Histologic patterns of glomerular changes

When evaluating a renal biopsy we must follow a systematic process that prevents us missing a lesion in any of the four histologic compartments: glomeruli, interstitium, tubules or vessels. To decide the primary site of the injury is not an easy task: injuries in a part of nephron affect the other compartments. We can have a vascular injury that when advancing the process will finish affecting glomeruli, interstitium, and tubules; or the injury can begin in glomeruli, which will affect the remaining histologic structures. Each compartment displays more or less characteristic histopathologic features that guide the pathologist in the task of determining the primary site of injury.

Most of morphologic patterns of renal injury do not correspond with only one disease. In addition to the morphologic features we need clinical data, immunofluorescence, and, in many cases, electron microscopy to adequately classify the renal alterations.

Patterns of glomerular lesions

Glomerular changes can be focal (only in some glomeruli) or diffuse (in all or almost all the glomeruli), and segmental (only a part of the glomerulus) or global (the entire glomerulus). The first step in the evaluation of the glomerular alterations is to determine the extension of the affectation.

The glomeruli can display global sclerosis; when this happens can be impossible to determine the cause of the damage. Ischemia originated glomerular sclerosis have a characteristic pattern of retraction and collapse of capillary tuft (PAS-positive) surrounded by fibrotic tissue (PAS-negative) (Figure 1). Sometimes can be demonstrated rests of a crescent, or remaining fragments of tuft fibrinoid necrosis, or immune deposits in not completely sclerosed tufts, which can orient to a glomerulonephritis as cause of the glomerular sclerosis.

Figure 1. Globally sclerosed glomerulus with ischemic origin of the injury. Remaining material of the capillary tuft is seen more intensively stained with PAS (left) and methenamine-silver stains (right) because sclerosed glomeruli contain abundant type IV collagen. The fibrous tissue that surrounds the sclerosed capillary tuft proliferates from the capsule and is negative, or weakly positive, with PAS and silver stains because contains a different collagen, predominantly type I. (PAS, and methenamine-silver, X300).

The focal and segmental glomerulosclerosis is a relatively frequent lesion produced by multiple causes. Only when there are clinical findings of glomerulopathy and other glomerulonephritis are ruled out, it can be diagnosed as a specific disease: focal and segmental glomeruloesclerosis. This glomerular lesion can be, or no, accompanied by hyaline deposits (Figure 2); the importance of this type of deposits, respect to the pathogenesis, is uncertain (see the chapter of focal and segmental glomeruloesclerosis). When this injury is not associated to clinical expression of glomerulopathy or it is assumed that it is secondary to other renal or systemic alterations, perhaps we would call it with other name like glomerular scar.

Figure 2. Glomerulus with segmental sclerosis (left superior corner). There are segments with preserved architecture. Notice small hyaline segments (positive with the PAS stain, arrow); they are proteins and other components. (PAS, X400).

In the spectrum of focal and segmental glomeruloesclerosis there are several morphologic aspects that will be seen in the respective chapter, but two are very characteristic: tip lesion and collapsing lesion. The first is defined by the presence of at least one segmental lesion involving the tip domain with either adhesion between the tuft and Bowman’s capsule at the tubular lumen or neck, or confluence of podocytes with parietal or tubular epithelial cells, at the tubular lumen or neck, it can be seen simply like an adhesion of the capillary walls to the Bowman’s capsule at the origin of proximal tubule or like a hyaline segment with podocyte hypertrophy and/or foam cells (Figure 3). Collapsing lesion is characterized by tuft collapse, global or segmental, accompanied of noticeable hypertrophy and hyperplasia of podocytes (Figure 4).

Figure 3. Adhesion and sclerosis at the “tip domain” of the tuft, in other words, at the tubular lumen or neck in the Bowman’s capsule (arrows) (although the origin of tubule is not perfectly identified here). This lesion has been associated with a less aggressive course in some works, but we do not found this “good prognosis” in our Hispanic patients. (Masson’s trichrome, X400).

