Purpose of review: For some time, the so-called vicious cycle has been believed to underlie progression of glomerular sclerosis. This mechanism describes a circumstance when loss of some glomeruli imposes injurious stress on the remnant glomeruli. Evidence from recent genetic approaches, however, has prompted revision of this classical view and now points toward a new direction of investigations.
Recent findings: Whereas experimental maneuvers that selectively injure mesangial cells have failed to induce glomerular sclerosis, genetic approaches that target visceral epithelial cells, or podocytes, in embryos and adult animals regularly produce glomerular sclerosis. Association between podocyte damage and glomerular sclerosis observed in many human diseases and animal models have identified podocyte injury as a common, if not universal, trigger leading to glomerular sclerosis. The process from podocyte injury to sclerosis is remarkably rapid, and the rate of progression depends upon the degree of initial podocyte injury. A single brief injurious stimulus on a podocyte activates a 'domino effect', whereby progressive damage of the initially hit podocyte spreads to involve cells that escaped the initial insult.
Summary: The mouse, a species highly useful for studying the function of specific gene products, is notoriously resistant to development of glomerular sclerosis in adulthood. However, recent genetic engineering in this species has overcome this disadvantage and brought about a new dimension to our understanding of the mechanisms involved in progressive glomerular sclerosis.