return to normal in time.
The podocyte abnormalities are associated with the
polyanionic charge which possibly explains the protein
leak.
Uriniferous Tubules.
The functional unit of the
kidney is the uriniferous tubule,
consisting of the nephron and
the collecting tubule, each of
which derived from a different
embryologic primodiums.
There are two types of the
nephrons: the cortical and
juxtamedullary
neurons,
classified by their location in the
kidney cortex and differing in
their structure and blood supply.
The nephron begins as a
distended,
blindly
ending
invaginated tubule, known as
Bowman’s capsule. Glomerulus
is not a part of the nephron.
The
region
of
continuation between
the renal corpuscle
and the uriniferous
tubule which drains
the Bowman’;s space
is called the urinary
pole.
The
ultrafiltrate
enters the urinary
space and leaves the
corpuscle
at
its
urinary pole through
the
pro-ximal
convoluted tubule.
Renal Corpusle
Proximal Tubules:
constitute much of the
renal cortex. Each of
them is about 60 mcm in
diameter
and
approximately 14 mm
long. It consists of a
highly tortuous region –
the
pars
convoluta
(proximal
convoluted
tubule) located near the
renal corpuscle, and a
straighter portion, the
pars recta (descending
thick limb of the Henle’s
loop).
Brush border of
microvilli enables the
tubules to reabsorb
about 70% of the
glomerular filtrate. The
membrane
contains
Na+,
H+,
and
Clantiport exchangers and
enzymes that digest
small amino acids.
Deep
basolateral enfoldings or
invaginations
greatly
increase surface area to
provide
effective
reabsorbtion of fluid
and solutes that pass
along it.
PROXIMAL CONVOLUTED TUBULE
Proximal tubule: Lumen
continuous with that of
glomerular
capsule
(Bowman's space). Large
cuboidal cells, abundant
eosinophilic cytoplasm,
and large round nuclei.
Brush border.
Neck: The first part of the
proximal tubule leading
away
from
the
Kidney, Cortex, x 612.
glomerulus. Narrow and
straight.
The ultrafiltrate from the glomerulus enters the urinary
space and is drained from there by the neck of the proximal
tubule. The simple cuboidal epithelium of the proximal tubule
adjoins the simple squamous epithelium of the parietal layer of
the Bowman’s capsule.
Kidney, Hematoxylin-Eosin.
Detail of renal corpuscle. Dark pink epithelium = proximal tubule.
Lighter pink (as at upper top left) = distal tubule. Renal corpuscle
with connection to proximal tubule at lower border.
Kidney, Hematoxylin-Eosin.
Dark pink = proximal tubule. Lighter, low cuboidal epithelium (as at
top left) = distal tubule.
NEPHRON, TEM
Proximal and distal convoluted tubules (EM). Proximal convoluted
tubules contain microvilli, intercellular junctions, mitochondria for
energy supply and infolded BM. Distal has no brush border.
Peritubular capillaries lie in the connective issue between tubules.
NEPHRON, TEM
Higher EM of proximal tubule with its brush border (arrow), which
indicates absorption by the cell.
NEPHRON, TEM
EM of base of epithelium of proximal convoluted tubule. Note
basement (basal) lamina and the great infolding of the cell membrane. The
many folds also provide increased cell surface for passage of absorbed fluid
and ions into the peritubular capillary below. The folds contain Na+K+
adenosine triphosphate complexes that pump Na+ out of the cell,
coupled to transport of glucose and amino acids. Sodium is followed by
chloride to maintain electrical nuetrality and by water to maintain osmotic
equilibrium.
Proximal
Convoluted
Tubule
of the Nephron
Cytoplasm of the epithelial cells is eosinophylic granular
cytoplasm. The folds of the BM, plus the many mitochondria lying in
them, tend to give the cytoplasm a striated look in light microscopy. The
cells have elaborated an intricate system of interlocking and
interwoven lateral cell processes. Thus, the lateral cell membranes
are unusually indistinguishable with the light microscope.