Figure 4. Almost global collapse of glomerular tuft. Notice the loss of capillary lumens and notorious proliferation of visceral epithelial cells (podocytes). Sometimes the proliferation is so intense that it can fill the Bowman’s space and be confused with an epithelial crescent. The collapsing lesion can be global or segmental. (methenamine-silver, X400).

Endocapillary proliferative glomerulonephritis

It is an increase of the number of tuft cells occluding the glomerular capillaries and usually accompanied by mesangial hypercellularity. The proliferating cells are endothelial, mesangial, and inflammatory cells infiltrating the tuft (leukocytes). The nature of cells contributing to glomerular hypercellularity is important pathogenetically, but the intraglomerular location of the cells is more important in terms of the diagnosis of GN. Glomeruli appear large, with expanded tuft, capillary lumens difficult to see or they are small, and there are noticeable increase of cells that have nuclei with variable size and form (Figures 5 and 6). When the inflammation is severe we can see tuft necrosis or karyorrhexis (nuclei fragmentation), fibrin (amorphous material that appear eosinophilic with H&E stain or red, due to the fuchsin, with the trichrome stain) and destruction of capillary walls (better demonstrated with silver stain) (Figure 7).

Figure 5. Global cellular proliferation with increase of the glomerular size; there is loss of capillary lumens and nuclei with variable aspect in form and size, suggesting that there are several proliferating cell types. In some cases the mesangial proliferation contributes to accentuate the lobules of the glomeruli: “lobulation”. (H&E, X400).

Figure 6. Severe cellular proliferation that obstructs the capillary lumen (endocapillary). Abundant polymorphonuclear leukocytes in the tuft: exudative form. This aspect suggests post-infectious GN. (H&E, X600).

Figure 7. Endocapillary proliferative GN with a segment of necrosis (among the arrows) that is eosinophilic (pink); it is accompanied by nucleus fragments (karyorrhexis) and polymorphous. In these lesions there is capillary wall rupture and they are frequently associated to crescents (H&E, X600).

Endocapillary hypercellularity is a true glomerulonephritis (GN) and when it is diffuse usually it is due to post-infectious GN, lupus nephritis, or membranoproliferative GN. Focal proliferativa GN (usually segmental) is, more frequently, due to lupus nephritis, IgA nephropathy, vasculitis, bacterial endocarditis, or other chronic infections.

If there are many polymorphonuclear leukocytes in the tuft, the lesion is known as exudative GN and it is suggestive of postinfectious GN or membranoproliferative GN (Figure 6).

Extracapillary proliferative glomerulonephritis (crescentic)

It is characterized by increased cellularity in the Bowman’s space due to proliferation of the parietal epithelium or infiltration of inflammatory cells. This lesion is termed crescent. By definition, it should be al least two cell layers thick and cover a third of the glomerular circumference (Figures 8, 9 and 10). It must be differentiated of podocyte proliferation that sometimes can produce a crescentic aspect. Crescents can be accompanied, or no, by endocapillary proliferation. Frequently crescents are associated to necrosis of the tuft and/or rupture of the capillary walls. When there is destruction of the Bowman’s capsule periglomerular granulomas can develop, but it do not indicate a granulomatous disease (Figure 10). Crescentic GN is diagnosed when more than 50% of the glomeruli are affected by extracapillary proliferation.

Figure 8. Extracapillary proliferation. Notice the collapsed tuft in the center of the microphotography. There is a clear space between tuft and the crescent. The green arrows point the Bowman’s capsule and the blue arrows point the internal edge of the crescent. This proliferation is formed by epithelial cells and mononuclear inflammatory cells. (H&E, X400).

Figure 9. Compressed tuft, in the center of the glomerulus, by a dense circumferential cellular proliferation that occupies the entire Bowman’s space. The arrows indicate rupture of capillary walls, a frequently associated phenomenon to the crescent. (Methenamine-silver, X300).

