Lecture 1: Renal Tissue and Cellular Structure

Question 1

What are the functions of the kidney?

Answer 1

maintain salt and water balance
excretion (filtration, reabsorption, secretion)
endocrine
metabolic

Question 2

What are some substances that are filtered by the kidney? Where does filtration occur?

Answer 2

products of protein metabolism

at glomeruli

Question 3

what are some substances that are reabsorbed by the kidney?

Answer 3

water, glucose, amino acids, sodium, bicarbonate reabsorbed in the tubules

Question 4

what are some substances that are secreted by the kidney?

Answer 4

drugs (penicillin, contrast dyes) by the ABC (ATP binding cassette) transporters shuttle lipophillic substances across the tubular membranes

Question 5

What are some of the endocrine effects of the kidney?

Answer 5

renin –> retention of Na+ and H2O
erythropoietin –> stimulates maturation of RBCs
local effects: prostaglandins and kinins - effect local blood flow through the kidney

Question 6

What are some of the metabolic effects of the kidney?

Answer 6

peptide hormones like PTH and insulin are degraded in the kidney
Vitamin D is converted to active form: 1,25 dihydroxyvitamin D3.
gluconeogenesis occurs during hypoglycemia
citrulline converted into arginine as part of the urea cycle’s process of NH4

Question 7

What are some of the microscopy techniques?

Answer 7

light microscopy, electron microscopy, immunofluorescnece microscopy

Question 8

Describe light microscopy.

Answer 8

H&E –> examine overall kidney morphology
Periodic Acid Schiff’s stains proteoglycans and appreciate thickness of basement membrane and ECM
Mallory trichome stain –> assess degree of collagen deposits in the interstitial matrix
silver stain visualizes fine collagen III in matrix

Question 9

Describe the use of electron microscopy

Answer 9

ultrastructural examination is essential for diagnosis of glomerular disorders

Question 10

Describe the use of immunofluorescence microscopy

Answer 10

antibodies labeled with fluorescein bind to immunoglobins, C3 or C4, which may accumulat in renal pathologies

Question 11

What are the physical elements that are examined in a renal biopsy?

Answer 11

glomeruli
tubules
interstitum
blood vessels

Question 12

what are the 6 main sections of the nephron?

Answer 12

bowman’s capsule
proximal tubules
thin limbs of the loop of Henle (descending and ascending)
thick ascending limb of the loop of Henle
distal convoluted tubule
collecting duct

Question 13

what blood vessels are bowman’s capsule related to?

Answer 13

the glomerulus for function of the tubular segment

Question 14

what blood vessels are the proximal and distal tubules related to?

Answer 14

peritubular capillaries for reabsorption and secretion

Question 15

what blood vessels are the loops of Henle related to?

Answer 15

vasa recta which produces a hypertonic interstitium to alter tonicity of excreted urine

Question 16

what are the descriptive terms used to describe locations of a nephron?

Answer 16

superficial, midcortical, juxtamedullary

based on the locations of the glomerulus

Question 17

what is the glomerulus?

Answer 17

the filtration unit of the kidney
it is formed by blood vessels, epithelium of the nephron tube (bowman’s capsule), intervening basement membrane and stalk of support cells (mesangial cells)
the entering afferent arteriole and exiting efferent arteriole cluster at the vascular pole of the glomerulus
at the opposite end the nephron tubule exits glomerulus at the urinary pole

Question 18

how much blood is filtered through the kidney?

Answer 18

20% of the blood volume leaving the heart
1L/min of the 5L/min total CO
produce 180 L of filtrate every 24 hours, but excrete 1-2 L:

Question 19

Describe the blood vessel.

Answer 19

afferent arterioles form glomerular capillaries where filtration occurs
capillary endothelium is fenestrated with no diaphragms spanning the pores
patent capillary loops are a histological landmark in light and EM
capillaries merge to form the efferent arteriole
arterioles are surrounded by smooth muscle that affect filtration pressure in the glomerular capillaries

Question 20

what is the bowman’s capsule ade of?

Answer 20

parietal epithelium - simple squamous forming the outer wall of the urinary space
at vascular pole it is continuous with visceral epithelium of Bowman’s capsule which reflects over glomerular tuft of capillaries
at the urinary pole, the pariental epithelium is continuous with the proximal tubule

Question 21

what is the visceral epithelium of Bowman’s capsule made of?

Answer 21

podocyes which have processes or trabeculae that interdigitate with neighboring podocytes forming a series of foot processes (pedicels)
the spaces between foot processes are spanned by a filtration slit diaphragm, which are a specialized junctional complex

Question 22

What is nephrin?

