2 - Histology of the Kidney and Urinary Tract

Question 1

What are the three main functions of the kidneys?

Answer 1

1. Excretory: produce ultrafiltrate from blood passing through. Excess water and ions, some drugs, toxins, and metabolites are excreted in urine.
2. Homeostatic: regulating and maintaining EC fluid volume and composition. Maintaining acid-base balance.
3. Endocrine: monitoring O2 carrying capacity of the blood via erythropoietin and regulating BP through RAA system.

Question 2

What are the components of the nephron? What is it’s function?

Answer 2

Components: renal corpuscle and renal tubule

Function: filter and fluid modifier - kidney makes an ultrafiltrate of the blood, but also recycles many components that are in the filtrate.

Question 3

What are the three main macroscopic components of the kidney?

Answer 3

Cortex: granular appearance. Linear arrays of tubules called medullary rays extending into the cortex.

Medulla: striated appearance; consists of 6-18 renal pyramids. The apex or tip of each pyramid is called a renal papilla.

Kidney lobe: consists of a renal pyramid and it’s surrounding cortex. Indisctint externally in adults.

Question 4

What is a kidney lobule? What is the capsule?

Answer 4

Lobule = medullary ray + cortical tissue (primarily nephrons) on either side

The tubules of these nephrons connect with the collecting ducts within the medullary rays

The capsule consists of mainly fibrous connective tissue and surrounds the kidney.

Question 5

How much blood goes to the kidney? What much is extracted/filtered?

Answer 5

Recieves 20-25% of the total cardiac output/minute. The total blood volume of the body passes through the kidneyts every 4-5 minutes.

125 mL of fluid is extracted fromthe blood each minute as filtrate, and 124 mL is reabsorbed in the tubules while only 1mL is excreted as urine.

Question 6

What are the 2 capillary systems of hte kidney’s microvasculature?

Answer 6

Tubular plexus - supplies tubules of the cortical nephron

Vasa recta - long capillary loops that supply tubules of juxtamedullary nephrons

Question 7

What is located in the space between the renal tubules?

Answer 7

The innerstitial (stromal) tissue is found in the renal cortex and medulla - the stroma is finer in the cortex.

• It’s made of interstitial connective tissue and interstitial cells (fibroblasts)
• interstitium is where ultrafiltrate goes before it’s taken up by the BVs

Question 8

What are the four components of the renal corpuscle?

Answer 8

1. Glomerulus
2. Visceral layer of the Renal Capsule (Bowman’s)
3. Parietal layer of the Renal Capsule
4. Mesangium

Question 9

What is the renal corpuscle? What are the poles? In what part of the kidney are these found?

Answer 9

Spherical, double layered sac that surrounds a network of capillaries (glomerulus)

Has a vascular pole where the arterioles enter and exit and a urinary pole that’s continuous with the proximal convoluted tubule.

Renal coerpuscles are found only in the cortex.

Question 10

What is the glomerulus? What supplies and drains it?

Answer 10

A network of capillary loops supplied by the afferent arteriole and drained by the efferent arteriole. The afferent arteriole is large in diameter, which creates a pressure differential that drives glomerular filtration.

Question 11

What is Bowman’s Capsule? What are the different layers and what cells make them up?

Answer 11

Double-layered epithelial sac around the glomerulus.

• The outer parietal layer is simple squamous epithelium.
• The visceral layer is also simple epithelium composed of cells called podocytes.
• The space between the two layers is the urinary space and is continuous with the proximal tubule; glomerular filtrate enters this space

Question 12

What are the components of the glomerular filtration barrier (ie the filtration membrane of the kidney)?

Answer 12

Capillary endothelium - discontinuous with numerous 70-100 nm pores that are permeable to water and solutes 6-8 kD or less and moderatly permeable to molecules 8-16 kD. The luminal surface has a negative charge because it’s coated with glycocalyx made of proteoglycans.

Basement membrane - primary barrier that prevents proteins from entering filtrate

Podocytes - with foot processes make basement membrane

Question 13

What is the structure and function of the glomerular mesangium?

Answer 13

Functions: physical support and regulation of glomerular blood flow

Cells and ECM that abut the inner surface of the glomerular basement membrane.

