H I S T O L O G Y
L E C T U R E
URINARY SYSTEM
OBJECTIVES:
Upon completion of study of this section the student should be able to:

List the parts and function of the urinary system and describe the roles of each
organ.

Identify and describe the substructures of the kidney as seen grossly in frontal
section.

Compare the kidney cortex and medulla in terms of structure and function.

Name the components and describe the function of the juxtaglomerular apparatus.

Trace the circulation of blood through the kidney.

Describe the ultrastructure of the filtration barrier of the renal corpuscle.

Trace the flow of fluid from Bowman’s space to a minor calyx, naming, in order, the
tubules through which it flows and describing any changes in fluid composition that
occur in each tubule segment.

Describe the structure of the walls of the calyces, renal pelvis, ureters, and urinary
bladder.

Name the parts of the male urethra and the epithelium found lining each part.

Compare the urethras of males and females.

Differentiate between cortex and medulla and identify all of the vascular and tubular
components as well as the components of the renal corpuscles and juxtaglomerular
apparatus in a slide or photomicrograph of the kidney.

Identify the components of the glomerular filtration barrier in a diagram or electron
micrograph of a renal corpuscle.

Identify the ureter, urinary bladder, and the parts of the urethra in histologic sections.
Urinary System

includes the kidneys, ureters, urinary bladder, and urethra, all of which function in
excretion.

functions to clear the blood of waste products and regulate the concentration of body
fluids.

kidney also produces the hormone erythropoietin (which stimulates erythrocyte
production) and the enzyme renin (which influences blood pressure and the
concentration of sodium in the body fluids).
Kidneys
Macroscopic Features
1. Location

paired, been-shaped organs situate retroperitoneally against the posterior abdominal
wall.
2. Hilus

a concavity on the medial border of the kidney where arteries, veins, lymphatics,
nerves, and the renal pelvis are present. The hilus is continuous with the renal sinus
(a central cavity containing fat).
3. Renal Pelvis

funnel-shaped expansion of the upper end of the ureter.

is continuous with the major calyces, which in turn subdivided into minor calyces.
4. Cortex and Medulla

easily identified in a hemisected kidney.

cortex appears granular and is located superficially.

medulla appears striated and lies deeper.

renal corpuscles and convoluted tubules comprise most of the cortex, while the
medulla is composed of several renal pyramids.

at the apex of each renal pyramid is the renal papilla with its perforated tip (area
cribrosa), which projects into the lumen of a minor calyx.

cortical tissue, the renal columns of Bertin, extend between adjacent renal
pyramids.
5. Renal Lobe

consists of a renal pyramid together with its closely associated cortical tissue.
6. Medullary Ray

is a group of straight tubules that project into cortex from the base of each renal
pyramid.
7. Renal Lobule

consists of a medullary ray (at its center) and the closely associated cortical tissue
surrounding it.

all nephrons in a single lobule are drained by the same collecting tubules.
Uriniferous Tubule
Components

two principal parts, the nephron and the collecting tubule.

each originates from separate primordia in the embryo.
1. Nephron

includes the renal corpuscle, proximal convoluted tubule, descending pars recta
(straight portion) of proximal tubule, thin segment, and ascending thick limb (of the
distal tubule), and distal convoluted tubule.

may be classified as cortical or juxtamedullary, depending on location of the renal
corpuscle.

cortical nephrons usually possess short loops of Henle in contrast to juxtamedullary
nephrons.

long loops of Henle, which extend deep into the medulla, are responsible for
establishing (by a countercurrent mechanism) the interstitial concentration gradient
that enables the kidney to form hypertonic urine.
Renal Corpuscle

includes two parts, the glomerulus (a capillary tuft) and Bowman’s capsule.

simple squamous epithelium that composes the outer wall of Bowman’s capsule is
known as the parietal layer (capsular epithelium).

modified simple squamous epithelial layer investing the glomerular capillaries is
referred to as the visceral layer (glomerular epithelium).