Answer 22

transmembrane proteins of the immunoglobulin superfamily which form homodimers to link neighboring foot processes

Question 23

what are the intracellular linkage molecules that interface between nephrin and actin cytoskeleton?

Answer 23

ZO-1, alpha-acctinin4, pododcin

Question 24

What are the pedicels coated with?

Answer 24

podocalyxin, which is a glycoprotein rich in negatively charge sialic acid

Question 25

what are pedicels important for?

Answer 25

glomerular filtration | fusion of the pedicels occur pathologically

Question 26

where is the glomerular basement membrane?

Answer 26

between the glomerular capillary endothelium and the podocyte foot processes

Question 27

what are the three layers of the glomerular basement membrane?

Answer 27

lamina rara externa: in contact with glomerular podocytes lamina densa: central dense layer lamina rara interna: contacts the capillary endothelium

Question 28

describe the central lamina densa

Answer 28

enriched with collagen type IV limiting porosity to less than 40,000 daltons

Question 29

describe the laminae rar interna and externa

Answer 29

contain anchoring proteins laminin and fibronectin as well as heparan sulfate-rich anionic proteoglycans repelling negatively charge plasma proteins

Question 30

what disease occurs as a result of a decrease in basement membrane heparan sulfate?

Answer 30

proteinuria, seen in diabetes

Question 31

what are the three histological layers comprising the glomerular filtration barrier?

Answer 31

fenestrated endothelium glomerular basement membrane foot processes with filtration-slit diaphragm

Question 32

what determine glomerular filtration rate?

Answer 32

hydrostatic pressure (BP) within the glomerular capillaries and oncotic pressure from the plasma proteins

Question 33

what fine tunes the GFR?

Answer 33

smooth muscle in the media of both the afferent and efferent arterioles and contractile intraglomerular mesangial cells

Question 34

what are intraglomerular mesangial cells?

Answer 34

connective tissue cells that are either smooth-muscle like mesangial cells or bone marrow-derived macrophages

Question 35

what are smooth muscle like mesanial cells?

Answer 35

contractile (express alpha-actin) and may decrease surgace area available for filtration sensative to angiotensin II they synthesize the ECM components: collagen (IV and V, more I and III, when activated by cytokines), fibroonectin, thrombospondin, laminin, vitronectin and proteoglycans, enzymes involted in turnover of ECM (metalloproteinases and serine-proteases) secrete PGs - vasodilation of glomerular arterioles secrete hormones, cytokine,s GFs and paracrine agents

Question 36

What do bone-marrow derived macrophages do?

Answer 36

phagocytose apoptotic cells and material lodged in the basement membrane utilize C3b or Fc receptors to bind opsonized particles they express MHC class II antigens on surgace and act as APC

Question 37

What is reabsorbed in proximal tubule?