• ECM contains fibronectin and collagen and specialized pericyte/smooth muscle cells

Question 14

What is the function of the convoluted portion of the proximal tubule? What type of cells are present?

Answer 14

(Green in picture) Begins at the urinary pole. Glucose, aas, and proteins are reabsorbed through facilitated transport.

• Cuboidal/columnar cells with granular cytoplasm and basal nuclei.
• Apical brush border with glycocalyx obscures lumen.
• Lysosomes, apical vescicles, and mitiochondria.
• Complex lateral interdigitations between epithelial cells make lateral cell membranes indistinguishable.

Question 15

What cells are located in the straight portion of the proximal tubule?

Answer 15

Cuboidal epithelium (Purple in picture)

Also found inthe thick descending limb of henle.

Question 16

What are the four parts of the loop of henle? Where is the loop of henle located?

Answer 16

1. Straight portion of the proximal tubule (thick descending limb)
2. Thin desxending limb
3. Thin ascending limb
4. Straight portion of the distal tubule (thick ascending limb)

Located in the medulla.

Question 17

Describe the two different types of nephrons and their loops?

Answer 17

Cortical nephrons: located external to the juzxtamedullaruy zone; they have short loops that only have a descending thin limb

Juxtamedullary nephrons: are long looped and have ascending and descending thin limbs

Question 18

What types of cells are found in the loop of henle? What about cell junctions?

Answer 18

Thick portions of the loop are lined with cuboidal epithelium, but thin segments are lined with simple squamous epithelium.

Thin descending limb is permeable to water.

Cell membranes in the ascending thin limb between epithelial cells are interdigitated, resulting in water impermeability.

Question 19

What is the countercurrent multiplier?

Answer 19

Urine concentration through differential resorption of water that yields hypotonic fluid in the distal tubule because the ascending limb of the loop of henle is impermeable to water, while the descending limb is somewhat permeable to water.

Question 20

What is the countercurrent exchanger and its function?

Answer 20

Maintains osmotic gradient established by countercurrent multiplier.

Accomplished becasue the arterioles around the descending limb of the loop have continuous endothelium and the venules around the ascending limb have fenestrated endothelium.

Question 21

What is the cell type and structure of the straight portion of the distal tubules (thick ascending limb)? What is the function of this region?

Answer 21

• Lined with cuboidal epithelium, scant microvilli, efficient tight junctions.
• Lateral and basal membrane interdigitations.
• Abundant mitochondria.
• Impermeable to water, Na and Cl resorbed. H+ secreted.

Question 22

What is the cell type and structure of the convoluted portion of the distal tubule (early distal tubule)? What is the function of this region?

Answer 22

Lined with cuboidal eithelium, scant microvilli, fewer basal interdigitations and fewer mitochondria than the straight portion.

Na+, CL-. and K are secreted.

Question 23

What cells makes up the collecting tubules (late distal tubule) And the dollecting ducts?

Answer 23

Cuboidal cells (principle - light, intercalated - dark)

Distinct cell borders

Question 24

What are the collecting tubules (late distal tubule)? What is the structure and function?

Answer 24

Transitional segment between the nephron and the collecting duct.

Anti-diuretic hormone (ADH) dependent segment where Na+ is reabsorbed and K+ is sescreted.

Epithelium contains principal cells.

Question 25

What cells are in with collecting ducts and what do they respond to?

Answer 25

**Lined with cuboidal to columnar epithelium.** **Principal (light) cells have one primary cilium and ADH sensitive AQP-2 water channels.** * In the presence of ADH, water diffuses out of the collecting duct and into the renal interstitium. This increases urine tonicity. * In the absence of ADH, water is excreted from the collecting ducts leading to pikyuria and hypotonic urine (diabetes insipidus).

Question 26

What is the function of the single, nonmotile primary cilium found on principal cells of the collecting duct?

Answer 26

It acts as a flow sensor. This is mediated by two proteins: polycystin 1 and 2. Defects in these proteins results in polycistic kidney disease.

Question 27

What are the componants of hte juxtaglomerular apparatus (JG)?

