What is a Lump vs a Hump

A student recently asked me what is the difference between “lumpy bumpy” and “humps”.  These are buzzwords that used to describe pathology on kidney biopsy samples.  While meant to be descriptive, they can also be a bit vague. (Double click on pictures to see full size)
1)  “Humps”
Type of imaging:  EM
What they are: Large subepthithelial electron-dense deposits (on the outside of the glomerular basement membrane).  Electron-dense deposits are much larger compared to other diseases, see the size of the deposit in comparison to the thickness of the GBM.  (Because post-infectious is associated with a nephritic syndrome you may be wondering why there are no sub-endothelial deposits, well…there are.  See the second picture.  The subendothelial deposits occur in the early phase and are often missed in biopsies)
Typically used to describe:  Post-infectious glomerulonephritis
Picture:
Post-infectious GN

 
2) “Lumpy-bumpy”
Type of imaging: Immuno-Fluorescence (IF)
What it is: A granular (coarse, irregular) pattern of staining.  Remember that IF is done by applying an antibody that specifically targets an antigen (e.g. IgG, IgA, complement, etc).  A fluorophore is attached to the antibdoy.  The attached fluorophore can then be detected via fluorescent microscopy.
Typically used to describe: IF pattern for MPGN or  post-infectious GN
Picture: Granular IgG and complement along the glomerular basement membrane

 
3) “Spike and dome”
Type of imaging:  Generally used to describe image on light microscopy (usually with a silver stain), but I’ve also seen it used on EM images
What it is: The basement membrane is severely thickened with abundant electron-dense deposits shown by arrowheads in second picture (EM). The deposits were originally on the outside of the GBM. The GBM now has grown to flank the deposits. These projections of the GBM correspond to the spikes seen by silver stains. The domes are the thin strips of GBM that go over the deposits.
Typically used to describe:  This term is specifically used for membranous nephropathy
Picture:


 
4) “Tram tracks”, also known as double contours of the basement membrane
Type of imaging: Light microscopy
What it is: It is thickening of the glomerular capillary wall and basement membrane.  When subendothelial immune deposits disrupt the endothelial-GBM layer, the endothelium produces a second layer of basement membrane, over time this thickens the GBM that can be seen as “tram tracking” on light microscopy.
Typically used to describe:  MPGN
Picture:  below are are 2 pictures with LM, the second picture is with silver staining to really bring out the GBM.  Also see page 250 of Rennke 3rd edition (Figure 9.10, panel B, light microscopy)

 
BTL