Bowman’s space (capsular space) is the narrow chalice-shaped cavity between the
visceral and parietal epithelial layers.

at the vascular pole the afferent glomerular arteriole enters and the efferent
glomerular arteriole leaves the glomerulus.

at the urinary pole the capsular space becomes continuous with the lumen of the
proximal convoluted tubule.
1) Podocytes
 are modified simple squamous epithelial cells comprising the visceral layer (glomerular
epithelium).

appear stellate and unusual in shape.

have several radiating primary processes that give rise to many secondary processes
(pedicels; foot processes).

pedicels interdigitate with similar processes from adjacent podocytes.

between adjacent pedicels are filtration slits, 20 to 30 nm in width (slit pores), that are
bridged by a layer of filamentous material referred to as a slit diaphragm (membrane).

pedicel surface Bowman’s space has a protein coating (podocalyxin) that stains with
cationic dyes and is believed to maintain the organization and shape of those processes.
2) Basal Lamina
 associated with the filtration barrier is unusually thick (0.1 to 0.15 um).

is located between the podocytes and the endothelial cells in the glomerulus.

has three distinct zones: the lamina rara externa (an electron-lucent area adjacent to
the epithelium), the lamina rara interna (an electron-lucent region adjacent to the
capillary endothelium), and the lamina densa (a thicker amorphous intermediate zone
that appears more electron dense).
3) Endothelium
 lining the capillaries is thin and fenestrated.

has no diaphragms extending across its large fenestrae (60 to 90 nm diameter).

contains most organelles in the thicker regions of unfenestrated cytoplasm.
4) Filtration Barrier
 functions to filter the blood plasma and is located in the renal corpuscle.

permits water, ions, and small molecules (ultrafiltrate) to enter capsular space.

prohibits protein molecules with a molecular weight greater than 69,000 or molecules
with a high net negative change from entering the capsular space.

components of the barrier include the fenestrated endothelium, the basal lamina, and
the filtration slits (with diaphragms) between pedicels.

current studies indicate that the glomerular basal lamina is responsible for filtration
selectivity, which depends not only on the size but also on the shape and charge of the
molecule.
5) Mesangium
 comprises the interstitial spaces of the glomerulus between the capillaries.

contains mesangial cells and extracellular matrix material.

is the area where phagocytic mesangial cells help maintain functional integrity of the
basal lamina in the filtration barrier by phagocytosing large protein molecules and/or
debris.

mesangial cells also contract to decrease surface area available for filtration, and they
have receptors for angiotensin II and atrial natriuretic factor.
Proximal Convoluted Tubule

leaves urinary pole of the renal corpuscle.

is the longest segment of the nephron, lines by a single layer of pyramidal-shaped
cells that have a well-developed brush border (microvilli).

proximal tubule cells have an endocytic complex (apical canaliculi, vesicles, and
vacuoles) actively involved in protein absorption.

later borders of proximal tubule cells have extensive interdigitations that interlock
them with one another.

cells have extensive basal plasma membrane infoldings, which compartmentalize
mitochondria.

at least 80% of the sodium chloride and water, and all of the glucose, amino acids,
and small proteins in the glomerular filtrate are absorbed in this part of the nephron.

hydrogen ions are secreted in exchange for bicarbonate ions, and organic acids and
bases are also secreted in this part of the nephron.
Descending Pars Recta (straight portion) of Proximal Tubule

is also lined by a simple cuboidal epithelium having a prominent brush border.

cells are shorter and less elaborate in shape than those of the proximal convoluted
tubule, but they have the same general features.

this region of the nephron is often damaged in acute renal failure and mercury
poisoning.

this segment constitutes the initial part (thick descending limb) of the loop of Henle.
Thin Limb of the Loop of Henle

is composed of a descending limb, a loop, and an ascending limb, all of which are
lined by a simple squamous epithelium.

cells in this epithelium have nuclei that bulge into the lumen, and their surfaces
possess only a few short microvilli.

the thin limb has separated into four distinct segment based on shape of cells, their
content of organelles, the depth of their tight junctions, and their water permeability.

is the region that forms the middle part of the loop Henle.
Ascending Thick Limb (straight portion) of Distal Tubule.

is the third (and final) component of the loop of Henle.

is lined by a simple cuboidal epithelium containing only a few microvilli.

its nuclei occupy an apical position in the cells.

mitochondria are compartmentalized within the interdigitations formed by the basal
and lateral infoldings.

cells transport ions from the lumen into the interstitium, and since this part of the
nephron has a impermeability to water, the luminal fluid becomes hypotonic to the
blood.
Distal Convoluted Tubule

begins at the macula densa.

