Interdigitating foot processes bridged by slit diaphragms
Expansion
Structure: a highly specialised visceral epithelial cell of Bowman’s capsule, with a cell body, major processes, and foot processes (pedicels) that interdigitate with those of neighbouring podocytes, wrapping the glomerular capillary.
The slit diaphragm bridges the gaps between foot processes and is the final size barrier of filtration. Its key proteins are nephrin and podocin.
Podocytes are terminally differentiated and cannot divide, so once lost they are not replaced. That single fact explains why glomerular injury tends to be progressive and why podocyte loss correlates with irreversible decline.
Foot process effacement is the common final pathway of podocyte injury: the processes retract and flatten, the slit diaphragms are lost, and proteinuria results.
- Minimal change disease: normal light microscopy, negative immunofluorescence, and effacement only on electron microscopy. The reason the disease is named as it is
- Focal segmental glomerulosclerosis: effacement with segmental scarring in some glomeruli
- Congenital nephrotic syndrome: nephrin mutation, Finnish type
The complete barrier has three layers: fenestrated endothelium excluding cells, the basement membrane providing the negative charge barrier, and the podocyte slit diaphragms providing the size barrier.