The Microscopic

Structure of the Kidney

Dr. Mahmud Abuauba MBChB, DCH, MD, PhD

Consultant Pediatrician & pediatric Nephrologist

Assistant Professor of Pediatrician

Zawia Kidney Center

5th

May 2018

الزاوية-ركز عالج أمراض الكلىم

برعاية

THE URINARY SYSTEM

Paired kidneys (2)

Paired ureters (2)

Unpaired urinary bladder (1)

Unpaired urethra (1)

SIX ORGANS

THE URINARY SYSTEM

THE KIDNEYS

CORTEX & MEDULLA

Coronal or long. section: two

regions:

Outer, darker, granular region

⚫ Renal cortex

⚫ Cortical labyrinth Cortical

⚫ Medullary rays arch

Inner lighter, striated region

⚫ Renal medulla ~10-18 medullary

pyramids

RENAL CORTEX

Renal lobule= medullary ray +neighbuoring cortical labyrinth,with medullary ray in the center oflobule bounded by interlobular As

Renal lobe= renal pyramid + renalcolumn + cortical arch

Renal lobe is a segment drains

through the renal papilla

RENAL MEDULLA

Composed of 10-18 conical-shapedmedullary tissue; renal or medullarypyramids

Base toward the CM border

Apex points toward the hilum, renalpapillae, project into minor calyx

Renal papillae perforated by 10-25opening of ducts of Bellini formingarea cribosa

Renal pyramids separated by cortical-like tissue (renal column of Bertin)

RENAL MEDULLA

Contains

Straight portion of PT & DT

Henle’s loop, ALH & DLH

Vasa recta

MCT

Papillary or ducts of Bellini

RENAL CORTEX

Cortical arch is portion of cortexoverlying renal pyramid

Formed of cortical labyrinth &medullary rays

Cortical labyrinth consists ofrenal corpuscle & convolutedtubules

Medullary ray, straight tubules ofseveral nephrons with single ormore collecting tubules

RENAL CORTEX

Contains

Glomeruli

PCT, DCT

Connecting tubules

CCT

URINIFEROUS TUBULE

Uriniferous tubule is the basic

functional unit of the kidney

Uriniferous tubule formed of

nephron & collecting tubule

UT entirely is epithelial in nature

UT are densely packed so the CT

of kidney (stroma) is scant

Uriniferous tubule

NEPHRON I

Nephron = renal corpuscle + renal

tubules

Each kidney has approx. a million

(0.7-1.3 million) nephrons

NEPHRON II

Several nephrons (10) are drainedby a single CCT

Several CCT join to form largertubule, MCT

Several MCT join to form ducts ofBellini, perforate & open at papilla

There are 250 collecting ducts

Each collects urine from 4000-5000 nephrons

NEPHRON III

Cclassifications

All nephrons are in the cortex

Cortical nephrons (85%); haveshort Henle’s loop, to outermedulla with short thin des HL &no thin Asc HL

⚫ Superficial nephrons

⚫ Mid-cortical nephrons

NEPHRON III

Juxtamedullary nephrons (15%=1/7);have very long Henle’s loop,sometimes descend to the tip ofpapilla

Have short thick des HL, long thinDec HL & thin Asc HL & thick Asc HL

To establish medullary hypertonicityfor concentrating urine

S1

S2S3

Cortical nephron

JM nephron

NEPHRON IV

Each nephron consists of:

⚫ Renal corpuscle

⚫ Renal tubule

RENAL CORPUSCLE

Each renal corpusclecconsists of:

⚫ Glomerulus, a tuft ofcapillaries

⚫ Bowman’s capsule, a double-walled epithelial capsule

Has two poles: vascular &urinary pole

Renal Corpusle

GLOMERULUS I

About 200-250 m

A tuft of capillaries

Invaginated into Bowman’s

capsule

Formed from AGA

AGAs are short, stright, enter

renal corpuscle at vascular pole

Filtrating surface area is 0.8 m2

GLOMERULUS II

Each AGA divides into 3-5branches; into a tuft of capillaries(20-40 loops) → glomerulus

Fenestrated endothelial cells (70-90 nm); no diaphragm, act as aphysical barrier

Capillaries coalesce again to formefferent glomerular arteriole

EGA is longer & narrower

Bowman’s capsule

Consists of two layers:

⚫ Visceral layer; internal; modified

epithelial cells called podocytes

⚫ Parietal layer; external, single layer

of squamous epith supported by

basal lamina & reticular fibers

In between, Bowman’s or urinary

space, receives ultrafiltrate

Neck of the PT at urinary pole

PODOCYTES

Modified epithelial cells of

visceral layer

Have cell body & primary

processes (arise from body) both

not in contact with the capillary

Secondary processes or pedicels

arise from the primary processes

Pedicels in intimate contact with

LRE

FILTRATION SLIT

Pedicels interdigitate forming

filtration slit≈ 25 nm (20-40) &

covered by slit diaphragm≈ 6 nm

thick

SD has circular pores of 3-5 nm

Acts as a physical barrier

Pedicels covered by glycocalyx

Glycocalyx contains sialoprotein

(podocalyxin)

Filtration Slit

Proteinuria

GLOMERULAR BASEMENT

MEMBRANE

Is produced by fusion of basal lamina

of endothelial cells & podocytes

As a physical & charge barrier

Consists of three layers:⚫ Lamina rara interna

⚫ Lamina densa; collagen IV

⚫ Lamina rara externa

Laminin, fibrenectin, heparan

sulphate

FILTRATION BARRIER

Is a barrier between blood in

capillaries & Bowman’s space

Consists of three layers:

⚫ Fenestrated endothelial cells

⚫ GBM

⚫ Filtration slits of podocytes

⚫ Glycocalyx

FILTRATION BARRIER

Lamina densa is a meshwork of

type IV collagen & laminin in a

matrix containing –vely charged

proteoglycan (heparan sulfate)

acts as a charge & physical

barriers

Laminae rarae contain fibronectin

that binds them to cells

FILTRATION BARRIER

Particles >10 nm in diameter don’t

cross GBM

While –vely charged proteins with

MWt > 69 kDa pass sparingly

The largest protein succeeded to

cross GBM has MWt 70kDa,

therefore small amounts of

albumin appear in urine

MESANGIUM

Located between glomerular

capillaries & consists of:

⚫ Mesangial matrix & mesangial cells

Mesangial cells:

⚫ Intraglomerular mesangial cells

⚫ Extraglomerular mesangial cells

Supporting, phagocytosis,

contraction, resorption

Mesangial cells

They are contractile & have Ang II

receptors that when activated the

glomerular blood flow is reduced

Also contain ANF receptors that

relax mesangial cells & ↑ RBF

Therefore, they regulate the RBF

& glomerular filtration

RENAL TUBULE

The second part

of the nephron

RENAL TUBULE

Is the part of uriniferous tubule

orginates at the urinary pole of

the renal corpuscle

Divided into 12 segments

Proximal tubule

⚫ S1, 1st 2/3 of para convoluta

⚫ S2, last 1/3 of para convoluta & much of para recta

⚫ S3, remainder of para recta

Loop of Henle

⚫ Thin descending limb

⚫ Thin ascending limb

Distal tubule

⚫ Medullary thick ascending limb

⚫ Cortical thick ascending limb

⚫ Distal convoluted tubule

Connecting tubule

Collecting tubule or duct

⚫ Cortical collecting tubule

⚫ Outer medullary collecting tubule

⚫ Inner medullary collecting tubule

S1

S2S3

Cortical nephron

JM nephron

S1

S2

S3

PROXIMAL TUBULE I

First part of renal tubule

Drains the urinary space

⚫ Pars convoluta = PCT

⚫ Pars recta = straight PT = DTkLH

Divides into three segments:

⚫ S1;1st 2/3 of pars convoluta

⚫ S2; last 1/3 of PC & much of PR

⚫ S3; remainder of pars recta

PROXIMAL TUBULE II

Confined to renal cortex

Is longer than DCT therefore more

numerous on histological cross section

It is surrounded by peritubular

capillaries

Composed of single layer of cuboidal

epithelial cells

PROXIMAL TUBULE III

Have brush border of microvilli & apical

canaliculi in between

These canaliculi help in formation of

pinocytosis to absorb macromolecules

as proteins

PROXIMAL TUBULE IV

Rich in mitochondria and it is highly

metabolically active,mainly S1

Because basolateral membranes have

abundant Na-K pump which

responsible for active reabsorption of

Na ions back into peritubular capillaries

HENLE’S LOOP

A U-shaped structure starts as

continuation of PR & ends as ATkLHL

Consists of three (or 5) parts:

⚫ Descending thick LHL

⚫ Descending thin LHL

⚫ Loop or turn of Henle

⚫ Ascending thin LHL

⚫ Ascending thick LHL

composed of single squamous

HENLE’S LOOP

HENLE’S LOOP

1/7 are JM nephrons

Consists of four types of cells:

l Type I only in midcortical &

superficial nephrons

l Type II,III,IV in JM nephrons

JM nephrons have long LHL

Cortical nephrons have short LHL

DISTAL TUBULE

Starts as continuation of ATnLHL

Subdivided into three parts:

⚫ Pars recta = ATkLHL (part of HL)

