Figure 64

A, High-power view of the medulla showing the wide interstitium and interstitial cells, which are abundant, varied in shape, and arranged as are the rungs of a ladder. B, Renal interstitial cells. The interstitium contains two main cell types, whose numbers increase from the cortex to the papilla. Type I interstitial cells are fibroblastic cells that are active in the deposition and degradation of the interstitial matrix. Type I cells contribute to fibrosis in response to chronic irritation. Type II cells are macrophage-derived mono-nuclear cells with phagocytic and immunologic properties. Type II

Cortex

Outer medulla

Inner medulla

Fibroblastic cells

Fibroblastic cells

Pericytes

Mononuclear cells

Mononuclear cells

Lipid-laden cells

Mononuclear cells

cells are important in antigen presentation. Their cytokines contribute to recruitment of infiltrating cells, progression of injury, and sustenance of fibrogenesis.

In the cortex and outer zone of the outer medulla, type I cells are more common than are type II cells. In the inner zone of the medulla, some type I cells form pericytes whereas others evolve into specialized lipid-laden interstitial cells. These specialized cells increase in number toward the papillary tip and are a possible source of medullary prostaglandins and of production of matriceal glyco-saminoglycans. A characteristic feature of these medullary cells is their connection to each other in a characteristic arrangement, similar to the rungs of a ladder. These cells have a distinct close and regular transverse apposition to their surrounding structures, specifically the limbs of the loop of Henle and capillaries, but not to the collecting duct cells.

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