1515thth The The UrinaryUrinary SystemSystem

Department of Histology Department of Histology and Embryologyand Embryology

1. Introduction2. Kidney

2.1 General organization※ 2.2 Nephron※ 2.3 Filtration apparatus of the kidney※ 2.4 Juxtaglomerular apparatus

3.Excretory passages

: components and functions

2.5 Kidney tubule function※ 2.5.1 Proximal convoluted tubules※ 2.5.2 The loop of Henle 2.5.3 Distal convoluted tubules 2.5.4 Collecting tubules and collecting ducts 2.6 Blood supply

Introduction

Components Functions

2.Synthesis and secretion the renin and erythropoietin

Production of the urine

3.Hydroxylation of 25-OH vitaminD3 to hormonally active 1,25(OH)2vitamin D3 to regulate the calcium balance.

1. The kidneys conserve body fluid andelectrolytes and remove metablic wastes

Kidney

Renal medulla

Renal cortexMedullary rays

Cortex labyrinth

Renal column

Renal (medullary) pyramids

Renal papilla

minor calyces

major calycespelvis

ureter

Capsule

parenchyma

lobe

lobule

Uriniferous tubule

Lobe: one renal pyramid and the cortical tissue at its base and sides

Lobule : a single medullary ray and the cortal tissue surrounding it

Uriniferous tubule:it consists of nephron and it collecting tubule

1. Fuctions

The 1st key point -Nephron

1) Filtration of most small molecules from blood plasma to form an ultrafiltrate of plasma.

2) Selective reabsorption of most of the water and some other moleculesfrom the ultrafiltrate,leaving behind excess and waste materials to be excreted

3) Secretion of some excretory products directly from blood into the urine.

4) Maintenance of the acid-base balance by selective secretion of H+ ionsinto the urine.

2.2 Renal tubules

Convoluted

Straight

Straight

Convoluted

Thin segment

Distal thick segment

Proximal thick segment

Loop of Henle

2. Structure

2.1 Renal corpuscle

The tubules of the loop of the Henle acompanied by a capillarynetwork.these vessels representthe vascular part of the countercurrent exchange systemthat regulates the concentration of the urine.

It forms the entire U-shaped portion of nephron.

1.1 Renal corpuscle

Glomerulus Bowman’s capsule

intraglomerular mesangium

(1) Glomerulus

Capillaries tuft

(2) Bowman’s capsule

Glomerulus

Afferent arteriole

Efferent arteriole

Urinary poleVascularpole

Parietal layer

Visceral layer

The basment memberane of the tubules and that of bowman’s capsule stains blue in Azan staining method

The 2nd key point - Filtration apparatus

Endothelium of the glomerular capillaries

Glomerular basement membrane,GBMVisceral layer of Bowmans’s capsule-- Podocyte

(Function of the nephron)

Primary process

Secondary process Fenestrated endothliun

GBMFiltration slit membrane

Diagram of podocyte

Scanning electron micrograph of Podocyte

Cell body

Primary process

Secondary process

Endothelium of the glomerular capillaries

1.It possesses numerous fenestrations.

1)These fenestrations are larger,more numerous,and more irregularin outline than fenestrations in other capillaries.

2)The diaphragm aren’t spaned the fenestrations in capillaries.

2.Endothelial cells possess a large number of water channels that allow the fast movment of water through the epithelium.

Renal filtration barrier

The fenestrated endothelium of the glomerular capillaries

The basal laminaThe filtration slits

Glomerular basement membrane,GBM

1. It is a thick(300-350nm) basal lamina that is joint product of the endothelium and the podocyte, PAS(+).

2. It acts as a physical barrier and an ion-selective filter.

Three portions of GBM:

The lamina rara externa

The lamina densa

The lamina rara interna

(electron-dense layer)

(more electron-lucent layer)

Lamina rara externa

Lamina rara interna

Lamina densa

location

Adjacent to the podocyte

Adjacent to the capillary endothelium

Sandwiched between the laminea rarae

components

Rich in polyanions

Similar to rara externa

Type IV collagen is organized into a network

function Impede the passage of negatively charged molecules

It act as a physical filter

The glomerular filtrate has chemical composition similar to that of blood plasma but contains almost no protein.

The glomerular basement membrane is a selective macromolecularfilter ,in which the lamina densa act as a pysical filter,whereas the anionic sites in the laminae rarae act as a charge barrier.

1) Particals greater than 10nm in diameter do not readily cross the basal lamina.

2) Neatively charged proteins with a molecular mass greater than

that of albumin(69kDa) pass across only sparingly.

3. Type of nephron

1. Superfacial nephron

(cortical nephron)

3. Juxtamedullary nephron

2. Midcortical nephron

Criterion :location of the renal corpuscles in the cortex

Clinical Considerations

Diabetes mellitus and Glomerulonephritis

The glomerular filter is damaged and markedly its filtering ability is reduced , it becomes much more permeable to proteins, with the subsquent release of protein into the urine.