has microvilli much shorter than the proximal microvilli, and their nuclei occupy an
apical position in the cytoplasm.

extensive lateral interdigitations compartmentalize mitochondria in basal cytoplasmic
infoldings.

cells actively transport sodium ions from the filtrate into the interstitium.
1) Macula Densa

is a specific region of the distal tubule, lying near the afferent glomerular arteriole.
is
one component of the juxtaglomerular apparatus.

cells are tall, narrow, and lined up closely together to form a row of nuclei that appear as
a “dense spot” by light microscopy.

cells are thought to monitor the fluid in the distal tubule and send a signal to the
juxtaglomerular cells (modified smooth muscle cells) located in the afferent arteriole.

signaling could occur via gap junctions present between these two cells types.
2. Juxtaglomerular (JG) Apparatus

is located at the vascular pole of the renal corpuscle.

consists of four structures: modified smooth muscle cells of the afferent arteriole,
of the efferent arteriole, the macula densa (of the distal tubule) and the
extraglomerular mesangial cells.
Function of Juxtaglomerular Apparatus

in response to a decrease in extracellular fluid volume (perhaps detected by the
macula densa) the JG cells release renin (an Enzyme).
Renin

acts in angiotensinogen in the plasma, converting it to angiotensin I.

in capillaries of the lung, angiotensin I is converted to angiotensin II, which causes
release of aldosterone from the zona glomerulosa cells in the adrenal cortex.
Aldosterone

stimulates distal tubule cells to retain sodium ions.

water follows the sodium, and the fluid volume is increased in the extracellular
compartment (thus correcting the initial problem.
Angiotensin II

is also a potent vasoconstrictor, which acts to elevate the blood pressure.
3. Collecting Tubules

have different functions, depending on their location in the kidney.

in the cortex and medulla they respond to antidiuretic hormone (ADH), also known
as vasopressin.

in the medulla they play a primary role in producing a concentrated urine (by
establishing a gradient due to the transport of urea from the tubular fluid into the
renal interstitium).
Cortical Collecting Tubules

are located primarily within the medullary ray, although a few arched collecting
tubules exist with the cortical labyrinth.

have two cell types, a light (principal) cell and a dark (intercalated) cell.
1) Light Cells
 are simple cuboidal in shape and have round centrally located nuclei.

a single central cilium (flagellum) extends into the lumen from the surface of each light
cell.
2) Dark Cells
 are fewer in number and have micorplicae (folds) on their surface.

apical cytoplasm of the dark cell contains many vesicles.
Medullary Collecting Tubule

is similar in structure to the cortical collecting tubule.

dark cells are present in outer medulla but absent in inner medulla.

principal (light) cells increase in height, and in the inner medulla are the only cell type
lining the collecting tubule.
Papillary Collecting Tubule (duct of Bellini)

large collecting tubule formed from converging collecting tubules.

consists only of principal cells and is a simple columnar epithelium.

lumen of the duct of Bellini measures 200 to 300 µm in diameter.

empties at the area cribrosa (a region having a sieve-like appearance) on the apex
of a renal papilla.
4. Renal Interstitium

is the connective tissue compartment in the kidney.

in cortical interstitium are two main cell types: fibroblasts and mononuclear cells.

in interstitium of the medulla, pericytes are present along the descending vasa recta.

another cell type having long processes and containing many small lipid droplets
(which probably contain a hormone that reduces blood pressure) is also common in
medulla interstitium.
5. Blood Supply of Kidney

is extensive (flow through both kidneys is about 1,200 ml/min.
Renal Artery

its branches enter the hilus, giving rise to interlobar arteries that travel between the
renal pyramids.
Interlobar Arteries

divide into several arcuate arteries that run along the corticomedullary junction, in a
direction parallel with the surface of organ.

small interlobular arteries arise from the arcuate arteries and enter the cortical tissue
to pass between lobules.
Interlobular Arteries

give rise to afferent (glomerular) arterioles, which supply the glomerular capillaries.
Efferent Glomerular Arterioles

leave the glomerulus and give rise to an extensive peritubular capillary network
that supplies the convoluted tubules.

from glomeruli of juxtamedullary nephrons form thin-walled vessels called vasa recta,
which are long straight capillaries that extend into the medullary pyramids.
Vasa Rectae

form hairpin loops and ascend to form a countercurrent system of vessels called
vascular bundle (rete mirabile).

