Figs. 33.2 33.3 ). The distal tubule begins a short distance beyond the macula densa and extends to the point in the cortex where two or more nephrons join to form a cortical collecting duct. The cortical collecting duct enters the medulla and becomes the outer medullary collecting duct and then the inner medullary collecting duct.
Each nephron segment is made up of cells that are uniquely suited to perform specific transport functions (see
Fig. 33.3 ). Proximal tubule cells have an extensively amplified apical membrane (the ultrafiltrate or urine side of the cell) called the brush border, which is present only in the proximal tubule. The basolateral membrane (the interstitial or blood side of the cell) is highly invaginated. These invaginations contain many mitochondria. In contrast, the descending and ascending thin limbs of the loop of Henle have poorly developed apical and basolateral surfaces and few mitochondria. The cells of the thick ascending limb and the distal tubule have abundant mitochondria and extensive infoldings of the basolateral membrane.
The collecting duct is composed of two cell types: principal cells and intercalated cells. Principal cells have a moderately invaginated basolateral membrane and contain few mitochondria (see
Fig. 33.3 ). Principal cells play an important role in reabsorption of NaCl (see ) and secretion of K+ (see ). Intercalated cells, which play an important role in regulating acid-base balance, have a high density of mitochondria (see
Fig. 33.3 ). One population of intercalated cells secretes H+ (i.e., reabsorbs HCO3 −), and a second population secretes HCO3 − (see ). The final segment of the nephron, the inner medullary collecting duct, is composed of inner medullary collecting duct cells, which have poorly developed apical and basolateral surfaces and few mitochondria.
All cells in the nephron except intercalated cells have in their apical plasma membrane a single nonmotile primary cilium that protrudes into the tubule fluid (
Fig. 33.4 ). Primary cilia are mechanosensors (i.e., they sense changes in the rate of flow of tubule fluid) and chemosensors (i.e., they sense or respond to compounds in the surrounding fluid), and they initiate Ca++-dependent signaling pathways, including those that control kidney cell function, proliferation, differentiation, and apoptosis (i.e., programmed cell death).