Proximal Convoluted Tubules, Periodic Acid-Shiff
Base Stain
PAS-staining of the
proximal
convoluted
tubule showing carbohydrate-rich
basal
laminae and microvilli.
The
brush
border
enhances reabsorbtion
of fluid and solutes from
the lumen through or
between the cuboidal
epithelial cells and into
capillaries.
These
tubules also secrete
organic bases and H+
into the lumen.
PARS RECTA OF
THE
PROXIMAL
TUBULE
Pars recta of the
proximal tubule descends in
medullary rays within the
cortex and then in the medulla
to become connected with a
loop of Henle at the junction of
the outer and inner stripes.
Cells of the pars recta
of the proximal tubule are low
cuboidal. On the contrary to the
cells the distal part of the
proximal tubule convoluted
tubule, they do not form apical
canaliculi for the protein
resorption,
contain
fewer
mitochondria
and
less
elaborate
intercellular
processes.
Upon
entering
the
medulla,
the
proximal tubule shows
an abrupt transition into
the descending thin
limb of Henle’s loop in
which
the
lining
epithelial cells are flat
with
nuclei
that
protrude into the lumen.
This appearance
persists in those loops,
which turn back to form
thin, ascending limbs.
Depending
on
whether a nephron has
a short or long loop of
Henle, the thin limbs are
1-10 mm in length.
LOOP OF HENLE
Kidney, Medulla.
Thin segment of Loop of Henle (in the middle), with a simple squamous
lining.
Kidney. Medulla.
Cross cuts in medulla. Two pale collecting tubules in the middle. Simple
squamous lining indicates thin loops of Henle. Compare these with
blood vessels, which contain r.b.c.'s. (Look for vessels up near top
center and to right; also in lower left quadrant of field.) Notice the blue
c.t. stroma in between the tubules.
Kidney. Hematoxylin-Eosin.
Large pale tubules are collecting tubules, with clear epithelial cell
boundaries. Brighter pink tubules are thick portions of loops of Henle;
these are basically like distal convoluted tubules in their histology, so
would be ascending limbs.
Distal Tubule
Distal tubule is
composed of the
three histologically
distinct segments:
thick ascending
Limb of Henle’s
loop; macula densa and distal convoluted tubule the
latter being the
last part of the
nephron.
The transition from thin to thick
ascending limb is recognized by the
appearance in the latter segment of
low cuboidal cells, at the US-level, by
abundant
mitochondria
and
invaginations of the basolateral
membrane.
These
feature
are
associated with active transport
mechanisms in which salt is
reabsorbed into the interstitium, to
produce a dilute tubule fluid and
hypertonic
interstitium.
The
ascending thick limb extends upward
toward cortex and returns to the
parent renal corpuscle. At the
contact
point
with
the
extraglomerular mesangial region,
the cells are narrow and clustered
side-by side to form the macula
densa.
DISTAL TUBULE
ENDOCRINE APPARATUS of the KIDNEY
Macula densa is a type of chemoreceptor that monitors
luminal Cl- concentration and is involved with adjustment of the
glomerular filtration rate. Together with the modified smooth muscle
cells of the afferent arteriole (juxtaglomerular cells containing specific
granules of renin)
and extraglomerular mesangial cells they
constitute juxtaglomerular apparatus of kidney.
ENDOCRINE APPARATUS OF THE KIDNEY
KIDNEY
Juxtaglomerular
cells, Mallory
Stain, x1614
Afferent arteriole: Terminal branch of the interlobular artery
entering the glomerulus. The renal afferent arterioles are volume
receptors and are sensitive to changes in perfusion (blood) pressure.
Juxtaglomerular cells: Granular variety of myoepithelioid cells in
the wall of the afferent arteriole. Replace the typical smooth muscle
cells of the tunica media of the artery.
FUNCTIONAL CORRELATES:
When cells of the macula densa detect a low sodium
concentration in the ultrafiltrate, they cause juxtaglomerular
cells to release the enzyme renin, which converts angiotensin
to angiotensin I a mild vasoconstrictor.
