Splitting of the Glomerular Basement Membrane.

THE problem of the splitting of the glomerular basement membrane originated with the observations of Jones (I953, I957) on the histogenesis of membranous thickening, made possible by means of a new method of staining (silver-methenamine). In his opinion this thickening was caused by the splitting of the basement membrane due to the deposition of a hyaline substance between the two layers. Jones' hypothesis was based on the fact that the basement membrane is constructed normally of two layers, one epithelial and the other endothelial in origin, being separated by a 'virtual space'. This space could become apparent in some pathological conditions. This concept of splitting of the basement membrane is not generally accepted. Electron microscopists have not been able to demonstrate similar lesions. On the contrary, electron microscopy has shown that the basement membrane in normal and in pathological conditions appears to be a single entity. We believe that only one of the two layers, as shown by the Jones' stain, represents the true basement membrane while the other layer corrresponds to a newly laid-down argyrophilic substance with staining properties similar to those of the basement membrane, but with different electron density. Churg and Grishman (I959), in their studies on the membranous glomerular lesion, were able to demonstrate three different morphological entities. The first was due to the 'splitting' of the true basement membrane while the second, also called splitting, was caused by the deposition of a substance between the endothelial cells ('lamina fenestrata') and the basement membrane. The third type consisted of the deposition of material between the epithelial cells and the basement membrane. This material, of which the irregular appearance is caused by the spike-like protrusion of the cytoplasm of the epithelial cells, was recognized by Movat and McGregor (I959) as being the basic lesion of the chronic membranous glomerulitis of Allen (I95I). Associated with the splitting is the presence of cellular elements scattered between the two layers.