Deposits in biological tissues are abnormal accumulations of various substances. These accumulations are not part of the normal cellular or extracellular matrix and can signify underlying medical conditions. When deposits form, they can disrupt the normal architecture and function of organs, potentially leading to disease progression. Understanding these deposits is important for diagnosing and managing various disorders.
Understanding Cellular Layers and Deposit Formation
The body is composed of various cellular layers that form barriers and linings within organs. Endothelial cells, for instance, form the inner lining of blood vessels, including capillaries within organs like the kidneys. These cells regulate the exchange of substances between the blood and surrounding tissues. Epithelial cells form the outer linings of organs and cavities, acting as protective barriers or specialized filtering units. In the kidneys, podocytes, a specific type of epithelial cell, wrap around the capillaries in the glomerulus, playing a role in filtering waste from the blood.
The term “sub” indicates a position beneath something. Therefore, “subendothelial” means beneath the endothelial cells, and “subepithelial” means beneath the epithelial cells. Deposits often arise from immune complexes, which are formations of antibodies bound to antigens. These immune complexes can become trapped in these distinct sub-cellular spaces, leading to inflammation and tissue damage. Other substances, such as proteins or complement components, can also contribute to these deposits.
Subendothelial Deposits
Subendothelial deposits are located beneath the endothelial cells that line blood vessels, particularly in the glomeruli of the kidneys. These deposits form between the endothelial cell layer and the glomerular basement membrane. Under an electron microscope, subendothelial deposits appear as electron-dense, irregular masses. They can distend the capillary lumen or lead to a “wire loop” appearance in the kidney’s filtering units.
These deposits are observed in conditions like Lupus Nephritis, a kidney inflammation caused by systemic lupus erythematosus, an autoimmune disease. In Lupus Nephritis, subendothelial deposits are a common finding, associated with more severe forms of the disease, such as Class III (focal proliferative) or Class IV (diffuse proliferative) lupus nephritis. Membranoproliferative Glomerulonephritis Type I is another condition where these deposits are found, characterized by changes in the glomerular basement membrane and cell proliferation. The composition of these deposits includes immune complexes, complement components, and various serum proteins.
Subepithelial Deposits
Subepithelial deposits are situated beneath the epithelial cells, specifically the podocytes, and on the outer aspect of the glomerular basement membrane in the kidneys. Under electron microscopy, these deposits present distinct appearances, such as “humps” or “spikes.” “Hump-like” deposits are large and rounded, protruding from the glomerular basement membrane towards the urinary space, and are seen with minimal associated basement membrane reaction. “Spikes” are projections of basement membrane material that grow around the deposits.
These deposits are seen in conditions like Post-streptococcal Glomerulonephritis, an inflammatory kidney disease that can follow a streptococcal infection. In this condition, “hump-like” deposits are a characteristic feature. Membranous Nephropathy is another disease associated with subepithelial deposits, where granular deposits are seen, interspersed by basement membrane spikes. C3 glomerulopathy, including C3 glomerulonephritis and dense deposit disease, can also present with subepithelial “hump-like” deposits.
Key Differences and Clinical Significance
Subendothelial and subepithelial deposits differ primarily in their precise location within the glomerular capillary wall and their characteristic microscopic appearance. Subendothelial deposits reside between the endothelial cells and the glomerular basement membrane, appearing as irregular, electron-dense masses that can cause widening of the subendothelial space. In contrast, subepithelial deposits are positioned between the podocytes and the glomerular basement membrane, presenting as distinct “humps” or “spikes” that protrude outwards.
These distinct locations and appearances are associated with different underlying disease processes. Subendothelial deposits are linked to diseases like Lupus Nephritis and Membranoproliferative Glomerulonephritis Type I, reflecting immune complex deposition within the vessel wall. Subepithelial deposits are seen in conditions such as Post-streptococcal Glomerulonephritis and Membranous Nephropathy, indicating immune complex formation or deposition on the outer aspect of the basement membrane. Differentiating these deposit types is important for accurate diagnosis, as it helps classify the specific kidney disease and informs the understanding of its potential progression.