these vessels drain into interlobular or arcuate veins, then into interlobar veins,
which at the hilus form the renal vein.
Outermost Layers of Cortex

are drained by superficial cortical veins, which join stellate veins and empty into
interlobular and arcuate veins.
Deeper Regions of the Cortex

are drained by deep cortical veins.
Excretory Passages
Components

include the minor and major calyces, the renal pelvis, ureter and urinary bladder.

each of these passages has three layers, a mucosa (consisting of transitional
epithelium lying on a subepithelial connective tissue), a muscularis, and an
adventitia.
1. Ureter

is the conduit between the renal pelvis and the urinary bladder.

epithelium is thicker and had more cell layers than that in the calyces.

upper two-thirds of ureter had an inner longitudinal and an outer circular layer of
smooth muscle.

lower one-third of ureter has an additional outer longitudinal layer of smooth muscle.

contraction of these muscle layers produces peristaltic waves, which propel urine
along so that it enters the bladder in spurts.
2. Urinary Bladder

is lined by a layer of transitional epithelium several cell layers thick.

lumen of the bladder has a scalloped contour in the relaxed state, due to the
domeshaped surface cells.

lining cells have a thick asymmetrical plasmalemma, which displays a unique
substructure (when viewed in the transmission electron microscope after freeze
fracture).

plaques in the form of hexagonally arranged subunits are observed, but their
functional significance remains obscure.

cytoplasm of the surface cell has flattened elliptical vesicles, which insert themselves
into the plasma membrane (to rapidly expand the area when the bladder is
stretched).

epithelium has a remarkable ability to change its morphology in the relaxed versus
the distended state.

a thin basal lamina underlies the transitional epithelium, and beneath it is fibroelastic
connective tissue.

muscularis of the bladder is thick and consists of smooth muscle arranged in an
inner longitudinal, middle circular and out longitudinal layer.
3. Male Urethra

conveys urine from the urinary bladder to the outside.

also serves as a passageway for semen during ejaculation.

is divided into prostatic, membranous and cavernous portions.

transitional epithelium lines the prostatic portion, which pseudostratified or
stratified columnar epithelium lines the remaining portions.

beneath a thin basement membrane is the subepithelial connective tissue, where
mucus-secreting glands (of Littre) may be found.

muscularis has an inner longitudinal and an outer circular layer.

at distal end of the cavernous urethra is the fossa navicularis, lined by stratified
squamous epithelium.
4. Female Urethra

is a short passageway from the urinary bladder to the outside.

is lined primarily by stratified squamous epithelium, but has patches of
pseudostratified columnar epithelium.

mucus-secreting glands (of Littre) are located in the subepithelial connective tissue.

