The distal tubule receives urine from the thin limb. Like the proximal tubule, the distal tubule is divided into two major sections. The ﬁrst is known as the thick ascending limb (also known as the distal straight tubule, or pars recta), and the second is known as the distal convoluted tubule (pars convoluta). Just before the transition to the distal convoluted tubule, the thick ascending limb touches its parent glomerulus, and the epithelial cells that make direct contact constitute a specialized structure known as the macula densa.
In short-looped nephrons, the thick ascending limb accounts for the entire ascending limb of the loop of Henle. In contrast, in long-looped nephrons, the thin limb accounts for the initial part of the ascending limb, then transitions to the thick ascending limb at the border of the inner and outer zones of the medulla.
Thick Ascending Limb
The thick ascending limb (TAL) plays an important role in the reabsorption of ions and is crucial for maintenance of the countercurrent multiplication system, described on Plate 3-12.
The cells of the TAL are low cuboidal. Their height decreases as the tubule progresses from medulla to cortex. Their apical surfaces are dotted with sparse, short microvilli. In the rat, there is a subset of “rough” cells, which have sparse microvilli, and “smooth cells,” which lack them altogether. The relative proportion of “rough” cells increases as the tubules approach the renal cortex. Because of the scarcity of microvilli, the distal tubules appear patent on light microscopy, facilitating the distinction from proximal tubules, which possess a well-developed brush border.
Below the apical surface are numerous small vesicles, which trafﬁc ion channels and transporters to the plasma membrane. The rough endoplasmic reticulum and Golgi apparatus, which synthesize these proteins, are prominent. The nuclei are located near the apical membrane and sometimes bulge out toward the lumen. The basolateral membranes are thrown into extensive, interdigitating processes and infoldings, which increase the surface area available for basolateral transport. As a result of this conﬁguration, the lateral cell borders appear indistinct on light microscopy sections. The basolateral processes are ﬁlled with mitochondria, which resemble striations on histologic sections, to provide energy for active transport. Interdigitating processes and infoldings from neighboring cells are joined together by tight junctions.
Unlike the rest of the distal tubular cells, the macula densa cells are columnar and lack extensive interdigitating basolateral processes. Their high nucleus to cytoplasmic ratio causes them to appear denser than neighboring cells. Their nuclei are positioned near their apical surface, above most of the cellular organelles. The basal surfaces of these cells interdigate with the adjacent extraglomerular mesangium through their shared basement membrane, reﬂecting their physiologic connection.
Distal Convoluted Tubule
At some distance after the macula densa, there is an abrupt transition from the low cuboidal cells of the TAL to the taller cuboidal cells of the distal convoluted tubule, which takes a tortuous course through a small area of cortex. The cells of the distal convoluted tubule have more numerous apical microvilli than those of the TAL, but their other features are similar.
At the end of the distal tubule, just before the transition to the collecting duct, there is a zone known as the connecting segment (or tubule). This segment lacks clear boundaries and mixes gradually with the previous and next segments. In general, however, the cells in this segment have less prominent interdigitating membrane processes and fewer mitochondria than those of the DCT. Principal and intercalated cells, which ﬁgure prominently i the collecting duct, begin to appear in this segment.