physical barrier to the passage of large proteins from the blood to the uri-
nary space. Glycosaminoglycans, particularly heparan sulfate, produce a
polyanionic charge that binds cationic molecules. The foot processes are
coated with a glycoprotein called podocalyxin, which is rich in sialic acid
and provides mutual repulsion to maintain the structure of the filtration
slits. It also possesses a large polyanionic charge for repulsion of large
anionic proteins. Patients with a mutation in the gene encoding for nephrin
suffer from proteinuria resulting in extensive edema. This syndrome is
known as congenital nephritic syndrome. Nephrin is the key protein com-
prising the slit diaphragm; it functions to inhibit the passage of molecules
through the filtration slits. It is an integral membrane protein, which is
anchored by other proteins to the cytoskeleton of the pedicel of the
podocyte.
257.The answer is e.(Junqueira, pp 385–386, 392–393. Kumar, pp
996–1004.)Tubulointerstitial nephritis uveitis (TINU) is an autoimmune
disease in which autoantibodies are targeted against the renal tubular cells.
The transmission electron micrograph illustrates a proximal convoluted
tubule cell, the primary site for reduction of the tubular fluid volume by
reabsorption from the glomerular filtrate. The elaborate microvilli at the
apical surface and the extensive endocytic vacuoles are “designed” for pro-
tein reabsorption and are the distinguishing features of proximal convo-
luted tubule cells. Numerous mitochondria, within basal folds, provide
energy for transport. In the distal tubule, there are very few microvilli.
The afferent arterioles contain the juxtaglomerular cells, modified arte-
rial smooth muscle cells that produce renin (answer d),a major factor in
blood pressure regulation. The thin loop of Henle is responsible for the
production of the countercurrent multiplier (answer b),which allows the
kidneys to produce a hyperosmotic medulla. The multiplier moves Na+and
Cl−out of the ascending limb (which is impermeable to water) and into the
medullary interstitial fluid. Subsequently, the descending limb, which is
permeable to water, takes up the Na+and Cl−from the interstitium. The
vasa rectae adjust their osmolarity to that of the medulla.
The distal convoluted tubule (DCT) has the highest concentration of
Na+, K+-ATPase. The DCT pumps Na+against a concentration gradient and
is relatively impermeable to water, leading to the production of a hypotonic
tubular fluid. The distal tubules empty into the connecting and collecting
ducts, which are permeable to water under the regulation of ADH (answer a).
Urinary System Answers 393