muscularis is composed of an inner longitudinal and an outer circular layer of smooth
muscle.
C L I N C A L
I N T E G R A T I O N
URINARY SYSTEM
Introduction:
A major function of the kidney is to remove waste products and excess fluid from the body.
These are excreted through the urine.
Apart from that, it produces hormones like Renin, erythropoietin etc and participates in
hydroxylation of Vitamin D and eventually in calcium absorption.
It is important to remember the normal histological appearance of the kidney to understand the
changes that take place during pathological processes.
Try your best to recall the structure of Renal corpuscle (Bowman’s capsule & glomerulus), the
modified visceral epithelium of bowman’s membrane as podocytes, their structural appearance
and the presence of filtration slits and the filamentous diaphragm that covers these slits.
The endothelial cells of the capillaries of the glomerulus formed by afferent arterioles, which
have fenestrations on their cell membrane, which are not covered by a membrane.
The significance of the filtration barrier, the factors affecting the passage of substances through
the filter to the urinary space.
Remember the role of intra glomerular mesangial cells as being phagocytic: have contractile
property , thereby can increase or decrease the rate of filtration , in response to angiotensin II
and atrial natriuretic factor, respectively. They are also capable of proliferation and secretion of
cytokines and growth factors.
Reminder of the layers of filtration barrier as:
1) Fenestrated epithelium
2) Lamina rara interna
3) Fused lamina densa
4) Lamina rara externa
5) Podocyte slit membrane
Due to the presence of the negatively charged heparin sulphate, negatively charged particles in
the blood are repelled. Particles larger than 69 Kdaltons-size of albumin are not filtered by the
membrane.
The importance of Juxta glomerular apparatus in the secretion of rennin and thereby
reabsorption of sodium from the DCT by the influence of aldosterone and ultimately
maintenance of blood pressure.
Identify the normal histological picture of renal corpuscle and apply the knowledge on the
diagrammatic picture shown on the slide.
The total intake of water, an average of 1 to 1.5 liters is distributed between the intracellular and
extracellular compartments and after metabolic activity in various parts of the body is excreted
in the stools (0.1L/day), sweat ( (0.1 L/day) , lungs(0. 3 L/ day) and urine (1-1.5 L/day).
Remember the substances that are reabsorbed in the various parts of the uriniferous tubule.
The absorption of 80% of sodium and chloride ions at the PCT and all of the amino acids and
glucose. The remainder of the water and sodium pass through various segments and get
reabsorbed and finally concentrated urine is excreted out.
Remember the influence of ADH (produced by the supra optic nucleus neurons and stored in
the posterior pituitary) on the collecting tubule for maximum reabsorption of water.
INTEGRATION CASES:
1) HYPERTENSION
2) DIABETES INSIPIDUS
3) MEMBRANOUS GLOMERULONEPHRITIS
4) IgA NEPHROPATHY
5) POLYCYSTIC DISEASE OF THE KIDNEY
DISEASES OF THE KIDNEY
Can be classified as:
Congenital- malformation- leading to obstruction leading to recurrent infections and ultimately
Chronic renal failure.
Hereditary- Poly cystic kidney, where the tubules are dilated and affect their functions
Acquired - Diseases due to an antigen or secondary to another disease like hypertension or
diabetes etc.
Mechanism of Injury:
Glomerular disease are mainly immunologically mediated
Tubulo Interstitial Diseases are caused by toxins or infectious agents.
Glomerular injury <<->> tubular destruction
Two forms of injury occur in the glomerular diseases:
1) Deposition of soluble circulating antigen-antibody complexes
2) Antibodies reacting insitu within the glomerulus.


The glomerular lesions usually consists of leucocytic infiltration in glomeruli and
proliferation of endothelial, mesangial and parietal epithelial [Podocyte] cells.
The presence of immunoglobulins and complement in these deposits are demonstrated
by immunofluorescence microscopy.
HYPERTENSION
Renal causes of Hypertension:
 Acute glomerulonephritis
 Renal artery stenosis
Others: Chronic renal disease, renal vasculitis, Renin producing tumors
Pathogenesis:
Decreased perfusion to kidneys – decreased solutes reaching the kidney – sensed by
macula densa – induces rennin production – leading to conversion of angiotensinogen to
angiotensin I and then converted to angiotensin II mainly in the lungs [POTENT
VASOCONSTRICTOR] – stimulates zona glomerulosa to produce aldosterone –increases
sodium reabsorption and there by increasing the blood volume and blood pressure.
Therefore, angiotensin II and aldosterone and two of the many factors that increases the blood
pressure.
In Renal Artery Stenosis, Decreased blood flow to the kidneys due to stenosis, is
misinterpreted by the JG cells as decreased blood pressure, therefore results in the rennin
cascade and ultimate rise in blood pressure.
DIABETES INSIPIDUS
Deficiency of the hormone –Anti diuretic hormone from the pituitary gland, leads to diuresis, as
the collecting ducts require this hormone for concentration of urine.
Clinical Case 1.
Terms to understand:
 Azotemia – increased blood urea, nitrogen and creatinine due to decreased GFR
[glomerular filtration rate]
 Uremia – azotemia + clinical signs and symptoms (ex. Itching)
 Hematuria – blood in urine
 Proteinuria – detectable amount of protein in urine
NEPHRITIC SYNDROME – glomerular disease marked by gross hematuria, mild proteinuria,
azotemia, edema and hypertension
NEPHROTIC SYNDROME refers to a clinical complex that includes:




Massive proteinuria > 3.5 g/day
Hypoalbuminemia < 3 g/dl
Generalized edema – most common clinical feature
Hyperlipidemia / lipiduria
Pathogenesis:
1) The initial event glomerular filtration barrier derangement resulting in increased
permeability to plasma proteins – 2) proteins escape from the plasma into the glomerular filtrate
–3) long standing proteinuria leads to depletion of serum protein (hypoalbuminemia) – 4)
resulting drop in oncotic pressure-fluid escapes from vascular tree into the interstitium (edema)
– reduced vascular volume activates rennin angiotensin cascade- salt and water retention –
more edema..5) Hypoalbuminemia triggers the synthesis of lipoprotein in the liver resulting in
hyperlipidemia which appears in the urine as lipid (lipiduria).
Diagnosis: MEMBRANOUS GLOMERULONEPHRITIS
Most common in adults. Occurs mainly in association with infections like syphilis, hepatitis B
virus and malaria: malignancies of colon, lungs etc ; Systemic Lupus erythematosus,
Rheumatoid arthritis etc
Histopathogenesis:
 Kidney is an “unfortunate innocent bystander” because it des not incite the reaction. It
is triggered by bacteria (streptococcal), viral (hepatitis B) parasitic (malaria), spirochetal
(treponema pallidum).
Characterized by Sub epithelial (lamina rara externa) deposits of antigen-antibody complexes
on the GBM (glomerular basement membrane).
 Leads to diffuse thickening of the GBM.
 Membrane complexes trigger the synthesis & liberation of proteases and oxygen
radicals by podocytes, mesangial cells and endothelial cells, which disrupts the integrity
of GBM leading to LEAKY BARRIER.-progresses to massive proteinuria –
hypoalbuminemia - anasarca.
Complications:
 Susceptibility to infection(due to loss of globulins in plasma)
 Chronic anemia
 Hypercoagulable state-leading to deep vein thrombosis, pulmonary embolism
(loss of anti thrombin III)
 End stage renal disease (ESRD)
Clinical case 2:
Diagnosis: Ig A NEPHROPATHY
Classical presentation with recurrent painless macroscopic hematuria. Follows viral illness by 24
– 48 hours, accompanied by flank pain secondary to renal capsular swelling. Gross hematuria
resolves in a few days.
Histopathogenesis:
Process is unclear. There is prominent deposition of Ig A and complement complexes in the
glomerular mesangial matrix following respiratory tract infection or GI infection.
 Leucocytic infiltration and cytokine release will stimulate mesangial proliferation and
hypercellularity.
 Complement activation leads to damage of glomerular filtering surface – hematuria
ADULT POLYCYSTIC KIDNEY
Commonest form of congenital cystic disease of kidney. Autosomal dominant.
Characterized by thinned out tubules. Glomeruli are normal. But eventually are affected.
77% suffer renal failure.
H I S T O L O G Y
L A B O R A T O R Y
Urinary System
OBJECTIVES:
Upon completion of study of this section the student will be able to:

Describe the structure of the kidney lobe and distinguish between cortical and
medullary organization.

Identify the regions of the nephron tubule and distinguish convoluted portions from
the loops of Henle.

Trace blood flow through the kidney and differentiate between vascular pathways
associated with cortical and juxtamedullary nephrons.

Describe the structural organization of the ureter and urinary bladder.
ANNOTATIONS

The urinary system is comprised of two kidneys, each linked to the bladder by a
ureter. Waste matter collected in the bladder is vented through the urethra. The
urinary system filters blood plasma and selectively eliminates soluble waste matter
while simultaneously balancing the body’s salt, water and pH requirements.

The kidney also serves as an endocrine gland, which releases renin and
erythropoietin, hormones that respectively affect blood pressure and red blood cell
formation.

The processing unit is the bean-shaped kidney. The kidney is ensheathed within a
connective tissue capsule. The tubular ureter and renal vessels enter the kidney
capsule at the concave hilum. The ureter enlarges within the kidney to form the
cavernous pelvis, which is subdivided by cup-shaped recesses called renal calyces.

The substance of the kidney between the calyces and outer capsule is subdivided
into two distinct zones, the medulla and cortex. The uriniferous tubules, which filter
the plasma and create urine, are found within the cortex and medulla.

Uriniferous tubules are composed of nephrons and collecting ducts. The nephrons
are derived from the metanephric intermediate mesoderm.

Collecting ducts are embryologically derived from the ureteric bud, an out growth of
the mesonephric duct. The observed zonation reflects the differential organization of
these tubules within the cortex and pyramidal medulla.