Angiotensin I is converted to angiotensin II by angiotensinconverting enzyme (ACE) which is a potent vasoconstrictor.
Angiotensin II influences the adrenal cortex to release
aldosterone increasing reabsorbtion of sodium and calcium.
Distal tubule: Cuboidal
cells without a brush
border, which stain less
intensely
than
the
proximal tubule cells. It
is about one third the
length of the proximal
tubule. The distal tubule
is continuous with the
collecting tubules.
Distal
Convoluted
Tubule
The tubules have no brush border, but their numerous
mitochondria (arrows) provide the adenosine triphosphate necessary
for active transport of NaCl reabsorbed from luminal fliud, which
amounts to 5-10% of the filtered load of NaCl. This nephron segment,
although relatively impermeable to water, is not homogeneous in
either morphology or function and is a transitional tubule that leads
to connecting segment or tubule.
Cortico-Medullary Region
Medullary rays (MR) extend down from the cortex into the
medulla and contain straight parts of proximal and distal tubules and
collecting ducts (CD) with wide lumens. The arteries (A) and veins (V) are
related to arcuate vessels that arise at the corticomedullary junction.
Medullary rays are prominent because they provide the route by which
the descending and ascending limbs of the loop Henle reach the inner
medulla. Juxtamedullary nephrons have the longest loops, but in the
human kidney most cortical nephrons have relatively short loops.
Distal convoluted tubule: Wider lumen, shorter cells, without brush
border. The ratio of cross-sectins of proximal to distal convoluted
tubules surrounding any renal corpuscle is 7:1
KIDNEY
Collecting tubules
represent transitional segments between distal convoluted
tubules and long
collecting ducts that
extend
to
the
papillary region of
the renal pyramid.
Cortical
collecting
tubules
show
a
cuboidal cell lining
which
becomes
taller,
to
form
columnar cells, as
the ducts descend
through the medulla.
Nephrons emptying into a collecting tubule.
Notice that the closer to the medulla a
glomerulus lies, the longer the loop of Henle
is. Also notice that the inner zone of the
medulla (lowest section of the picture)
contains only thin limbs of the loop of
Henle, plus collecting ducts. This is the area
where the counter-current mechanism for
urine concentration (carried out between the
tubules and the surrounding peritubular
capillaries) is most active. To make his
drawing clear, the artist has made one
accommodation that is not quite accurate
histologically; namely, within the cortex, the
straight positions of the nephrons (that is,
the thick and thin portions of the loops of
Henle) should lie immediately next to the
collecting duct, thus making up the
medullary ray. The glomeruli and convoluted
portions of the nephrons would then lie on
either side of the ray. The ray is the central
axis of a lobule.
NEPHRON
Medullary
Ray,
transverse
section
Collecting ducts (CD) with distinct cell outlines, straight
portions (pars recta) of the proximal tubules with brush border
(PCD) and thick ascending limbs (TAL) of the loop of Henle. The
TAL extrudes Na+ into the interstitium, but it is impermeable to
water so the osmolarity of the tubular fluid decreases as it
approaches the cortex and the distal tubule.
Collecting
Duct
A longitudinal section of collecting duct (CD) showing
orderly cuboidal epithelium and prominent plasma membrane
borders between cells. These ducts absorb water and urea and
partly determine urine volume and concentration. Permerability
is regulated by antidiuretic hormone, and also aldosterone.
Collecting Tubules of the
Medulla
Many
collecting
tubules with wide lumen are
present
in
the
inner
medulla. In the interstitium
are medullary interstitial
cells that support a rich
vascular plexus of capillary
loops and the thin limbs of
the loop of Henle.
Collecting ducts are
permeable to water in the
presence of antidiuretic
hormone,
which
also
increases
urea
permeability; the urea is
recycled in the nephron,
and contributes to the high
osmolarity of the inner
medulla.
There is close relationship between collecting ducts (CD),
thick ascending limb (TAL), and thin limb (tL) of the loop of Henle and
the peritubular capillaries or vasa recta (VR). Water moves from thin
limbs to interstitium (I) in response to high osmolarity of the
interstitial region created by Na+ movement from the TAL, the cells of
which contain many mitochondria to energize the active transport of
Na+ through the base of the tubule. Collecting ducts reabsorb water
from lumen to interstitium in response to antidiuretic hormone.
