Spherules on the Glomerular Basement Membrane: An Ultrastructural Variant of Membranous Nephropathy with Antigenic Implications

Dear Editor,

Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults and is defined by diffuse subepithelial immune deposits along the glomerular basement membrane (GBM).¹ Advances in antigen discovery, including PLA2R, THSD7A, and NELL1, have enabled an antigen-based classification of MN.² Despite these molecular insights, ultrastructural morphology continues to provide important diagnostic and pathogenetic information. Rare reports have described subepithelial deposits composed of discrete, uniformly sized spherules resembling nuclear pore complexes on electron microscopy (EM).³

Microspherular MN is an uncommon ultrastructural variant described by Kowalewska et al., Choung et al., and Ren and Hou, who reported the largest cohort (26 cases).^3-5 These studies established clinicopathologic heterogeneity but did not systematically integrate antigenic profiling with ultrastructural staging. A related differential diagnosis is podocyte infolding glomerulopathy (PIG), a rare entity characterized by podocyte cytoplasmic invaginations and intramembranous vesicular structures within the GBM, typically lacking classic membranous architecture.

We present a 16-case series of MN with spherular deposits, and provide systematic clinicopathologic, antigenic, and ultrastructural characterization of this uncommon variant, emphasizing its distinction from PIG.

We retrospectively searched renal biopsy archives (2012–2022) for cases diagnosed as MN on light microscopy and immunofluorescence that demonstrated spherular deposits on EM. Sixteen biopsies met the inclusion criteria. Clinical and laboratory data were retrieved from records. Antinuclear antibody (ANA) testing and viral serology (HBsAg and HCV) were performed in all cases. Extended autoimmune testing (ENA profile, complement levels, rheumatoid factor, and Sjögren’s serology) was performed when clinically indicated but was not uniformly available due to the retrospective design. Immunohistochemistry (IHC) for PLA2R, THSD7A, NELL1, and EXT1 was available in 12 cases. EM was performed on glutaraldehyde-fixed tissue with epoxy embedding, and spherule diameters were measured on high-magnification images.

The cohort comprised 16 patients (10 male, 6 female) with a mean age of 48 ± 12 years (range 12–85). All presented with nephrotic-range proteinuria or nephrotic syndrome, with preserved renal function in 12 of 14 evaluable patients. Positive serological findings were identified in 2 of 16 patients (12.5%), but none had clinical features fulfilling diagnostic criteria for systemic lupus erythematosus, rheumatoid arthritis, or Sjögren’s syndrome at the time of biopsy or during available follow-up. Baseline clinical and morphologic features are summarized in Supplementary Table 1.

Light microscopy demonstrated diffuse capillary-wall thickening with epimembranous spikes. Immunofluorescence showed granular capillary-wall deposition of IgG (2–3+) with frequent C3, without light-chain restriction or extraglomerular deposits, fulfilling established criteria for MN.

EM revealed uniform spherical deposits in subepithelial and intramembranous locations. These spherules measured 25–130 nm (mean ∼90 nm), were discrete and homogeneous, and lacked the cylindrical morphology, central channel, or substructure characteristic of nuclear pores. Podocyte foot processes were diffusely effaced. Control MN cases lacked this phenotype. No case demonstrated podocyte cytoplasmic infolding, GBM invagination, or intramembranous microvesicular structures characteristic of PIG. Representative ultrastructural findings are shown in Figure 1.

Close

Figure 1: Composite electron micrographs of spherular deposits in membranous nephropathy. (a) Low-power EM shows uniform spherular subepithelial deposits decorating the glomerular basement membrane. (b) Higher magnification reveals aggregates of spherical deposits embedded within the thickened GBM, accompanied by diffuse podocyte foot-process effacement. (c) Measurement of individual spherules demonstrates diameters in the 80–100 nm range, highlighting their discrete and homogeneous nature. (d) Another representative case illustrates dense clusters of uniform spherules replacing classical amorphous immune deposits in the subepithelial zone.

Export to PPT

IHC (12 cases) showed PLA2R positivity in 8 cases and THSD7A positivity in 2 cases, while all tested cases were negative for NELL1 and EXT1. Thus, a substantial subset expressed known MN antigens, whereas the remainder were antigen-negative. Limited follow-up (median 18 months) showed partial remission in 7 patients, complete remission in 2, and persistent proteinuria in 7.

Based on ultrastructural assessment, cases were classified according to Ehrenreich-Churg staging as Stage I (n = 5), II (n = 6), and III (n = 5); no cases showed Stage IV changes. Spherular deposits were identified across both early subepithelial and later intramembranous phases, indicating that this morphology represents a persistent ultrastructural phenotype rather than a transient developmental stage.

This study demonstrates a reproducible ultrastructural variant of MN in which spherules replace classical amorphous deposits. Earlier reports suggested possible entrapment of nuclear pore complexes,³ but our observations argue against this interpretation. Nuclear pores are 80–120 nm cylindrical structures with a central channel and eight-fold symmetry, whereas the deposits in our cohort were spherical, homogeneous, and lacked substructure, consistent with prior studies.^3-5

The relationship to antigen status is noteworthy. Detection of PLA2R and THSD7A in a substantial subset confirms that spherular morphology occurs within bona fide immune-complex mediated MN rather than representing a separate disease process. However, several cases were antigen-negative. In the context of expanding antigen discovery in MN,^2,6,7 this raises the possibility that the spherular phenotype may be associated with an as-yet unidentified antigen. Thus, this study uniquely integrates ultrastructural morphology with antigenic profiling in microspherular MN.

Alternative explanations include immune complexes with distinct physicochemical properties independent of antigen specificity. A parallel exists in antenatal MN due to anti-neutral endopeptidase antibodies, where recurrent disease supports a circulating antigen-driven mechanism.⁸

Clinically, patients resembled primary MN, with nephrotic syndrome, preserved renal function, and the absence of convincing secondary associations, similar to idiopathic MN.⁹ Importantly, none developed clinical evidence of systemic autoimmune disease during available follow-up, reducing the likelihood of clinically significant secondary MN despite incomplete serologic evaluation.

Recognition of this pattern has diagnostic implications. Misinterpretation as nuclear pores or an artifact is possible; awareness may prevent under-recognition and may prompt expanded antigen testing in antigen-negative MN.

This condition needs to be distinguished from PIG, which is characterized by podocyte cytoplasmic infoldings, intramembranous vesicular inclusions, and variable immune-complex association.¹⁰ None of these features were identified in our cohort. Instead, all cases demonstrated classic membranous nephropathy morphology, supported by antigen positivity in a substantial subset, confirming classification as MN rather than podocytopathic infolding pathology.

The limitations of this study include a retrospective design, a modest sample size, incomplete antigen testing, a lack of uniform extended autoimmune serologic workup, and the absence of proteomic analysis. However, the consistent ultrastructural phenotype across 16 cases supports a reproducible entity.

Future studies should include multicenter cohorts, broader antigen screening, and mass spectrometry-based antigen discovery to determine whether spherular deposits represent a marker of a novel antigen. In an era dominated by antigen-based classification, ultrastructural morphology remains a valuable discovery tool. Spherular deposits represent a distinctive ultrastructural manifestation within membranous nephropathy and should be distinguished from podocyte infolding glomerulopathy.