S interstitial cells

The connective tissue of the kidney parenchyma, called interstitial tissue, surrounds the nephrons, ducts, and blood and lymphatic vessels. This tissue increases considerably in amount from the cortex (where it constitutes approximately 7% of the volume) to the inner region of the medulla and papilla (where it may constitute more than 20% of the volume).

In the cortex, two types of interstitial cells are recognized: cells that resemble fibroblasts, found between the basement membrane of the tubules and the adjacent peritubular capillaries, and occasional macrophages. In their intimate relationship with the base of the tubular epithelial cells, the fibroblasts resemble the subepithelial fibroblasts of the intestine. These cells synthesize and secrete the collagen and glycosaminoglycans of the extracellular matrix of the interstitium.

In the medulla, the principal interstitial cells resemble myofibroblasts. They are oriented to the long axes of the tubular structures and may have a role in compressing these structures. The cells contain prominent bundles of actin filaments, abundant rough endoplasmic reticulum (rER), a well-developed Golgi complex, and lysosomes. Prominent lipid droplets in the cytoplasm appear to increase and decrease in relation to the diuretic state. Some evidence suggests that these cells may secrete a hormonelike material that reduces blood pressure, but this sub-

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