Podocytic infolding glomerulopathy(
PIG)
What is
PIG? – This is a pathology based finding seen rarely in the EM section of the
report. It is a form of glomerulopathy,
in which microspheres or microtubular structure or both are associated with
infolding of cytoplasmic processes of podocytes into the glomerular basement
membrane. This type of glomerulopathy is
not included in the World Health Organization’s classification of glomerular
diseases but has been noticed in few cases reports and large case series from Japan.
Associations:
In the
largest series from Japan, most patients
might have a subtype of lupus, nephritis class V, or membranous
glomerulonephritis. MCTD, Sjogren's syndrome and tumor lysis syndrome was also found. Interestingly, hydronephrosis
was associated with most patients seen with this entity. There have been cases
associated with Myeloma as well.
Electron micrograph
shows cytoplasmic processes of podocytes
infolded into the GBM. At the end of an infolded cytoplasmic process of podocyte,
microspheres or microtubular structures or both were found. b The infolded cytoplasmic
process of a podocyte went through the middle layer of the GBM, which was
accompanied by thickening of the lamina densa.
The
pathogenic mechanism of PIG is unknown. Hydronephrosis was found in three of 25
patients. However, no study has shown that experimental
hydronephrosis can induce podocytic infolding.In the large series from Japan,
all had GBM thickening, and IF was negative in many, followed by some that had
only C1q,c3 and some with a full house pattern. Membranous GN is the most common light microscopy
finding followed by FSGS and MPGN. In an earlier report, some
might have called this entity Membranous
glomerulopathy with spherules.
Some
researchers performed an immunohistochemical study of complement C5b-9
complexes in several human kidney diseases and have shown positive reactions on
round extracellular particles and on striated membranous structures in the GBM.
Therefore, the
mechanism of podocytic infolding might be related to the role of special types of
complement activation in situ on the microstructure. We don’t know if infolding of the podocyte cytoplasm may
be a pattern of GBM/podocyte disruption rather than a true disease entity. However,
the diffuse extent and severity of PIG raise the suspicion of defects in the
repair mechanism.
Worth
a read as we learn about this new entity!
Here
are some interesting references
