Electron Microscopy in Renal Pathology
Moderators: Dr. J. Allan Tucker and Dr. Jahn M. Nesland
Section 3 -
Ultrastructural Evaluation of Renal Allograft Biopsies
J. Allan Tucker
University of South Alabama
Mobile, AL USA
Renal transplantation has become a common practice at many centers. Needle biopsies of renal
allografts are frequently performed for allograft dysfunction and sometimes for surveillance. Table 1
lists some of the causes of renal allograft dysfunction to be considered in the evaluation of a renal
Table 1 - Evaluation of Renal Transplant Biopsies
Adapted from reference 1
- Acute Tubular Necrosis
- Hyperacute Rejection
- Acute Antibody-Mediated Rejection
- Acute Cellular Rejection
- Chronic Rejection
- Drug Toxicity/Reaction
- Hypertensive Renal Disease
- Recurrent/De Novo Glomerular Disease
Electron microscopy may, at times, contribute to the diagnosis in all of these areas. Electron
microscopy particularly contributes to the evaluation of the following four categories:
- Chronic rejection
- Recurrent/de novo glomerular disease
Rejection is an important cause of renal allograft dysfunction and a common point of evaluation of
renal allograft biopsies. Many centers apply the criteria of the Banff 97 working classification of
renal allograft pathology .
By Banff 97 criteria, acute cellular rejection is recognized as a combination of interstitial
inflammation and infiltration of the tubular epithelium by lymphocytes (tubulitis) and/or infiltration of
the arterial intima by lymphocytes (intimal arteritis). The criteria for acute/active rejection are
provided in table 2.
Table 2 - Acute/Active Rejection 
|Type (Grade) ||Histopathological findings|
| IA ||Cases with significant interstitial infiltration (>25% of parenchyma affected) and foci of moderate tubulitis (>4 mononuclear cells/tubular cross section or group of 10 tubular cells)|
| IB ||Cases with significant interstitial infiltration (>25% of parenchyma affected) and foci of severe tubulitis (>10 mononuclear cells/tubular cross section or group of 10 tubular cells)|
| IIA ||Cases with mild to moderate intimal arteritis (v1)|
| IIB ||Cases with severe intimal arteritis comprising >25% of the luminal area (v2)|
| III ||Cases with "transmural" arteritis and/or arterial fibrinoid change and necrosis of medial smooth muscle cells (v3 with accompanying lymphocytic inflammation)|
The Banff 97 classification also provides criteria for the evaluation of chronic/sclerosing allograft
nephropathy, which is based on a combination of the extent of interstitial fibrosis and of tubular
atrophy, as outlined in table 3 .
Table 3 - Chronic/Sclerosing Allograft Nephropathy 
| Grade ||Histopathological findings|
|Grade I ||Mild interstitial fibrosis and tubular atrophy without (a) or (mild) with (b) specific changes suggesting chronic rejection|
|Grade II ||Moderate interstitial fibrosis and tubular atrophy (a) or (b) (moderate)|
|Grade III ||Severe interstitial fibrosis and tubular atrophy and tubular (severe) loss (a) or (b)|
Chronic rejection can result in these patterns of chronic/sclerosing allograft nephropathy, but a
number of other disease states can produce similar changes, including renal ischemia, hypertension, drug
effects, infection, increased ureteral pressure, and non-immune inflammatory processes .
Chronic/sclerosing allograft nephropathy, then, is a general term which does not specifically indicate
chronic rejection. Each grade includes subdivision as (a) or (b) depending on whether specific changes
suggesting chronic rejection are identified. By light microscopy, two major specific changes can be
seen. The first is a characteristic arteriopathy consisting of arteriosclerotic-type thickening of the
arterial intima, which is nonspecific, but which assumes a picture indicative of chronic rejection when
mononuclear cells are present in the thickened intima. The other change which can be identified by light
microscopy is the presence of chronic transplant glomerulopathy, with alterations in the glomeruli that
include mesangial interposition which results in double contours of the basement membrane of peripheral
capillary loops on PAS-stained sections.
