FIGURE 20.11 • Transmission electron micrograph of a glomerulus in the region of the urinary pole. The nuclear and perinuclear regions of the endothelial cells (EC) that line the glomerular capillaries (C) bulge into the vascular lumen. On the outer surface of the capillaries are the processes of the podocytes (P). External to the podocytes is the urinary space (U). Bowman’s capsule (BC) is shown on the left; it is continuous at the dashed line (marked by arrowheads) with the tubule cells of the proximal tubule (PTC). Note the numerous mitochondria (M) in the base of these cells and the brush border (BB) at the apex, projecting into the urinary space. The nuclei of three adjacent mesangial cells (MC) can be seen in the upper right of the micrograph. 4,700.
protein is reabsorbed by endocytosis in the proximal convoluted tubule. Albuminuria (presence of significant amounts of albumin in the urine) or hematuria (presence of significant amounts of red blood cells in the urine) indicate physical or functional damage to the GBM. In such cases (e.g., diabetic nephropathy), the number of anionic sites, especially in the lamina rara externa, is significantly reduced.
The filtration slit diaphragm acts as a size-selective filter.
The narrow slit pores formed by the foot processes of podocytes and the filtration slit diaphragms act as physical barriers to restrict the movement of solutes and solvents across the filtration barrier. The discovery of specific proteins that form the slit diaphragm has led to new insights into the function of the filtration apparatus in the kidney. Most of the proteins found in the diaphragm are crucial for normal development and function of the kidney. The slit diaphragm architecture accounts for a true size-selective filter properties, which determine the molecular sieving characteristics of the glomerulus. Several mechanisms prevent clogging of the filtration slit diaphragms. These include, the negative charges of glycosaminoglycans in the GBM, negative charges of the podocyte cell membrane, and the phagocytic function of mesangial cells in the renal corpuscle.
FIGURE 20.12 • Scanning electron micrograph of a glomerulus. a. Low-magnification image revealing the tortuous course of the podocyte-covered glomerular capillaries. 700. b. A higher magnification of the area in the rectangle in a. Note the podocyte and its processes embracing the capillary wall. The primary processes (1 ) of the podocyte give rise to secondary processes (2 ), which in turn give rise to the pedicels. The space between the interdigitating pedicels creates the slit pores. 14,000. Inset. This higher magnification of the area in the rectangle reveals the slit pores and clearly shows that alternating pedicels belong to the secondary process of one cell; the intervening pedicels belong to the adjacent cell. 6,000.
FIGURE 20.13 • Transmission electron micrograph of a glomerular capillary and adjacent podocyte. The pedicels of the podocytes rest on the basal lamina adjacent to the capillary endothelium, and together, the three components—capillary endothelium, basal lamina, and podocyte—form a filtration apparatus. 5,600. Inset. The large arrows point to the fenestrations in the endothelium. On the other side of the basal lamina are the pedicels of the podocytes. Note the slit diaphragm (small arrows) spanning the gap between adjacent pedicels. 12,000.
Changes in different components of the filtration apparatus influence the functions of one another.