Answer 27

**Renin-producing JG cells:** specialized SMCs in the wall of the afferent arteriole. **Extraglomerular mesangial (lactis) cells:** connected to JG cels via gap junctions **Macular densa:** columnar cells of the DCT that detect Na and Cl concentration for the JG cells, resulting in alterations of the filtration rate and auto-regulation of blood volume via the RAA system.

Question 28

What are the three layers of the calyces, pelvis, and ureters?

Answer 28

* **Mucosa:** transitional (uro)epithelium; lamina propria contains abundant elastic tissue * **Muscularis**: smooth muscle. ~2 layers in the upper 2/3 of the ureter, 3 layers in the lower 1/3 * **Adventitia** - fibrous connective tissue

Question 29

What is the cellular structure of the urinary bladder?

Answer 29

**Transitional epithelium** * appears pseudostratified when empty * appears stratified when filled 3 layers of smooth muscle.

Question 30

How does the urethra differ in men and women?

Answer 30

**Men:** * 15-20 cm, 3 parts (prostatic, membranous, and penile) * Transitional - pseudostratified squamous epithelium * Shared urinary and reproductive systems **Women:** * 3-5 cm * Transitional - pseudostratified squamous epithelium * Urinary system only

Question 31

The nuclei within the glomerulus belong to three cel types. Name these cell types?

Answer 31

Capillary endothelial cells, podocytes, and mesangial cells.

Question 32

What components make up the filtration barrier through which fliud passes in going from glomerular capillaries into the urinary space?

Answer 32

Fenestrated capillary endothelium Fused basal laminae of capillary endothelial cells and podocytes. Diaphragm-covered filtration slits between podocyte foot processes.

Question 33

The plasma ultrafiltrate drains from the urinary spae into what structure?

Answer 33

The proximal tubule.

Question 34

What is the cell type, lumen characteristics, cell borders, nuclei location, cytoplasmic staining in the proximal tubule?

Answer 34

**Cell type:** cuboidal **Lumen:** occluded **Cell borders:** indistinct **Nuclei**: few, basally located **Cytoplasmic staining:** eosinophilic granular

Question 35

What is the cell type, lumen characteristics, cell borders, nuclei location, cytoplasmic staining in the distal tubule?

Answer 35

**Cell type**: cuboidal **Lumen**: open, wide, smooth contour **Cell borders**: indistinct **Nuclei:** many, centrally located **Cytoplasmic staining:** pale

Question 36

What is the cell type, lumen characteristics, cell borders, nuclei location, cytoplasmic staining in the collecting duct?

Answer 36

**Cell type:** cuboidal to columnar **Lumen**: open, scalloped edge because cells buldge inward **Cell borders:** distinct **Nuclei**: many, centrally located **Cytoplasmic staining:** pale

Question 37

What is the function of the cells in the proximal tubule?

Answer 37

PT cells resorb Na+ and Cl-, water follows passively. Glucose, amino acids, and proteins are resorbed by facilitated transport.

Question 38

The macula densa is part of a larger structure that includes specialized cells in a portino of the afferent arteriole. Name this larger structure?

Answer 38

Juxtaglomerular apparatus.

Question 39

What is the secretory product of the juxtaglomerular cells of the afferent arteriole?

Answer 39

Renin.

Question 40

How are the collecting tubules distinguished from proximal tubules and distal tubules?

Answer 40

**By their large lumens and the distinct borders between adjacent cells:** * cuboidal to columnar cells have pale cytoplasmic staining and centrally located nuclei * lumen will have scalloped edges because apical lumen buldges into the lumen

Question 41

What are the three tunics of the ureter? Describe the middle layer in the upper 2/3 of the ureter?

Answer 41

**Mucosa, muscularis, and adventitia.** In the upper 2/3 of the ureter, the muscularis layer has an inner longitudinal layer and an outer circular layer. * Another outer longitudinal layer is present int he bottom 1/3 of the ureter (so it goes longitudinal-circular-longitudinal) This is the opposite arrangement to what is found in the tubular organs of the GI tract.

Question 42

Within the lamina propria and the muscularis are refractile, pink staining fibers. These fibers are much more coarse in the muscularis. What are these fibers composed of?

Answer 42

Elastin.

Question 43

Because the bladder is a saccular organ, it has ___ layers of smooth muscle in its wall.

Answer 43

3