⚫ Macula densa

⚫ Pars convoluta = DCT

DT has single cuboidal epithelial

cells, no brush border, no canaliculi

ATkLHL is highly metabolic

DISTAL TUBULE

Macula densa

⚫ At junction of ATkLHL & DCT

⚫ It is part of juxtaglomerular

apparatus

DCT

⚫ From MD to connecting tubule

⚫ Cudoidal cells, No brush border

⚫ Shorter than PCT, ratio 1:7

Macula densa

Is a part of DT at junction of

ATkLHL & DCT

⚫ Also part of juxtaglomerular

apparatus

Consists of modified columnar

cells with closely packed nuclei

Sensitive to ionic content & water

of tubular fluid

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CONNECTING TUBULE

It is a short segment of renaltubules that connects DCT tocortical collecting tubules

DCT of several nephrons join toform short connecting tubule

Several connecting tubules join toform collecting tubule

S1

S2S3

Cortical nephron

JM nephron

S1

S2

S3

COLLECTING TUBULE

Consists of three parts:

⚫ CCT

⚫ IMCT

⚫ OMCT (papillary duct, Bellini ducts)

It is not part of the nephronembryologicaly different

The epithelium of CT is sensitiveto vasopressin or ADH to ↑ waterreabsorption

Development of the renal pelvis, calyces, collecting

tubules

A: at 6 weeks, B: at the end of the 6th

wk, C: 7 weeks

& D: newborn

COLLECTING TUBULE

CCT consists of two types of cells:⚫ Principal cells (P cells), Na reabsorption

& water reabsorption 2ndary to ADH

⚫ Intercalated cells (I cells); AB balance

MCT formed by union of severalCCT; has same cells as CCT

Papillary CT or ducts of Belliniformed by union of several MCT &open at area cribosa; has onlyprincipal cells

Prof. Dr. Peter AGRE

Nobel Laureate

Nobel Prize in

Chemistry 2003

RENAL INTERSTITIUM

It is the space between uriniferous

tubules & blood & lymphatic

vessels

Composed of:

⚫ scanty loose CT; collagen & elastic

fibers in both cortex & medulla

⚫ Three types of cells; fibroblast;

macrophages & interstitial cells

RENAL INTERSTITIUM

Interstitial cells have numerous

lipid droplets & synthesize &

secrete medullipin I

Medullipin I is converted (liver) to

medullipin II; a potent vasodialtor

which lowers BP

Also secrete PGs (E2), prostcyclin

JUXTAGLOMERULAR

APPARATUS

Is a combination of specialized

tubular & vascular cells located at

vascular pole of glomerulus

Consists of:

⚫ Juxtaglomerular cells of the AGA

⚫ Macula densa of the distal tubule

⚫ Extraglomerular mesangial or Lacis

cells

Juxtaglomerular apparatus

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Juxtaglomerular apparatus

JUXTAGLOMERULAR CELLS

Specialized, modified (VSMC)

myoepithelaial cells; mainly

tunica media of AGA as it enters

glomeruls, occasionally EGA

Contain secretory granules; renin

Renin is a proteolytic enzyme that

converts angoitensinogen into

angiotensin I to regulate BP

JUXTAGLOMERULAR CELLS

JGCs receive sympathetic nerve

via 1 adrenergic receptors

Acts as a baroreceptor (BP)

Sensitive to change of BP

So regulate arterial blood

pressure

MACULA DENSA

Is a speciliazed short segment of

DT, as ATkLHL passes near its

own corpuscle

Located at vascular pole between

AGA & EGA

Composed of modified epithelial

cells that are tall, narrow & have

dense closely packed nuclei

Appears as dark spot by LM

MACULA DENSA

They are not innervated

Act as chemoreceptors (Na&Cl)

Sensitive to Na & Cl load in DT

The JG cells & MD cells are in

intimate contact, because no

basal lamina between them

LACIS CELLS

Extraglomerular mesangial cells

Polkissen or polar cushions

Are one type of mesangial cells

forming interstitium of JGA

Located between AGA, EGA & MD

Are in intimate contact with

intramesangial cells

Function is unknown (renin)

Bladder & Urinary passages

Conduct & store urine till excreted to

exterior

Mucosa of calyces, renal pelvis,

ureters, UB consists of transitional

epit with basal lamina

Muscle fibers of UB run in every

direction until reach bladder neck

then arranged into 3 layers

Adventia covers them externally

Bladder & Urinary passages

Internal long layer becomes circular

around prostatic urethra, true

involuntary urethral sphincter

It extends to external meatus in ♀

Middle layer ends at bladder neck

Outer layer ends at prostate in ♂ & at

external meatus ♀

THANKS FOR

ATTENTION