renal pearls 1

i have my therapy (internal medicine) exam coming up, so the next few posts are going to be related to topics i need to cover for the exam.

let's start with my second least favourite system: the renal system.

the nomenclature of kidney diseases alone give you a good idea of what it represents:

diffuse: it just means EVERY glomerulus is involved.
focal: only some glomeruli (in one foci) is involved.
focal segmental: the diseased is localized to the kidney and only some of the glomeruli are affected. technically, it should be called focal focal, but you if try saying that out loud fast a couple of times, you'd realise why they chose to call it focal segmental instead.
proliferative: increased number of cells. it's a microscopic presentation of lots of nuclei.
membranous: thickened membranes.
membranoproliferative: both increased cell count and thickened membrane.

before i explain the twins of renal diseases (nephritic and nephrotic syndromes), let's do a quick overview of the physiology of the glomeruli:

podocytes line the bowman's capsule. they have slitpores between them. they synthesize the glomerular basement membrane (GBM).

what keeps albumin out of the urine normally? the strong negative charge of the BM.
what causes this strong negative charge? a GAG called heparan sulfate.

so if when there's damage to the visceral epithelium, there's automatically a damage to the BM and therefore a spill of protein into the urine.

say, i gave you the results of a renal biopsy of an abnormal kidney, and it shows a linear pattern on immunoflourescent IgG tag, what should you be thinking of?

GOODPASTURE'S SYNDROME, a HSII where there are IgG directed against the BM of glomeruli (and alveoli in lungs). so, you get a linear pattern because IgG will attach to all BM.
 
but say i gave you instead a biopsy showing a granular bumpy pattern? what's big and gets in the way of things?

IMMUNE COMPLEXES, which are ab-ag complexes (ie at lupus: anti-DNA + DNA). these IC float around and then deposit at certain places (HSIII). because they consist of both ag and ab, naturally they are larger than just plain abs and can be of different sizes, depending on what forms the complex, so they don't fit nice and neat in the glomeruli. 

you'd get subendothelial deposits in lupus because the IC is too big to go past the BM. in post step GN, you'd get subepithelial deposits because the ICs are small and soluble enough to pass the BM.

you can't see the ICs on immunofluorescence (won't be stained), but you can on EM (they're electron dense).

to be continued...