Each kidney is covered by a thin fibrous capsule consisting mainly of dense, irregular collagenous connective tissue with occasional elastic fibers and smooth muscle cells [2]. The capsule, which passes inward at the hilum, forms the connective tissue covering of the sinus and becomes continuous with the connective tissue forming the walls of the renal pelvis and minor calyces. A hemisected view of the kidney reveals a darker, outer region, the cortex, and a paler, inner region, the medulla. The difference in color reflects the distribution of blood. About 90-95% of the blood passing through the kidney is in the cortex, the remaining 5-10% is in the medulla [4].In humans, the renal medulla consists of 10-18 conical or pyramidal structures, the medullary pyramids (also referred to as renal pyramids)which contain the nephric tubules. The apical portion - called the papilla- of each medullary pyramid projects into a minor calyx. The tip of the papilla is perforated with the openings of 10-25 papillary ducts, the final collecting ducts of the uriniferous tubules, and is therefore called the area cribrosa (Latin for sieve-like) [4].Parallel arrays of tubules, the medullary rays, arise from the base of the medullary pyramids and penetrate the cortex. Neighboring pyramids are separated from each other by material resembling the cortex, the cortical columns. The portion of the cortex overlying the base of each pyramid is known as a cortical arch. A renal pyramid, with its associated cortical arch and cortical columns, represents a lobe of the kidney [2].
The connective tissue of the kidney parenchyma is called interstitial tissue. It increases in amount from the cortex, where it constitutes about 7% of the volume, to the inner region of the medulla, where it constitutes about 20% of the volume [4]. Two kinds of interstitial cells are found in the cortex. The first kind, found between the basement membrane of the tubules and the adjacent peritubular capillaries (to be discussed), resembles fibroblasts and secretes the collagen and glycosaminoglycans of the extracellular matrix of the interstitium. The second type of cell is a macrophage [5].
The principal interstitial cells in the medulla resemble myofibroblasts. They contain prominent bundles of actin filaments, abundant rough ER, well-developed Golgi, lysosomes and lipid droplets [4]. It is thought these cells are the precursors of prostaglandins [5].
