Functional HIstology of the Kidney Flashcards
What is the glomerulus?
A knot of capillaries
Seen here by SEM (scanning electron microscopy)
What are the three stages of filtration?
- Fenestration- pores between the capillary endothelial cell of the glomerulus
- Thick, fused basement membranes of capillary endothelial cells and podocytes
- Filtration slit membrane between podocyte foot processes (~4nm pores)
What happens to blood that enters from the afferent arteriole?
Blood comes in through the afferent arteriole
Goes around the capillaries, get filtered
Blood is filtered from the inside outwards
Exits as filtered blood through the efferent arteriole
What is the function of the proximal convoluted tubule?
Function- reabsorption from ultrafiltrate (the fluid coming from the corpuscle)
And it does it in several ways:
Active transport:
Small molecules against a concentration gradient, e.g. glucose, amino acids, Na+ ions
Pinocytosis:
Macromolecules, especially proteins
(proteins are degraded within lysosomes and amino acids recycled to blood)
Passive flux:
Small molecules with a concentration gradient, e.g. H2O, Cl- ions
What are the features of the proximal CT epithelial cells?
Microvilli (high surface area); lytic enzymes on surface to break down macromolecules
Pinocytotic vesicles carrying macromolecules to lysosomes
Many lysosomes to recycle macromolecules
Many mitochondria to fuel active transport (especially near the basolateral Na+ pumps)
What is the thin arm of the Loop of Henle?
Function- reabsorption from ultrafiltrate
By passive flux only (i.e. osmosis)
A thin loop of Henle, epithelial cell
Thin, squamous epithelium to allow passive fluxes
A minimum of organelles doesn’t need the energy
What is the thick ascending loop of Henle?
Thick ascending loop of Henle and distal conv. tubule
Function- blood homeostasis
Regulated active transport and ion exchange (Na+/K+, H+/HCO3-)
What does the distal CT epithelial cell look like?
Cuboidal epithelium – thicker than squamous, to reduce passive fluxes and accommodate organelles
Many mitochondria to fuel active transport. Can show as a pale or striped basal area in H&E-stained sections.
Few, short microvilli (unlike PCT cells)
What is the function of the collecting duct and collecting tubule and its epithelial cell?
Functions:
Transport of urine to the ureter
Water homeostasis- passive reabsorption of water, regulated through epithelial permeability
Collecting duct epithelial cell:
Cuboidal to columnar epithelium, to prevent unregulated passive flux of water (and urea)
Dense membranes at cell contacts.
Function unclear – probably also helping to prevent passive flux.
What is the function of the juxtaglomerular apparatus cells?
Macula densa - senses [Na+] in the DCT fluid. Appears to signal to….
Juxtaglomerular cells - release renin – more so in response to lower [Na+] in DCT. Renin indirectly increases vascular tone and sodium resorption.
Lacis cells – function unknown. (Signalling between the other two?) Also called extraglomerular mesangial cells.
What is a transitional epithelium?
A special stratified epithelium, found only in ureters and bladder
Specialized to be impermeable to urine
Changes appearance on stretching
Appearance is like stratified squamous epithelium, except biggest cells are apical (on the outside)
(Stratified squamous epithelium = big basal cells, flat apical cells)
Why can the impermeability of the transitional epithelium be a problem?
Another oddity; plaques of specialised (urine-resistant) plasma membrane in apical cells of transitional epithelium
However its impermeability makes the bladder poorly accessible to leukocytes- hence the urinary tract is a common site of bacterial infection