Electron microscopy can extend the ability to recognize chronic rejection in allografts with
chronic/sclerosing allograft nephropathy, especially as the characteristic arteriopathy of chronic
rejection is often not seen in affected allografts. Evaluation of the peritubular capillaries and the
glomeruli ultrastructurally can reveal early evidence of chronic rejection that is not otherwise
Peritubular capillaries normally exhibit a single, continuous basement membrane, and very slight
lamellation of the membrane is commonly seen. Ultrastructural evaluation of allografts, however, may
reveal extensive lamellation of the basement membrane of peritubular capillaries
lamellation can be seen in conditions other than chronic rejection and can sometimes be seen in native
renal biopsies. Nonetheless, a very prominent degree of lamellation in the proper setting is highly
indicative of chronic rejection. Ivanyi et al. concluded that when seven or more layers of basement
membrane were identified, the finding was quite specific for chronic rejection . Even if a portion of
renal cortical tissue put aside for electron microscopy from an allograft biopsy does not yield any
glomeruli, evaluation of the peritubular capillaries can be a useful and often the sole method of
demonstrating specific changes of chronic rejection, changing a chronic/sclerosing allograft nephropathy
grade from an "a" to a "b" subtype.
While fully developed chronic transplant glomerulopathy can be identified by light microscopy. In
such cases, electron microscopy reveals mesangial interposition and/or subendothelial lucency/flocculent
Also, as with the peritubular capillaries, distinct lamellation of the glomerular
basement membrane may be seen . Glomeruli with early, mild chronic transplant glomerulopathy,
however, often appear completely unremarkable by light microscopy. Some allograft biopsies with
unremarkable-appearing glomeruli by light microscopy will reveal areas of early mesangial interposition,
with mesangial cell cytoplasm extending from the paramesangium into a portion of the capillary loop, and
may similarly reveal areas of mild subendothelial accumulation of lucent to flocculent material. Again,
these early changes, undetectable by light microscopy, may be the only factor available to result in the
change of the chronic/sclerosing allograft nephropathy grade from an "a" to a "b."
Renal allografts are susceptible to infection, including viral infection. One common type of viral
infection in these allografts is BK polyomavirus infection . The renal tubular epithelial cells are
particularly prone to this infection, and infected cells will sometimes display nuclear enlargement with
smudging of the chromatin. Immunohistochemistry can confirm the presence of BK polyomavirus infection.
Ultrastructurally, viral particles can be seen in the nucleus and also sometimes in the cytoplasm when
the particles leak from the nucleus. These particles are generally round and crowded and measure about
40 to 45 nm . One particularly useful morphologic feature for ultrastructural identification is that
the particles will sometimes form crystalline arrays. Some laboratories uniformly perform
immunohistochemical staining for this virus in allografts, and electron microscopy is often only
confirmatory. Not all cases of infection, however, will produce the characteristic nuclear changes by
light microscopy, and immunohistochemical stains, while sensitive, will not always yield optimal slides.
Electron microscopy, then, may at times be a primary method of diagnosis. Further, electron microscopy
may reveal other viral particles as well, such as cytomegalovirus
Ultrastructural examination may from time to time reveal any number of additional miscellaneous
findings. For example, we have observed cases in which tubuloreticular inclusions were present in
endothelial cells. These structures are often associated with systemic lupus erythematosus or HIV
infection, though they are not specific . While some authors report that tubuloreticular inclusions
may be seen as a rare finding even in apparently normal kidneys , in our cases, the tubuloreticular
inclusions were common and were seen in subsequent biopsies from the same patient as well. These
patients did not have a positive ANA or a positive HIV test, and the significance of this finding remains
to be seen, but such miscellaneous findings detected only with electron microscopy are no doubt of
significance in some instances.
Recurrent/De Novo Glomerular Disease:
A renal allograft is subject to all of the same diseases as a native kidney. Further, for a patient
undergoing transplantation for glomerulopathy, the original glomerular disease can sometimes recur in the
allograft. Electron microscopy can be extremely useful in these cases.