Mesangium

mesangial matrix

The function of the mesangial cell 1 Phaocytosis to clean the GBM

2 Structure support to provide support for the podocytes in the areas where the epithelial basement memberane is absent or incomplete. 3 Secretion interleukin-1 and platelet-derived growth factor (PDGF),which play a central role in response to to glomerular injury.

Structure

Juxtaglomerular cells

The 3rd key point -Juxtaglomerular (JG) apparatus

Macula densa cells

Extraglomerular mesangial cells

2. Components

1. Location:

Lying directly adjacent to the afferent and efferent Arterioles at the vascular pole of the renal corpusle

They are senstive to the ionic content and water volum of the tubular fluid,producing molecular signals that promote the liberation of the ezyme renin in the circulation .They have a cytoplasm full of secretory granules. Secretion of JG

Cells played a role in the maintance of blood pressure.

3. Function:

Contain secretory granules of renin

Chemoreceptorperceiving the change of Na +

in distal tubule lumen

Renin-angiotension-aldosterone-system

2.Structure and Function

Chemical composition difference between primary urine(ultrafiltrate) and final urine

The two kidneys produced 125ml of filtrate per minute,but just only 1ml is released into the ureters as urine.

The glomerular filtrate has chemical composition similar to that of blood plasma but contains almost no protein.

The final urine contains water, electrolytes as well as waste products, such as urea, uric acid, and creatinine,and breakdown products of various substances.

Where did it go to? Where are they from?

Kidney tubule function

The 1st key point-Proximal convoluted tubule

Function : It is the initial and major site of reabsorption

Structure :The cuboidal cells of it have the elaberate surface specializations associated with cells Engaged in absorption and fluid transport.

1. A brush border 2. A juctional complex3. Folds located on the lateral surface of the cells 4. Interdigitation of basal process of adjacent cells

5. Basal striations

The 1st key point-Proximal convoluted tubule

1.A brush border

It greatly increases the free cell surface area, correlating with itsabsorptive capacity.

1)At the electron microscopiclevel , the microvilli constitutethe brush border apically.

2)The PAS staining method has been used to demonstrate the prominent brush border.

The 1st key point-Proximal convoluted tubule

The cytoplasm immediately beneath the brush border contains many pinocytotic veicles V and lysoomes L which are involved in reabosorption and degradation of small amounts of protein that have leaked through the glomerular filter. reaborbed solutes are transported into surrounding cap with attenuated endothelium E resting on a very thin basement memberane.

2. Folds located on the lateral surface of the cells

They creates interdigitating cytoplasmic process of adjoining cells.

no discrete limites can be observed (in the LM) between cells of the proximal tubule .

They increase the lateral surface area of the cell and are particularlyprominent in epithelia that are engaged in fluid and eletrolyte ransport

In active fluid transport ,sodium ions are pumpedout the cytoplasm memberane by Na+/K+ATPase located in the memberane.

Na+-K+-ATPase

Location :It is a kind of transmembrane proteins that are localizedIn the lateral folds of the plasma membrane.

Function : The active transport of Na+ out of the cell is acompanied by facilitated transport into the cells of Cl- , glucose and amino acids by means of transport proteins .almost 100% of filrated glucose and amino acids is reabsorbed by the PCT 。

Clinical thinking: When the amount of glucose in the filtrate exceeds the absorbing capacity of the proximal tubule, urinebecome more abundant and caintain glucose.

3. Iterdigitation of basal process

Some of the interdigitating process extend the full height of the cell.the process are long in the basal region and create an elaberate extracelluar compartment adjacentto the basal lamina.

They increase the basal surface area of the cell and are particularlyprominent in epithelia that are engaged in fluid and eletrolyte ransport.

The cytoplasm of PCT epithelial cells stain intensely due to a high content of organelles, principally mitochondria.

4. Basal striations

Mitochondria M are present in the cell within the interdigitating process P. M are responsible for the appearance of the basalStriations in LM.

5. A juctional complex

It consists of a narrow ,tight junction that seals off the intercelluar space from the lumen of the tubule and a zoula adherens that maintains the adhension between neighboring cells .

The tight junction at the apical end of the intercelluar space prevents fluid from moving in the opposite direction.

The 2nd key point-Loop of henle

Components:

The pars recta of the proximal tubule

The thin descending limbThe thin ascending limb

The thick ascending limb(the distal staight tubule)

Function : It is to produce an increasing osmotic gradient from the cortex

to the lip of renal papilla by the counter-current multiplier mechanism.

1. The pars recta of the proximal tubule

The cells of it are not as specialized for absorption as are those of the proximal convouluted tubule.

1) They are shorter, with a less well developed brush border and with fewer and less complex lateral and basal-lateral processes.2) The mitochondria are smaller than those of the cells of the convoluted segment and are randomly distributed in the cytoplasm.3) There are fewer apical invaginations and endocytotic vesicles, as well as fewer lysosomes.

The 2nd key point-Loop of henle

Structure :

2. The thin limb, T

The thin limbs T are lined by a flattened squamous epithelium Which has no capicity for active transport .The thin desending limb allows free diffusion of H2O

but impermeable to NaCl .

The thin asending limb has also active function but allows passive diffussion of NaCl into the interstitium, not H2O.

