The molecular structure and composition of each component of the glomerular filtration barrier has important consequences for adjacent components of the barrier. For instance, molecular changes in the GBM not only modify the contribution of this layer but also modify the rate at which solutes and solvents pass through the endothelium of glomerular capillaries on one side and the visceral layer of the Bowman’s capsule on the other. In addition, it is important to understand that glomerular filtration barrier is not a passive but an active structure that can remodel itself and modify its own permeability.
Simple squamous epithelium constitutes the parietal layer of Bowman’s capsule.
The parietal layer of Bowman’s capsule contains parietal epithelial cells and forms a simple squamous epithelium. At the urinary pole of the renal corpuscle, the parietal layer is continuous with the cuboidal epithelium of the proximal convoluted tubule (see Figs. 20.7 and 20.11). Proliferation of parietal epithelial cells is a typical diagnostic feature in certain types of glomerulonephritis (inﬂammation of the glomerulus). For an example of such a disease, see Folder 20.2. The space between the visceral and parietal layers of Bowman’s capsule is called the urinary space or Bowman’s space (see Fig. 20.11). It is the receptacle for the glomerular ultrafiltrate (primary urine) produced by the filtration apparatus of the renal corpuscle. At the urinary pole of the renal corpuscle, the urinary space is continuous with the lumen of the proximal convoluted tubule.
The renal corpuscle contains an additional group of cells called mesangial cells. These cells and their extracellular matrix constitute the mesangium. It is most obvious at the vascular stalk of the glomerulus and at the interstices of adjoining glomerular capillaries. Mesangial cells are positioned much the same as pericytes, in that they are enclosed by the GBM (Fig. 20.16). The mesangial cells are not confined entirely to the renal corpuscle; some are located outside the corpuscle along the vascular pole, where they are also designated as lacis cells and form part of what is called the juxtaglomerular apparatus (see Fig. 20.7).
Important functions of the mesangial cells follow:  Phagocytosis and endocytosis. Mesangial cells remove trapped residues and aggregated proteins from the GBM and filtration slit diaphragm, thus keeping the glomerular filter free of debris. They also endocytose and process a variety of plasma proteins including immune complexes. Maintaining the structure and function of glomerular barrier is the primary function of the mesangial cells. Structural support. Mesangial cells produce components of mesengial matrix, which provide support for the
FIGURE 20.14 • Diagram of the fltration slit diaphragm.