The apex of each medullary pyramid, the papilla, projects into a minor calyx, while
the base of the pyramid abuts the cortex. The collecting ducts, blood vessels and
limbs of the nephron within the medulla are arranged in highly organized vertical
linearity. Portions of the linear tracts radiate through the cortical region forming
medullary rays.

Surrounding the medullary rays in the cortex are randomly arranged glomeruli and
convoluted portions of the nephron. To further understand the differential
appearance of the kidney cortex and medulla, consider the following analogy: In the
medulla the uriniferous tubules resemble tightly clustered parallel bundles of
uncooked, dry spaghetti; those portions of the nephrons that extend into the cortex
resemble the jumbled organization observed on a plate of freshly cooked pasta (bon
appetit, Paul - please hold the clams, thanks, BL).

In today’s lab we will be examining randomly sectioned portions of a kidney. Since
the organ is somewhat spherical, it is important to observe the section at low
magnification and attempt to determine the orientation of the tissue.

Compare your section to that illustrated in the diagram below, can you orient yourself
and distinguish cortex from medulla (note: most sections reveal the cortex and do not
show much of the parallel arrays that characterize the medulla. In most sections,
masses of unilocular adipose are the only component of the medullary region that
can be identified). Please find other slides to examine if yours does not have enough
detail.
MICROSCOPIC STUDIES
There will be 3 slides used in this exercise:
1. Kidney
2. Ureter
3. Bladder
1. Kidney:

The capsule surrounding the cortex is composed of a thin layer of eosinophilic
dense irregular connective tissue. The subjacent cortex is characterized by
convoluted portions of the nephrons and the spherical masses, which represent the
vascular glomeruli.

The glomeruli are capillary tufts that indent within the vesicular cavity of Bowman’s
capsule, the blind-ended termination at the proximal end of the nephron. At the
vascular pole of the capsule, entering afferent arterioles supply the glomeruli and
exciting efferent arterioles drain the capillary network.

The epithelium of Bowman’s capsule is subdivided into two regions: the visceral
epithelium comprised of podocytes is that portion that envelops the surfaces of the
indented glomerular vessels.

Filtration-slits (observed in the electron microscope) between podocyte processes
allow the plasma filtrate of the capillaries to enter the urinary space within the
capsule.

Podocyte nuclei are larger, possess an elliptical-spherical shape and are more
euchromatic than the glomerular endothelial cell nuclei. Identify them.

The peripheral walls of Bowman’s capsule are lined by simple squamous epithelial
cells that comprise the parietal epithelium.

Surrounding the glomeruli are segments of the convoluted portions of the nephron.
There are two convoluted portions that are structurally distinct: the proximal
convoluted tubule (PCT) and the distal convoluted tubule (DCT). The PCT is
composed of a thick-walled simple cuboidal epithelium. The luminal surface is lined
by a microvillous border that usually appears foamy or indistinct when not fixed
wall (as in your slides!).
The eosinophilic cytoplasm of the PCT is irregular and not “boxlike,” consequently in
a section nuclei may not be observed for each cell that surrounds the lumen. In
addition, the lateral surfaces of the cells are interdigitated so that membrane
boundaries between adjacent cells are not visible.

The DCT tubules resemble the PCT except for one major difference, there is no
microvillar border on the luminal surface. Therefore the cells are thinner and exhibit
a smooth lining around the lumen. The nuclei are distributed in the same manner as
in the PCT, however, in these thinner cells they appear to project into the lumen.

Special regions of the DCT form intimate associations with the afferent arterioles that
supply the glomeruli. In the area of contact the DCT cells are tightly packed together
and their clustered nuclei comprise the macula densa. The macula densa is an
osmoreceptor region that reads sodium content of the DCT urine.

Scattered among the nephron tubules are portions of the collecting ducts. These
ducts are distinguished by their classic box-like cuboidal epithelium. The cells
possess a very pale cytoplasm and their nuclei are evenly distributed around the
lumen. The relatively straight cell margins between adjacent cells makes their
intercellular borders visible in the light microscope (borders are not observed
between PCT and DCT cells).

When observed in longitudinal section, the linear collecting ducts form the medullary
rays that arise in the medulla and extend through the cortex. the medullary rays
represent the centers of the renal lobules. All of the lobules whose collecting ducts
empty through the papilla of one renal pyramid constitute a renal lobe.