Answer 37

``` 65-70% water, Na, K all glucose, amino acids, citrate, small peptides 80% bicarbonate 40-50% Ca2+ 80 - 95% of PO4- ```

Question 38

what are segments of the proximal tubule?

Answer 38

physiogically (different transporters): S1, S2, S3 | anatomically: proximal convoluted tubule and proximal straight tubule

Question 39

describe the convoluted portion of the proximal tubule.

Answer 39

coil and occupy much of the cortex around the glomeruli | proximal straight tubules are part of the medullary ray (cluster of straight tubules in the cortex) en route to medulla

Question 40

describe the proximal straight tubule

Answer 40

part of medullary ray; cluster of straight tubules en route to medulla

Question 41

what some of the features of the proximal tubule epithelium?

Answer 41

simple cuboidal with large prominent nucleus and | surface specializations

Question 42

what are some features of the apical plasma membrane

Answer 42

it folds as tall, regular microvilli = brush border carriers inserted iinto this increased surface area for secondary active transport of reabsorbed amino acids, glucose, citrate and bicarbonate ions the ions are couples with Na+ ions in positively charged carrier complexes apical tight junctions are less frequent than other tissues, desmosomes are infrequent

Question 43

what are claudins?

Answer 43

they are in the tight junctions of proximal tubule epithelium that create a leaky epithelium that permits paracellular movemet of water and Ca2+ from lumen to interstitial space below epithelium

Question 44

what is the endocytic apparatus?

Answer 44

it is in the apical cytoplasm and consists of vesicular invaginations between bases of microvilli, clathrin coated vescles, large vacuoles lysossomes and condensin vacuoles peptides from filtrate are endocytosed here and degraded before release outside the basolateral side

Question 45

what is megalin

Answer 45

a specific endocytosis receptor for albumin and small proteins recovered in the proximal tubule it is downregulated by TGF0beta during inflammatory conditions which contribute to proteinuria

Question 46

What drugs inhibit endocytosis contributing to proteinuria?

Answer 46

HMG-Coa inhibitors, like statins

Question 47

Describe the basolateral plasma membrane

Answer 47

extensive folding Na+K+ATPase transporters pump Na+ out of cell into basolateral extracellular space cytoplasmic mitochondria occupy foldes between basolateral invaginations providing ATP for active transport

Question 48

what are the portions of the loop of Henle?

Answer 48

thin descending limb thin ascending limb medullary thick ascending limb (mTAL) cortical thick ascending limb (cTAL)

Question 49

Describe the variation in the lengths of the loops of Henle.

Answer 49

superficial glomeruli have very short loops of Henle | juxtamedullary glomeruli have very long loops that extend to tips of the medullary papillae

Question 50

what are some features of the loop of henle?

Answer 50

establish hypertonic interstitium in the medulla, which is essential to production of concentrated or hypertonic urine there is a countercurrent exchange between the tubules and the vasa recta which maintains the hypertonic interstitum

Question 51

What are some physiological features of the thin portions of the loop of Henle?

Answer 51

Simple squamous epithelium Thin descending limb is permeable to water and urea but impermeable to Na+ and Cl- Water and urea passively move frome the tubular fluid into interstitium as thin limb descends into hypertonic interstitium of medulla Water moves paracellularly through shallow tight junctions and intracellularly through aquaporin channels Into the ascending thin limb, the permeability characteristics reverse: impermeable to water and highly permeable to Na+ and Cl-. Salt exists the tubule as flow ascends into relatively less hypertonic regions of the medulla Movement of water prevented by extensive tight junctions with claudins that seal and lack aquaporins No active transport of Na+ or Cl- in the thin limb

Question 52

What is the main function of the thin limbs?

Answer 52

Along with the accompanying vasa recta, it maintains and stabilizes the hypertonic gradient of the medullary interstitium

Question 53

Where does the transition to the thick ascending limb of the loop of Henle take place?

Answer 53

Near the hairpin turn in short looped nephrons and at the transition from the outer to inner medullary zones in long looped nephrons

Question 54

Describe the thick ascending limb.

Answer 54

The luminal contents are hypotonic when they exit. Interstitum becomes hypertonic. Simple cuboidal epithelial with numerous invaginations of the basolateral plasma membrane which is the site of Na+K+ATPase active transport pumps The cytoplasm between the folds contains mitochondria (more round than ones in proximal tubule) Electrical gradient powers ion movement Apical plasma membrane: slightly irregular: contains Na+K+2Cl sympotrter: net movement of Ca+ and Cl- from lumen into interstium

Question 55

What inhibits the Na+K+2Cl- symporter?

Answer 55

Loop diuretics like furosemide

Question 56

How is the apical membrane distinguished?

Answer 56

Extensive tight junctions, which block paracellular movement of water since there are no aquaporins Thick ascending limb is impermeable to water, claudins of the tight junction permit passage of Mg 2+, Ca2+ and Na+

Question 57

What is the macula densa?

Answer 57

Last segment of the cortical thick ascending limb It returns to the glomerulus Component of the juxtaglomerular apparatus – responsible for the secretion of rennin

Question 58

What is the distal convoluted tubule?

Answer 58

They are in the cortex intermingled with proximal convoluted tubules Shorter length and a lower cuboidal epithelium Apical surface is devoid of microvilli, resulting in a smooth lumen Toght junctions are broader than proximal tubule with sealing claudins – precluding paracellular water movement Impearmeable to water Basolateral surface is highly folded with Na+K+ pumps and accompanying mitochondria The basolateral active transport pumps are the driving force for the reabsorption of Na+ and Cl- Responds to aldosterone to reabsorb Na+ and PTH to reabsorb Ca2+

