What is the functional significance of fenestrations in glomerular capillaries, and which cells contribute to the filtration barrier?

• A. Fenestrations Block All Filtration; Barrier Includes Keratinocytes
• B. Fenestrated Endothelium Allows Selective Filtration; Barrier Comprises Endothelial Fenestrations, Basement Membrane, And Podocyte Filtration Slits
• C. Fenestrations Have No Role; Barrier Is Mesangial Matrix Only
• D. Fenestrations Cause Leakage; Barrier Includes Smooth Muscle Cells

Answer: (B)

The main idea is that filtration across the glomerulus is a three-layer process, with the endothelial fenestrations playing a key opening role but not acting alone. The fenestrated endothelium allows plasma water and small solutes to pass into Bowman's space while keeping blood cells from crossing. However, the barrier that truly determines what gets filtered is composed of three parts: the endothelial fenestrations, the glomerular basement membrane, and the filtration slits between podocyte foot processes (the slit diaphragms). The basement membrane provides a physical and charge-based barrier, helping restrict large and negatively charged proteins, while the slit diaphragms of podocytes add another layer of size and charge selectivity. Together, these components permit rapid filtration of small solutes and water but prevent passage of cells and most proteins. Mesangial cells support the capillaries and regulate flow, but they are not the main part of the filtration barrier.