Histology of urinary system

HISTOLOGY OF URINARY SYSTEM

Dr.GURUDASAN01.04.13

INTRODUCTION The urinary system

consists of Kidneys Ureters Urinary bladder Urethra

MAIN FUNCTIONS OF URINARY SYSTEM Kidneys filter

ToxinsMetabolic wastesExcess waterExcess ions

Disposes nitrogenous wastes from blood UreaUric acidCreatinine

Regulate the balance of water and electrolytes, acids and bases.

MACROSCOPIC FEATURES

Kidney has two

regions

Cortex: It lies just

beneath the

connective tissue

capsule.

The cortex extends at

the margins between

each pyramid as Renal

columns.

Some of the striated

patterns from the base of

the pyramid may extend

into the cortex, which are

called as Medullary rays.

KIDNEY LOBE:

It is defined as a renal

pyramid with its overlying

cortex and laterally

associated renal columns.

LOBULE OF KIDNEY – IT IS DEFINED AS MEDULLARY RAY WITH ITS LATERALLY ASSOCIATED CORTICAL TISSUE.

GENERAL STRUCTURE OF NEPHRON It is the functional unit of

kidney. Parts: Renal corpuscle Proximal convoluted

tubule Loop of Henle Distal convoluted tubule Collecting duct

Renal corpuscle:- The blind end of each nephron is expanded in the cortical region into a double walled cup called Bowman’s capsule.

- It consists of outer parietal layer (Simple squamous)

- Inner visceral layer(specialised epithelial cells – podocytes)

- Bowman’s space – space between parietal and visceral layer

Glomerulus is the tuft of capillaries fed by afferent arteriole and drained by efferent arteriole (vascular pole).

Afferent arteriole – Large

Efferent arteriole – small Varying diameters of

arterioles facilitate active filtration due to pressure gradient.

Glomerulus has fenestrated endothelium.

Mesangial cells – specialised supporting connective tissue cells in capillary network. These are satellite cells with contractil eand phagocytic property.

Their phagocytic property helps to remove large proteins and filtration residues.

The glomerular capillaries are closely invested by podocytes.

PODOCYTES: These are cells of visceral

layer of renal corpuscles that envelope glomerular capillaries.

They have long cytoplasmic processes called primary processes, which in turn gives rise to short secondary foot processes or pedicles.

The secondary processes interdigitate with adjacent podocytes and form elongated gaps called filtration slits/pores.

JUXTAGLOMERULAR APPARATUS JUXTAGLOMERULAR CELLS:

- Modified smooth muscle cells present in Tunica media of afferent arteriole.

- Sensitive to pressure of blood in the afferent arteriole.

- Secretes renin

MACULA DENSA:

- Specialised region in the wall of DCT which comes in contact with JG cells.

- Cells are taller and their nuclei are denser and close to one another.

- Sensitive to concentration of sodium ions in the fluid present in DCT.

LACIS (NETWORK) CELLS:

Extra glomerular mesangial cells found at the vascular pole of the renal corpuscle.

- May be involved in production of erythropoietin.

COMPONENTS OF GLOMERULAR FILTER During filtration, the glomerular

filtrate passes through three layers.

I) Fenestrated endothelium of glomerular capillary – Acts as coarse filter preventing cellular elemets and allowing only plasma.

II) Glomerular basement membrane – It is a selective macromolecular filter preventing passage of particles greater than 10 nm in diameter.

III) Filtration slitsThis makes glomerular filtrate similar to plasma wihout plasma proteins(macromolecule).

GFR – 125 ml/min. 124 ml is reabsorbed in renal tubules, 1 ml is released as urine

PROXIMAL CONVOLUTED TUBULE Confined to renal cortex Cuboidal epithelial cells

with long microvilli (fuzzy appearance)

Resorption of water, ions and solutes

LOOP OF HENLE Arises from PCT in cortex,

dips down into medulla as descending limb and loops back as ascending limb and continuous with DCT at the cortico-medullary junction.

Thin segment – Simple squamous epithelium (permeable to water and sodium)

Thick ascending limb cuboidal epithelium ( impermeable to water)

DISTAL CONVOLUTED TUBULE Confined to the renal

cortex Simple cuboidal epithelium Selective secretion and

resorption of ions – This is coupled with secretion of Hydrogen and potassium ions ( one hydrogen or potassium ion is secreted for every sodium ion reabsorbed) – this is controlled by aldosterone.

DCT is involved in maintenance of acid-base balance.

DIFFERENCES BETWEEN PCT AND DCT

DIAMETER: It is more (about 45 – 60µm)

Lining cells are pyramidal and broad

Has brush border Lumen of tubule is irregular

and star shaped Cytoplasm is darkly stained

with Eosin and cell outlines are not clear.

Nuclei are large, spherical and basally or centrally placed.

DIAMETER: It is less ( About 25 – 50 µm)

Lining cells are cuboidal

Has no brush border Lumen of tubule is large

and regular Cytoplasm is lightly

stained with Eosin and cell outlines are clear.

Nuclei are small, spherical and located more in apex.

PROXIMAL CONVOLUTED TUBULE DISTAL CONVOLUTED TUBULE

PROXIMAL CONVOLUTED TUBULEDISTAL

CONVOLUTED TUBULE

COLLECTING DUCT It begins in the medullary ray

as the continuation of DCT. As it enters medulla it is

joined by several other collecting tubules to form larger ducts (Ducts of Belini).

These are lined by simple cuboidal epithelium with distinct cell boundaries and clear pale cytoplasm.

Normally not permeable to water, but in the presence of ADH it becomes permeable

THE URETERS Slender tubes about 25

cm (10 “) long leaving each renal pelvis

One for each kidney carrying urine to the bladder

Descend retroperitonealy and cross pelvic brim

Enter posterolateral corners of bladder

Run medially within posterior bladder wall before opening into interior

This oblique entry helps prevent backflow of urine

Three basic layers Mucosa:Transitional

epithelium of mucosa stretches when ureters fill

Muscle layer: Inner longitudinal, outer

circular layers Inferior 3rd with extra

longitudinal layer)Stimulated to contract

when urine in ureter: peristaltic waves to propel urine to bladder

Adventitia (external)

– Loose conn. Tissue with blood vessels, lymphatics and nerves

Collapsible muscular sac Stores and expels urine Lies on pelvic floor

posterior to pubic symphysis Males: anterior to rectum Females: just anterior to

the vagina and uterus

URINARY BLADDER

Bladder wall has three layers (same as ureters)Mucosa with

distensible transitional epithelium and lamnia propria (can stretch)

Thick muscularis called the detrusor muscle 3 layers of highly

intermingled smooth muscle

Squeezes urine outFibrous adventitia

CLINICAL ASPECTS

Glomerulonephritis:In DM, degenerative changes in glomeruli leads to thickening of glomerular basement membrane and damage to the podocytes and alteration of slit pore membrane. As a result glomerular filter becomes more permeable to proteins and subsequent release of proteins in urine (proteinuria)

RENAL CALCULI:- Calcium salts and uric acid

are excreted in glomerular filtrate. These salts are less soluble in water and later crystallize to form stones

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