Question 59

What are the parts of the collecting duct?

Answer 59

Connecting segment Cortical collecting duct Medullary collecting duct Responsible for final adjustments to the colume and tonicity of the excreted urine Controlled by vasopressin and aldosterone Initially lined by simple cuboidal epithelium, as one progresses to the medullary papilla the epithelium becomes tall columnar and then pseudostratified at the exiting papillary ducts

Question 60

What does EM show of the collecting duct?

Answer 60

Principle (light) cells and intercalated (dark) cells

Question 61

What are principal cells?

Answer 61

Principal cells have a pale cytoplasm, small random mitochondria, few basal infoldings, very few microvilli and single cilium, which is a flow sensor mechanically linked to a Ca2+ channel Respond to aldosterone by reabsorbing Na+ and secreting K+ Respond to ADH by increasing luminal permeability to water

Question 62

What are intercalated cells?

Answer 62

More electron dense cells (dark in EM), secrete H+ and play a large role in acidification of urine

Question 63

What is the goal of the structures of the juxtaglomerular apparatus?

Answer 63

Control GFR to not flood out the capacity of the distal nepron preventing excessive salt and water losses The apparatus adjusts renal hemodynamics, during periods of increased or decreased perfusion and activates the RAAS cascade.

Question 64

What are the sensors in the juxtaglomerular apparatus?

Answer 64

Afferent arteriole stretch recptors and macula densa Cl-/osmolality receptors

Question 65

What are the components of the juxtaglomerular apparatus?

Answer 65

Afferent arteriole Macula densa Extraglomerular mesangial cells Efferent arteriole

Question 66

Describe the afferent arteriole

Answer 66

Smooth muscle in the media are composed of juxtaglomerular cells which contain intracellular actin, myosin, and dense bodies like all smooth muscle cells, contain more RER, Golgi and a store of secretory vesicles (containing renin). The JG cells respond to a decrease in arteriolar stretch due to decreased BP by secreting renin granules. With elevated BP, the JG cells produce a myogenic response to vasoconstrict the afferent arteriole

Question 67

Describe the macula densa

Answer 67

The base of macula densa cells interdigitate with underlying extraglomerular mesangial cells and with JG cells. Nuclei are forced apically, toward the lumen, making this tubular section easy to identify. Respond to flow related alterations in level of Cl- (osmolality) in the fluid being delivered from the thick ascending limb. When Cl flow rates are high, the macula densa blocks release of renin by the JG cells. The macula densa acts on the JG cells of the afferent arteriole directly through the release of adenosine and NO, and indirectly via Angiotensin II and thromboxane

Question 68

Describe extraglomerular mesangial cells

Answer 68

Lie between the afferent arteriole, efferent arteriole, the macula densa and the glomerulus. Interact with the acula densa and JG cells assisting in regulation of renin secretion and vasoconstriction

Question 69

Describe the efferent arteriole

Answer 69

Smooth muscle contraction of the efferent artiole increases hydrostatic pressure within the glomerulus In low pressure/flow – ang II causes constriction preserving GFR (vasoconstriction modulated by PGs which vasodilate the efferent arteriole to preserve blood flow to medulla)

Question 70

What is the renal interstitium

Answer 70

Connective tissue cells that are found between all of the tubular and vascular elements of the kidney

Question 71

Describe the renal interstitium in the cortex.

Answer 71

It has fibroblast like cells that make ECM and contain contractile filaments. They may secret erythropoietin. May have phagocytic cells that are derived from the monocyte line

Question 72

What are the interstitial cells that are found in the medulla

Answer 72

Type 1 fibroblast-like, with many lipid droplets, SER, and mitochondria with vesiscular cristae. It is the site of PG synthesis Type II which are rounded cells with many free ribosomes and lysosomes, but no lipid dropelets. Unknown function Type III similar to pericytes and are in the basement membrane of the vasa recta. The may act as a source of regenerative cells

Question 73

Describe the blood flow within the kidney

Answer 73

Intrarenal blood flow: 80% to outercortex, 10 to 15% to inner cortex, 5 to 10% go to medulla

Question 74

Where are the glomeruli in the kidney?

Answer 74

85% in outercortex, 15% in the inner cortex

Question 75

What parts of the kidney are most susceptible to ischemia?

Answer 75

The low flow areas. During hypovolemia, blood flow is diverted to the inner cortex

Question 76

Describe the branching of the renal arteries

Answer 76

Renal arteries --> interlobar arteries --> arcuate arteries --> interlobular arteries --> afferent arterioles (may come from intralobular arteries) --> glomerular capillary tuft --> efferent arterioles--> peritubular capillary plexus (superficial glomerulus) or vasa recta (juxtamedullary glomeruli) --> both reform the arcuate veins

Question 77

Where does the renal arteries branch into interlobar arteries?

Answer 77

Base of medullary pyramids and penetrate the medulla between renal lobes

Question 78

Describe the arcuate arteries.

Answer 78

They are division of the interlobar artiers that turn a right angle to lie at the junction of the cortex and medulla. They do not anastomose. Responsible for supplying a specific portion of the cortex

Question 79

Describe interlobular arteries

Answer 79

Arise from arcuate and penetrate cortex between medullary rays. May give rise to afferent arterioles