Figure 1918

Electron micrograph of a distal convoluted tubule cell. The apical surface of the cell displays some microvilli (Mv), but they are not sufficiently long or numerous to give the appearance of a brush border (compare with Fig. 19.15). The nucleus and Golgi apparatus (C) are in the upper portion of the cell. Mitochondria (M) are chiefly in the basal region of the cell within the interdigitating processes (IP). As in the proximal tubule cell, the mitochondria account for the appearance of basal striations in the light microscope. A basal lamina (BL) is seen adjacent to the basal surface of the cell, x 12,000.

Distal Convoluted Tubule

The distal convoluted tubule exchanges Na+ for K+ under aldosterone regulation

The distal convoluted tubule, located in the cortical labyrinth, is only about one third as long (~5 mm) as the proximal convoluted tubule. This short tubule is responsible for

• Reabsoiption of Na+ and secretion of K+ into the ultra-filtrate to conserve Na+

• Reabsorption of bicarbonate ion, with concomitant secretion of hydrogen ion, leading to further acidification of the urine

• Conversion of ammonia to ammonium ion, which then enters the urea cycle that counteracts the toxic effects of ammonia

Aldosterone, secreted by the adrenal gland and released under stimulation by angiotensin II, increases the reabsorption of Na! and secretion of K1. These effects increase blood volume and blood pressure in response to increased blood Na+ concentration.

Collecting Tubules and Collecting Ducts

The collecting tubules as well as the cortical and medullaiy collecting ducts are composed of simple epithelium. The collecting tubules and cortical collecting ducts have flattened cells, somewhat squamous to cuboidal in shape. The medullary collecting ducts have cuboidal cells, with a transition to columnar cells as the ducts increase in size. The collecting tubules and ducts are readily distinguished from proximal and distal tubules by virtue of the cell boundaries that can be seen in the light microscope.

Two distinct types of cells are present in the collecting tubules and collecting ducts:

• Light cells, also called collecting duct or CD cells, are the principal cells of the system. They are pale-staining cells with true basal infoldings rather than processes that inter-digitate with those of adjacent cells. They possess a single cilium and relatively few short microvilli (Fig. 19.19). They contain small, spherical mitochondria. These cells possess an abundance of ADH-regulated water channels, AQP-2, which are responsible for water permeability of the collecting ducts. In addition, aquaporins AQP-3 and AQP-4 are present within the basolateral membrane of these cells.

• Dark cells, also called intercalated (IC) cells, occur in considerably smaller numbers. They have many mitochondria, and their cytoplasm appears denser. Micropli-cae, cytoplasmic folds, are present on their apical surface, as well as microvilli. The microplicae are readily observed with the SEM but may be mistaken for mi-

0 0

Post a comment