The following three cases are offered as an example of the importance of electron microscopy in the
diagnosis of de novo glomerular disease.
A 53 year old woman who was 6 years status post renal transplantation for diabetes mellitus presented
with a rising creatinine from 2.3 to 4.0.
On routine sections, the glomeruli exhibited a possible slight increase in mesangial matrix but
otherwise appeared unremarkable. On PAS-stained sections, a suggestion of an increased mesangial matrix
was also seen, and, in addition, definite glomerular basement membrane duplication was seen. The
findings, then, indicated chronic transplant glomerulopathy, as might be expected in this allograft.
Ultrastructural examination revealed very dramatic mesangial interposition. In addition, however,
numerous subepithelial to intramembranous deposits were seen, some of which exhibited a microspherical
substructure. The findings were conclusive for membranous glomerulopathy which was superimposed on
chronic transplant glomerulopathy.
A 62 year old woman who was 5 months status post renal transplantation for adult polycystic kidney
disease presented with a rise in creatinine and proteinuria.
The glomeruli exhibited mild hypercellularity and an increase in mesangial matrix. PAS stain further
demonstrated areas of apparent complexity of the glomerular basement membrane. The light microscopic
findings, then, were suggestive of chronic transplant glomerulopathy, but the time course did not fit
well for this entity.
By electron microscopy, mesangial interposition was indeed present, but in addition striking
subendothelial and mesangial deposits were seen. Immunofluorescence staining was performed, revealing
positive staining for IgG and C3. No tubuloreticular inclusions were seen, and the patient did not have
a positive ANA. The findings, then, were diagnostic of de novo membranoproliferative glomerulonephritis.
The patient was a 49 year old man who was 15 years status post renal transplantation. The "underlying
disease was unknown." The patient presented with a rise in creatinine and nephrotic range proteinuria (6
By light microscopy, the glomeruli appeared normocellular but exhibited some consolidation, probably
from an increase in mesangial matrix, and some exhibited segmental to global glomerulosclerosis.
Arteriolar thickening was noted. PAS-stained sections revealed similar findings as well as some possible
basement membrane duplication. The findings, then, again indicated chronic transplant glomerulopathy and
In the portion of tissue for electron microscopy, a glomerulus was not identified, but ultrastructural
examination was performed. The tubules exhibited striking thickening of the tubular basement membrane,
with areas measuring up to six microns in thickness. Though this finding is not specific, it raises the
possibility of diabetes mellitus. Further, a minute portion of a somewhat crushed glomerulus was present
at the edge of the tissue section which was not identified on the one micron sections. Examination of
these capillary loops reveal no mesangial interposition, but marked thickening of the glomerular basement
membrane was observed, with the thickness uniformly measuring greater than 1,000 nm. This finding
indicated diabetes, which would also account for the arteriolar thickening. The clinician was contacted,
and upon further review of the history, it was identified that the patient did have diabetes mellitus.
The case, then, represented diabetic nephropathy occurring in an allograft.
Because of the important contributions of electron microscopy in the evaluation of these allograft
biopsies, electron microscopy is performed routinely on these samples at our institution. Electron
microscopy proves particular useful in the following areas:
- Identification of
specific features of chronic rejection
- Lamellation of basement membranes of peritubular capillaries
- Chronic transplant glomerulopathy
BK polyomavirus and other viral diseases
- Recurrent/de novo
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- Herrera GA, Isaac J, Turbat-Herrera EA. Role of electron microscopy in transplant renal pathology. Ultrastruct Pathol. 1997; 21: 481-498.
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- Haas M, Aronson AJ, Bartosh SM. Progressive postinfectious glomerulonephritis with multiple tubuloreticular inclusions in an HIV-negative patient. Am J Kidney Dis. 1997; 30: 725-728.
- Ghadially FN. Microtubule group: microtubuloreticular structures (lupus-type and others). In: Ultrastructural Pathology of the Cell and Matrix, 4th Ed. Boston: Butterworth-Heinemann; 1997: 524-537.