Figure 10. In this case the crescent has evolved with collagen proliferation, which is demonstrated by the fibres stained with silver stain: green arrows (they are also easily demonstrated with trichrome stain). The lesions in this stage are called fibroepithelial crescents. When the entire crescent has been replaced by fibrous tissue and it has not epithelial cells, is called fibrous crescent and it is a chronic, irreversible lesion. In the microphotography also there is a zone with Bowman’s capsule destruction: red arrow. (Methenamine-silver, X300).

Crescentic proliferation can be seen in anyone of the endocapillary proliferative GN or it can be found as pure extracapillary proliferation. In this last case usually is associated to (or due to) a small vessels vasculitis (Wegener’s granulomatosis, microscopic polyangiitis, or Churg-Strauss disease), or can be an idiopathic crescentic GN. Crescentic GN can be also seen in anti-glomerular basement membrane disease.

As seen, the term crescentic glomerulonephritis (crescents in more than 50% of glomeruli) is not a specific diagnosis, but, a manifestation of glomerular damage due to many causes.

Mesangial cellular proliferation

Mesangial hypercellularity has an ample rank, from slightest, difficult to demonstrate without histomorphometric analysis (by computer) until most severe forms with glomerular distortion or marked lobulation of the tuft (Figures 11 and 12). In many cases hypercellularity is accompanied by increase in the extracellular mesangial matrix (Figure 13). Mesangial proliferation can be produced by several mechanisms and can be an unspecific response to many glomerular insults. In many cases immune deposits in the mesangium are demonstrated. This pattern of injury can be seen in lupus nephritis (class II), IgA nephropathy, minimal change disease (and its possibly related: IgM nephropathy), focal and segmental glomerulosclerosis, some cases of membranous GN, in the rare diseases C1q nephropathy and mesangial GN with C3 deposits, resolving postinfectious GN, and others.

Figure 11. In this glomerulus we can see cellular proliferation in diverse mesangial areas. We consider proliferation when there are more than 3 nuclei in a mesangial area. The arrows point areas with increase of cellularity, in this case: mild proliferation. (H&E, X.400).

Figure 12. . Just as in the previous photo, we can see, in this image, cellular proliferation limited to the mesangium, with well preserved peripheral capillary walls and without obstruction of its lumens, although these appear diminished. Unlike to the previous one, in this photo we see a severe increase of the cellularity, which give a lobulated aspect to the tuft. See the relative homogeneity of nuclei and compare with the endocapillary proliferation in Figures 5 and 6. (H&E, X400).

Figure 13. With stains that highlight the mesangial matrix (silver and PAS) is notorious the increase of the extracellular components of the mesangium that usually accompany mesangial hypercellularity. Pure increase of the matrix (without cellular increase) can be seen in some types of glomerular injury. (PAS, X400).

Mesangial matrix increase without hypercellularity

Expansion of the mesangial matrix (production of components of the mesangial extracellular matrix) is a response to many injuries against glomeruli. Mesangial matrix expansion is also evidenced when there are pathologic deposits of proteins or circulating substances: fibrillary/immunotactoid glomerulopathy, amyloidosis, light and heavy chain deposition disease, and diabetes mellitus.

When there is disruption of the mesangial matrix with capillary lumen expansion (microaneurisms), the lesion is called: mesangiolysis, and it is more frequently demonstrated in haemolytic-uremic syndrome, diabetes and malignant hypertension (Morita T, et al. Mesangiolysis: an update. Am J Kidney Dis. 1998;31:559-73 [PubMed link]).

Glomerular basement membrane (GBM) alterations

GBM can be altered by immune or nonimmune mechanisms. The membranous transformation, seen in primary or secondary membranous GN, is the reaction of the GBM to subepithelial deposits (between GBM and the visceral epithelial cell). These deposits generate basement-membrane-like material production that tries to surround them leading to formation of perpendicular projections (or “cuplike” projections) that are seen as “spikes” (Figure 14).

Figure 14. GBM reaction to the subepithelial deposits is seen as perpendicular projections to the GBM in its external part: “spikes”, demonstrated here with silver stain (arrows). The immune deposits (no visible here) are between spikes. (Silver, X1000).