Medulla of the
Kidney
Vasa Recta
Vasa recta are columns of capillaries that are prominent in the
medulla. They are a mixture of descending vessels that originate from
glomerular efferent arterioles, which extend into the inner medulla
and return as venous or ascending vessels, that drain into
corticomedullary veins. In the outer medulla, the vasa recta form
vascular bundles mainly surrounded by thick and thin limbs of
Henle’s loop.
Structure & Function of the Uriniferous Tubule
region of
uriniferous
tubule
structure
major functions
renal
corpuscle
simple squamous filtration
epithelium, fused
basal laminae,
podocytes
filtration barrier:
endothelial cell, fused
basal laminae, filtration
slits
proximal
tubule
simple cuboidal
epithelium with
brush border
and vbasolateral
striations
sodium pump in
basolateral membrane;
ultrafiltrate is isotonic
with blood
resorption of 67-80% of water,
sodium, and chloride
(reducing volume of
ultrafiltrate), resorption of
100% of protein, amino acids,
glucose & bicarbonate
miscellaneous comments
descending simple squamous completely permeable to water
thin limb of epithelium
& salts (reducing volume of
Henle’s
ultrafiltrate)
loop
ultrafiltrate is hypertonic
with respect to blood,
urea enters lumen of
tubule
ascending
simple squamous
thin limb of epithelium
Henle’s
loop
ultrafiltrate is hypertonic
with respect to blood;
urea leaves renal
interstitium and enters
the lumen of tubule
impermeable to water,
permeable to salts; sodium &
chloride leave tubule to enter
renal interstitium
Structure & Function of the Uriniferous Tubule
Region of
uriniferous
tubule
Structure
Major functions
Miscellaneous comments
ascending
thick limb
of Henle’s
loop
simple
cuboidal
epithelium
impermeable to water;
chloride & sodium
leave tubule to enter
renal interstitium
ultrafiltrate becomes hypotonic with
respect to blood; chloride pump in
basolateral cell membrane is
responsible for the establishment of
osmotic gradient in interstitium of
outer medulla
macula
densa
simple
columnar
epithelium
monitors sodium level
contacts & communicates with
& volume of
juxtaglomerular cells
ultrafiltrate in lumen of
distal tubule
distal
convolted
tubule
simple
cuboidal
epithelium
responds to aldosterone ultrafiltrate becomes more hypotonic
by resorbing sodium & (in presence of aldosterone); sodium
chloride from lumen
pump in basolateral membrane;
potassium is secreted into lumen
collecting
tubule
simple
cuboidal
epithelium
in the presence of ADH,
water and urea leave
the lumen to enter the
renal interstitium
urine becomes hypertonic in the
presence of ADH; urea in
interstitium is responsible for
gradient of concentration in
interstitium of the inner medulla
The Vasa Recta
The vasa recta have a slow blood flow and operate a
countercurrent exchange mechanism of water and solutes between
their plasma and the interstitial fluid. These capillaries supply
nutrients and remove wastes from the interstitium, but they do not
wash away the solutes within the medulla necessary to maintain the
gradient of salinity. Descending or arterial capillaries (A) are smaller
with thicker walls compared to ascending or venous capillaries (V).
Apex
of a Renal
Pyramid
Collecting ducts terminate, with prior fusions (F), at a papilla
to form wide ducts of Bellini which open, sieve-like (arrows), at the
area cribrosa. The cavity of the minor calyx (MC) is lined by transitional epithelium, & conveys urine from papilla to the renal pelvis.
Papillary ducts (of
Bellini). Arise by convergence
of collecting tubules in the
medulla near the pelvis. Have
large lumina lined by tall
columnar cells and open at the
area cribrosa at the apex of
the papilla.
Cytoplasm of
epithelial cells is clear; nuclei
are dark and basally located.