The vasa recta take up water from the medullary interstitium and return it to the general circulation.

2. The thin limb, T

The interstitial fluid in the medulla is hyperosmotic

Water diffuses out of ,salt diffuses into the nephron at this site

Passive momment of water into the connective tissue,and of salt and urea into the thin desending limb.

3.The thick ascending ,A

Active transport of NaCl again occurs hereThe appearance of the cuboidal epithelium

1) Basolateral process interdigitate with each other forming an extensive intercellular space in a similar maner to the PCT.

2) The active transport process is fuelled by ATP produced by many mitochondria found in these process .

3) It is also impermeable to water which maybe related to its thick glycocalyx composed of the glycoprotein, tamm-horsfall protein.

The 3rd key point-

The distal convouluted tubule DCT

DCT maybe differentiated from PCT by

1) the absence of brush border (PAS-positive)2) A larger more clearly defined lumen 3) more Nuclei per cross section (since DCT cells Are smaller than PCT cells)4) paler cytoplasm5) Sections of DCT are much less numeroursthan sections of PCT since the DCT is a much more shorter segment of the renaltubule than the PCT

Light microscope:

The main features of Electron micrograph

1. Lateral cell interdigitations and large numbers of mitochondria.

2.The basal plasma memberanecontains the Na+-K+ATPase.

3.It have only a few irregular Microvilli at the luminal surface

4.The overlaying cytoplam containsLarge number of tiny vesiles.

Function of DCT

The distal convoluted tubule exchanges Na+for K+into the ultrafiltrate to conserve Na+ under aldosterone regulation.

1. Reabsorption of Na+and secret K+into the the ultrafiltrate to conserve Na+

2. Reabsorption of bicarbonate ion

3.Conversation of ammonia to ammonium ion.

collecting tubule

They are readily ditinguished by Virtue of the cell boundaries that can be seen in the LM

The simple columnar epithelium consist two types of cells:

1) Light cells are princinple cells of the system.

2) Intercated cell : occur in considerably smaller numbers.

1. Structure:

2.Function 1) They concentrate urine by paasive reabsorption of H2O Into the medullary interstium following te osmotic gradient Creeated by the counter-current multiplier system of the Loop of henle.

2) The amount of H2O reabsorbed is controled by antidiuretic hormone(ADH) secreted by the posterior pituitary in response to dehydration.3) They can secrete H+.

Medical application

Aldosterone deficiency in adrenalelectromized animalsand in humans with Addison diseaseresult in an excessive loss of sodium in the urine.

5. Renal blood circulation

Renal artery

Interlobar arteries

Interlobular arteries

Arcuate arteries

Afferent arterioles

Efferent arterioles

Interlobular veins

Renal veins

Interlobar veins

Arcuate veins

Capsule capillary network Stellate veins

Glomeruluscapillary network

Section from kidney which has been perfused with a red dye in order to demonstrate the renal blood supply.Nephrons remain unstained.

1.An interlobular artery IA can be seen branching to form the afferentAterioles AA of a glomerulus G.

2.The efferent aterioles EA leavingG is of much smaller diameterthan the AA, to maintain pressure withinG cap for plasma to be filtered into Bowman’s space BS. Blood pressure

whithin G is controlled by variation of the diamter of the AA and EA

3.In the superficial and midcortex as shown here , EA give rise toa network of cap which surround the renal tubules RT.Towards the medula EA give rise to the vasa recta.moleculesreabsorbed from G filrtrate returned tothe general circulation via this cap net which drain into the renal venous system.

Bladder and urinary passages

Structure of the wall :

They have same basic histologic Structure:

1. Mucosa :Transitional epithelium

Lamina propria of loose –to –denseconnective tissue

2.The muscular layer

3.Adventia

Review Test

1. A nephron includes all of the following components EXCEPT

(A) a renal corpuscle (B) a distal convoluted tubule (C) a thin limb of the loop of Henle (D) a collecting tubule (E) pars recta of the proximal tubule

2. Which cells form the visceral layer of Bowman’s capsule

(A) Medullary interstitial cells (B) Mesangial cells (C) Podocytes (D) Cells lining the proximal convoluted tubule (E) Juxtaglomerular cells

3. Renal filtration barrier consists of all of the following components EXCEPT

(A) The fenestrated endothelium of the glomerular capillaries (B) The basal lamina (C) The filtration slits with diaphragms between pedicels (D) Medullary rays

Summary

The paired kidneys and ureters and the unpaired bladder and urethra.

1.The components of the urinary system

2. The principal function of the urinary system

the maintenance of water and electrolyte homeostasis, which requires that any input into the system is balanced by an equivalent output.

It permits passage of water, ions, and small molecules from the blood stream into the capsular space but prevents passage of large and/or most negatively charged proteins, thus forming an ultrafiltrate of blood plasma in Bowman’s space.

The structure of theRenal filtration barrier

It is composed of the fenestrated endothelium of the glomerular capillaries, the basal lamina, and the filtration slits with diaphragms between pedicels.

The function of theRenal filtration barrier

homework

Give me a summary on what we talked about the structure of the renal tubule associated with its function in the class.

Have a good time !