Subendothelial deposits induce circumferential extension of the mesangium into the glomerular capillary walls (mesangial interposition; it can include mesangial cells) that generates synthesis of GBM-like material. This neoformed material is located between the endothelial cell and the deposit or the interposed mesangial cell. Thus, a double contour of the basal membrane is formed, being the internal contour the neosynthesized one (Figures 15 and 16).

Figure 15. This case corresponds to type I membranoproliferative GN. There is cellular proliferation and images in double contour (sometimes termed: tram-tracking, splitting, or duplication of the GBM) (arrows). (Methenamine-silver, X400).

Figure 16. Many and extensive subendothelial deposits. The immune deposits are positive with the fuchsin used in trichrome stain. The double contour forms due to synthesis of basement-membrane-like material between the endothelial cells and the deposits. (Masson’s trichrome, X400).

Double contours are classically found in type I and III membranoproliferative GN, but it can be also seen in lupus nephritis, cryoglobulinemia and some diseases in which subendothelial immune deposits are not demonstrated: thrombotic microangiopathy and chronic transplant glomerulopathy (Figure 17).

Figure 17. Glomerulus from a kidney allograft with chronic transplant glomerulopathy. The double contours found in this alteration (arrows) are not caused by subendothelial immune deposits. Cellular proliferation is not seen in these cases. (Methenamine-silver, X400).

In the GBM is also possible find organized deposits as in amyloidosis (Figure 18) and fibrillary/immunotactoid glomerulopathy.

Figure 18. Acellular deposits in interstitium. Congo-red demonstrate positivity for amyloid. On the left we see the reddish colour that gives the staining, but the true positivity is marked by apple green birefringence when polarized lenses are used (right). (Congo-red stain, X200; on the left with normal lenses; on the right with polarized lenses).

Diffuse thinning of the GBM: thin basement membrane disease.
Irregularity and multilayering or splitting of the GBM
: Alport’s syndrome.
At present, these two diseases tend to be grouped with the name type IV collagen nephropathy.

Diffuse thickening of the GBM: diabetes mellitus, hypertension.

Ischemic changes in glomeruli can be demonstrated by retraction, wrinkling, and folding of the capillary walls; similar changes can be also observed in basement membranes of the tubules.

Immunopathologic patterns

In addition to immunoglobulins (Igs) and/or complement components deposited in a disease, the pattern of deposition is also important. Linear deposition in the capillary walls is observed in anti-GBM disease (mainly IgG) (Figure 19a), and unspecifically IgG in diabetes mellitus (Figure 19b). Granular parietal deposition of several Igs and complement is seen in membranoproliferative GN (subendothelials) (Figure 20a), in post-infectious GN (subepithelials) (Figure 20b), in membranous GN (subepithelial and intramembranous) (Figure 20c).

Figure 19a. Linear Positivity for IgG demonstrated by immunofluorescence. The capillary walls are seen well demarcated by a continuous, well-defined line. This pattern is observed in anti-GBM disease and in diabetes mellitus (IgG and albumin), being in this last case an unspecific immunostaining. In this image we see a crescent compressing the glomerular tuft (Immunofluorescence for IgG, anti-human-IgG antibodies marked with fluorescein, fluorescence microscopy, X400).

Figure 19b. Linear Positivity for IgG in diabetes mellitus: "pseudolinear" (Immunofluorescence for IgG, anti-human-IgG antibodies marked with fluorescein, fluorescence microscopy, X400).

Figure 20a. Granular subendothelial deposits. This pattern is seen in some glomerular diseases with immune complexes deposits. The photo corresponds to a case of lupus nephritis (Immunofluorescence for C3, anti-humn-C3 antibodies marked with fluorescein, fluorescence microscopy, X400).

Figure 20b. Granular positivity for C3 in a case of post-infectious GN. This pattern isdescribed as "bumps and humps" or "lumpy-dumpy", traditionally associated with postinfectious GN (Immunofluorescence for C3, anti-humn-C3 antibodies marked with fluorescein, fluorescence microscopy, X400).