The collecting ducts penetrate the medullary region where they are surrounded by
parallel arrays of tubules, the loops of Henle that are derived from the nephrons.
Each loop is composed of a thick walled descending limb (continuous with the PCT), a
narrow-walled thin limb and an intermediate diametered ascending limb (becomes continuous
with the DCT).
 Question: How can you distinguish a PCT from the descending limb or a DCT from
an ascending limb?
 Answer: The PCT and DCT are convoluted tubules and do not express straight,
elongated segments. The descending and ascending limbs are only associated with
the medullary rays and consequently are straight and parallel to one another and
their associated the collecting ducts.

Vascular supply: The renal artery enters the kidney at the hilum and large
interlobar artery branches extend from the medulla and penetrate between
pyramidal lobes. At the cortex/medullary border, the interlobar arteries branch
forming arcuate arteries that run parallel to the cortical medullary boundary.

Narrower interlobular arteries extend from the arcuate arteries and penetrate the
cortex running between adjacent medullary rays. Intralobular vessels terminate as
arborized stellate vessels under the connective tissue capsule.

Afferent arterioles branch off of the intralobular vessels and supply the adjacent
glomeruli. Efferent arterioles exiting cortical glomeruli provide for a peritubular
capillary network that supplies the convoluted tubules and also serves in returning
recycled plasma elements to the system. Peritubular vessels drain into intralobular
veins and the venous return path parallels the arterial supply.

Nephrons with glomeruli close to the cortical-medullary boundary are structurally
different than cortical nephrons and are referred to as juxtamedullary nephrons.
These nephrons possess long loops of Henle that extend deep into the medulla and
provide for the counter current multiplier system.

Efferent arterioles of the juxtamedullary glomeruli run into the medullary as vasa
rectae (straight vessels) that parallel the medullary rays.

The vasa rectae provide the counter current exchange system and are structurally
indistinguishable from the thin limbs of the loops of Henle in most light microscopy
preparations.
Identify and check-off each of the following:
 Capsule
 Glomerulus
 Parietal epithelium
 Distal convoluted tubule
 Macula densa
 Medullary ray
 Descending limb
 Cortex
 Medulla  Bowman’s capsule
 Visceral (podocyte) epithelium
 Proximal convoluted tubule
 Collecting Duct
 Afferent and efferent arterioles
 Ascending limb
Juxtamedullary nephrons
2. Ureter:

The ureters conduct urine from, the kidneys to the bladder. Urine is conducted to the
bladder by peristaltic waves created by the smooth muscle in the wall of the ureter.
The muscle bundles, which are helically wound around the lumen, are disposed in
two layers.

The inner is generally longitudinally and the outer muscles are more circularly
arranged. Near the junction with the bladder, there may be an additional (third)
smooth muscle layer, which contains longitudinally aligned smooth muscle bundles.

The lumen is lined by a transitional epithelium, which possesses a specialized apical
layer of facet (dome) cells. The epithelium is characteristically thrown up into stellate
folds which unfolds as urine is passed down the tube.

In your preparations the epithelial cells are not preserved well and the epithelium is
frequently disrupted.

Between the transitional epithelium and muscle layers is the thin layer of loose
connective tissue (lamina propria). Can you readily distinguish the smooth muscle
from the connective tissue?

(Hint: the CT has lots of ground substance, the muscle cells are densely packed and
reveal little matrix and ground substance). Surrounding the muscle layers is the
vascular adventitial connective tissue.
Identify and check-off each of the following:
 Transitional epithelium
 Lamina propria
 Longitudinal smooth muscle  Circular smooth muscle  Adventitia
3. Bladder:

The wall of the bladder resembles that of the lower portion of the ureter. It consists of
a transitional epithelium-lined mucosa. The lamina propria contains a rich bed of
vessels beneath the epithelium. Examine this layer and distinguish arterial from
venous vessels.

The muscularis contains three layers of smooth muscle: a longitudinal layer closest
to the lumen, followed by a circular layer, and the outermost layer (which may not be
too developed in your sections) is composed of another longitudinal layer.
Identify and check-off each of the following:
 Transitional epithelium  Lamina propria  Inner longitudinal smooth muscle
 Middle circular smooth muscle
 Outer longitudinal smooth
muscle