40) Functional histology of the kidney

Question 1

What are the two parts of the Loop of Henle?

Answer 1

• Thin arm which descends and ascends half way up

- Thick arm which only ascends

Question 2

What is the collecting tubule?

Answer 2

• The part of the tubule that joins with the collecting duct

Question 3

What is the renal corpuscle?

Answer 3

• It is a structure containing the Bowman’s capsule and the glomerulus

Question 4

What cells cover the glomerular capillaries?

Answer 4

• There is a specialised layer of cells which cover the glomerular capillaries called podocytes
• They have projections that branch off them. This branching keeps going until we reach the end (called the feet)
• The feet are interdigitating with other feet (i.e. they are interlocked) which creates slits between them to allow for filtration of blood

Question 5

What are the three stages of filtration?

Answer 5

• Fenestration: Which are gaps between endothelial cells that line the walls of the capillary in which blood can flow through. Allows for larger molecules through but filters out excessively large substances
• Basement membrane: Which is thick and where the endothelial cells and podocytes are attached to. It allows smaller molecules to pass through by filters out larger compounds
• Filtration slits: Found between the feet of the podocytes which allow molecules that are even smaller to pass through while filtering out larger compounds

(As we move down the stages of filtration the levels of fineness increases (i.e. the molecules they allow through gets smaller and smaller)

Question 6

What is the function of the proximal convoluted tubule?

Answer 6

• Reabsorption of ultrafiltrate

Question 7

What are the mechanisms by which material is reabsorbed in the proximal convoluted tubule?

Answer 7

• Active transport: Small molecules against a concentration gradient (e.g. glucose or Na+)
• Pinocytosis: Macromolecules (e.g. proteins which are degraded within lysosomes and amino acids are recycled in the blood)
• Passive flux: Small molecules down a concentration gradient (e.g. H20)

Question 8

What is the structure of the proximal convoluted tubule epithelial cells?

Answer 8

• Microvilli: Long finger like projections that increase surface area but also contain lytic enzymes on the surface to break down macromolecules
• Pinocytotic vesicles: Carrying macromolecules to lysosomes
• Lysosomes: Found in great abundance to recycle macromolecules
• Mitochondria: Found also in abundance (especially at the basal surface) to fuel active transport

Question 9

What is the function of the thin arm of the loop of Henle?

Answer 9

• They reabsorb ultrafiltrate

Question 10

What are the mechanisms by which material is reabsorbed in the thin arm of the loop of Henle?

Answer 10

• Passive flux only (i.e. osmosis)

Question 11

What is the structure of the cells of the thin arm of the loop of Henle?

Answer 11

• Thin squamous epithelium (in size) to allow passive fluxes to occur
• A minimum number of organelles

Question 12

What is the function of the thick arm of the loop of Henle and the distal convoluted tubule?

Answer 12

• Blood homeostasis

Question 13

What are the mechanisms by which material is reabsorbed in the thick arm of the loop of Henle and the distal convoluted tubule?

Answer 13

• Through active transport and ion exchange

Question 14

What is the structure of the cells of the distal convoluted tubule?

Answer 14

• Cuboidal epithelium (in size): Thicker than squamous to reduce passive fluxes and accommodate organelles
• Mitochondria: Found in abundance to fuel active transport
• Microvilli: Tiny amounts found which are short

Question 15

What is the function of the collecting duct and collecting tubule?

Answer 15

• Transport urine to the ureter

- Water homeostasis as it allows the passive reabsorption of water which is regulated through epithelial permeability

Question 16

What is the structure of the cells within the epithelial of the collecting duct?

Answer 16

• Cuboidal to columnar (in shape): Prevents unregulated passive flux of water (and urea)
• Dense membrane: This occurs at the site of cell contacts as it may help prevent passive flux

Question 17

What is nephrotic syndrome?

Answer 17

• When glomerular basement membranes become damaged and leaky causing proteins to be lost in the urine
• In some cases the basement membrane becomes thickened which may be the deposition of immunoglobulins (autoimmunity)

Question 18

How does high blood pressure cause kidney damage?

Answer 18

• High blood pressure damages glomeruli which is replaced by a hyaline material
• A hyaline material is one that is glassy and usually indicates that the cellular matter has been replaced by proteinaceous material

Question 19

How do distal and proximal convoluted tubules differ?

Answer 19

• Distal tubules are smaller cells with a smoother surface that appears paler
• Proximal tubules are bigger with wispy material within the lumen (which is fixed protein)

Question 20

What is the juxtaglomerular apparatus?

Answer 20

• Where the distal convoluted tubule loops back and meets the arterioles of the same nephron

Question 21

What are the different cell types in a juxtaglomerular apparatus?

Answer 21

• Juxtaglomerular cells: Lining the arteriole which contain renin granules
• Lacis cells: In the middle
• Macula densa: On the distal convoluted tubule

Question 22

What is the function of the macula densa?

Answer 22

• They sense the [Na+] in the distal convoluted tubule fluid

- They send signals to juxtaglomerular cells

Question 23

What is the function of the juxtaglomerular cells?

Answer 23

• Receive signals from the macula densa and in doing so release renin
• They release more renin when there is a lower [Na+] in the distal convoluted tubule.
• Renin indirectly increases vascular tone and increases sodium reabsorption

Question 24

What is the function of the lacis cells?

Answer 24

• It is speculated that they have a role in signalling between the two cells
• They are also called extraglomerular mesangial cells

Question 25

What are the structures within the ureter?

Answer 25

• Transitional epithelium: Which surrounds the lumen
• Dense connective tissue: Located around the transitional epithelium (also called lamina propria)
• Layers of smooth muscle: Found around the dense connective tissue and allows peristalsis to occur
• Adventitia: Which is the tissue located around the layers of smooth muscle and is the outermost layer

Question 26

What is transitional epithelium?

Answer 26

• A specialised stratified epithelium which is found only in the ureters and bladder
• It is specialised to be impermeable to urine and changes appearance on stretching
• Its appearance is like stratified squamous epithelium except the biggest cells are apical (whereas in stratified squamous epithelium the biggest cells are basal)

Question 27

What is found on the apical cells of the transitional epithelium?

Answer 27

• There are plaques of specialised (urine-resistant) plasma membrane
• When the bladder is distended they are impermeable, rigid and protect the cells from urine (which is toxic)
• When the bladder is relaxed they fold up forming interplaque regions and will also invaginate to form pits which allows the volume of the bladder (and ureter) to decrease