Figure 20c. Granular positivity for IgG demonstrated by immunofluorescence. There are many “grains” of diverse size in the capillary walls (parietals). In some capilaries a "reticular" appearence can be seen. The photo corresponds to a case of membranous GN, with the more typical appearence of subepithelial deposits (Immunofluorescence for IgG, anti-humn-IgG antibodies marked with fluorescein, fluorescence microscopy, X400).


Mesangial deposits are granular and they can be found in IgA, IgM, and C1q nephropathies, in C3 mesangial GN, lupus nephritis class II, resolving postinfectious GN, and others.

Figure 21. IgA Positivity in mesangial areas in a case of IgA nephropathy. See that capillary walls are not marked by the immunostaining. Only there is a positivity that draws irregular areas that correspond to the mesangium. (Immunofluorescence for IgA, anti-human-IgA antibodies marked with fluorescein, fluorescence microscopy, X400).

Combinations of immunopathologic patterns can be found in several diseases with immune complexes deposition like lupus.

Negativity for immune deposits is also an important feature in renal pathology; this fact is evidenced in diseases as minimal change disease and crescenti pauciimmune GN that, by definition, does not have deposition of Igs or complement (or if there is deposition this is weak).

The immunoglobulin type, the complement fraction, or the light chain deposited are also important in the differential diagnosis.

Figure 22. Tubulointerstitial positivity for Igs or complement can be see in some autoimmune disease, mainly lupus, and exceptionally in other diseases. (Immunofluorescence for IgG, anti-human-IgG antibodies marked with fluorescein, fluorescence microscopy, X400).

Electron microscopy

Ultrastructure helps to determine very important features in the definition of glomerular diseases and its differential diagnosis.

The location of electron-dense deposits (generally immune) with respect to the GBM: subepithelials, subendothelials, intramembranous or mesangials (Figures 23 to 25).

Figure 23. Electron-dense subepithelial deposits in membranous GN (arrows). Notice also the projections of basement membrane-like material surrounding deposits. EM, original magnification, X6.000.

Figur3 24. Subendothelial deposits in membranoproliferative GN (red arrows). Notice also a capillary segment with double contour. EM original magnification, X6.000.

Figure 25. Mesangial electron-dense deposits in a case of IgA nephropathy (arrows). EM, original magnification, X6.000.

[Subepithelial deposits in membranous GN (link)] [Another image with subepithelial and intramembranous deposit (link)]

One important issue in ultrastructural assessment is the aspect of podocytes and foot process. A frequent alteration is foot process effacement or fusion; this feature can be found in any disease causing nephrotic syndrome, but it is a characteristic feature in minimal change disease (Figure 26).

Figura 26. Complete loss of podocyte processes (effacement or "fusion"). The blue arrow indicates an area of detachment (loss) of the podocyte. EM, original magnification, X8.000.

[Foot processes effacement or fusion (link)] [Another image in MCD (link)]

The characteristics of the GBM: diffuse thinning (thin basement membrane disease) (Figure 27), irregularities, multilayering and fragmentation (Alport’s disease) (Figure 28), and diffuse thickening (diabetes mellitus) (figure 29) are key diagnosis features in ultrastructural pathology.

Figura 27. Diffuse thinning of glomerular basement membranes. The homogeneous thin appearance suggests the alteration, however, it is necessary to measure its thickness and, of course, correlate with clinical features: this finding does not necessarily mean "disease". EM, original magnification, X4.000.

Figura 28. Glomerular basement membrane very irregular, with areas of thinning and other very thickened in a case of Alport disease. EM, original magnification, X10.000.

Figura 29. Diffuse thickening of the glomerular basement membrane (note the result of measuring its thickness) in a case of diabetes mellitus; This change, though unspecific, can help in the early diagnosis of diabetic nephropathy. ME, original magnification, X10.000.

Myelin figures in Anderson-Fabry’s disease, microfibrils in fibrillary/immunotactoid glomerulopathy, and other “almost specific” features are very important and constitute another reason for ultrastructural study in kidney pathology.

See the section: Electron microscopy in renal biopsies.