The tops of cells tend to bulge
into the lumen.
Minor calyx: Subdivision of a
major calyx in the pelvis of the
kidney,
minor calyx is an
infolded tube forming a
double-walled cup. The inner
wall of the calyx fits over the
papilla of a pyramid. The
transitional epithelium of the
minor calyx is continuous
with the columnar epithelium
of the papillary ducts.
KIDNEY, Papilla, area
cribrosa, minor calyx , x162
Area cribrosa: The sieve-like appearance
of the papilla is produced by a large
number of collecting tubules passing
through it.
CLINICAL CORRELATIONS
A decrease in afferent arterial volume secondary to low perfusion
pressure results in the release of renin.
Renin is an enzyme that is released into the blood and acts upon
blood proteins to produce a potent vasoconstrictor, angiotensin,
which can, under abnormal conditions, elevate blood pressure to
dangerous levels.
Hypertension of renal origin in humans can be cured by removal
of the diseased or ischemic kidney.
Renin also affects blood volume and osmolarity by initiating a
chain of events leading to the release of the hormone aldosterone
by the cells of the zona glomerulosa of the adrenal cortex.
Aldosterone acts upon the renal tubules to enhance sodium
reabsorption.
Proximal
Ureter, the
transverse
section
The ureter is lined with with typical transitional epithelium
(T), fibroelastic lamina propria (LP), and circula muscularis
externa (ME). Urine passes along urtere in peristaltic waves when
the lumen changes from stellate to ovoid.
Distal
Ureter,
the
transverse
section
Distal ureter has a thick smooth muscle coat (M,
sometimes seen in three layers) and thin lamina propria. Through
autonomic nerve supply, the muscle ensures that peristaltic
forces deliver urine from the ureter into the bladder.
Key Features of the Urinary System
Kidney
Primary Location
Features of
Epithelium
Other Features
Proximal convoluted
tubule
Cortex
Large granular, darkstaining cells; brush
border; not all cells
show a nuclear profile
Stellate or irregular
lumen
Distal convoluted
tubule
Cortex
Light-staining
columnar cells; each
cell shows nuclear
profile
Smooth luminal
surface, large,
circular in outline
Renal corpuscle poles Cortex
(Bowman’s capsule,
glomerulus
Glomerular epithelium Vascular and urinary
– light-staining nuclei thick basal lamina
Thin segment (loop of
Henle)
Medulla
Light-staining tubule
lined by simple
squamous epithelium
Narrow lumen
Thick segment (loop
of Henle)
Medulla
Light-staining
cuboidal cells
Wider lumen
Collecting tubule
Medulla
Light-staining
columnar cells,
distinct
Extrarenal Passages
Epithelium
Supporting Wall
Ureter (proximal
two-thirds)
Transitional (4-5
cell layers)
Inner longitudinal
and outer circular
smooth muscle
Ureter (distal onethird)
Transitional (4-5
cell layers)
Inner longitudinal
and outer circular
smooth muscle
Bladder.
A low magnification showing non-distended, folded mucosa (M) and
small strands of smooth muscle (S) in the lamina propria (LP), which is
a normal finding but is not described as a muscularis mucosae. The
external smooth muscle, showing three interwoven divisions (1,2,3),
functions to allow bladder filling and contraction during micturition
reflex.
Bladder.
Transitional epithelium of the bladder is impermeable to urine and for
this depends upon a thick plasma membrane that faces the lumen,
together with intercellular tight junctions. The dome-shaped
appearance of the superficial cells is typical of relaxed bladder mucosa,
which is usually 4-5 cell layers in deapth and contains cuboidal and
columnar cells.
Transitional epithelium from a distended urinary bladder (right);
compare with the contracted wall (left). When stretched, the epithelium
becomes very thin, with fewer layers of cells, and the surface cells tend
to be flattened. There's only the one layer of flatter cells, however,
which is quite different from the appearance of stratified squamous
epithelium (with which this might be confused.)
Transitional epithelium -- diagnostic for urinary tract. The
surface cells are characteristically dome-shaped and puffy.