An In Depth Look At the Collecting Tubule

1. An In Depth Look At the Collecting Tubule Group 30

2. Overview of the Nephron: • Each kidney is made up of approximately 1 million nephrons. Each nephron is composed of a renal corpuscle (a spherical filtering component) and a tubule extending from it. • The tubule extending from the renal corpuscle is made up of a single layer of epithelial cells lying on a basement membrane. The structure and histological characteristics of the tubule differ at different segments. From the renal corpuscle the filtrate travels through the tubule in this order: • Proximal Tubule → Henle’s Loop→ Distal Convoluted Tubule → Collecting Duct System

3. Structure of the Collecting Duct System: • From the Distal Convoluted Tubule the filtrate continues through short connecting tubules or arched collecting tubules in the cortex. These tubules continue on to the medullary ray where they become collecting ducts. • Collecting ducts within the cortex= cortical collecting ducts. • When cortical collecting ducts reach the medulla they are known as medullary collecting ducts. • These ducts continue on to the apex of the pyramid and converge to make larger collecting ducts known as papillary ducts that open into the minor calyx.

4. The Structure of the Collecting Duct System (continued)… • Composed of a simple epithelium • Collecting tubules and cortical collecting ducts consist of cuboidal cells. • The medullary collecting ducts are composed of cuboidal cells that transition into columnar cells. • The collecting tubule/duct system is characterized from other parts of the tubule by clear intercellular boundaries as seen via the light microscope.

5. The Two Cell Types of the Collecting Duct System: • Principal Cells: • Major cell type of the collecting duct system • Pale staining • True basal infoldings • Single cilium and a few short microvilli • Small, spherical mitochondria • Have ADH-regulated water channels • Become taller as the duct pass from the from outer to inner medulla • Intercalated Cells: • Dark cells • Only represent a small percentage of cells in the collecting duct system • Contain microplicae (cytoplasmic folds on the apical surface) and microvilli • Do NOT have basal infoldings but do have interdigitations with neighboring cells. • Vesicles in the apical cytoplasm • Number of intercalated cells decrease from outer to inner medulla until none are present in the renal papilla.

6. EM of Collecting Duct: • The astericks represent intercalated cells with their microplicae • The principal cells can be distinguished by their single cilium projecting into the lumen of the tubule.

7. EM of Collecting Ducts: • View of simple cuboidal epithelium lining the collecting ducts. • Note the interdigitations between cells to increase surface area for fluid reabsorption.

8. Functions of the Collecting Duct System: • Na+ Reabsorption • K+ Secretion • K+ Reabsorption • Active Transport of Na+ • Acid-Base Balance • ADH-dependent water reabsorption

9. Ion transport in the Collecting Duct Aldosterone

10. Na+/K+ Transport in Collecting Duct Differences in the potassium excretion over the normal physiological range are due mostly to differences in the amount of K+ secreted by the principal cells. This secretion involves active transport of K+ into the cell across the basolateralmembrane and passive exit across the luminal membrane. Increased extracellular [K+] stimulates aldosterone production. Aldosteroneincreases potassium secretion by stimulating the Na/K ATPases and the potassium channels in principal cells. It also stimulates the lumenal Na+ channels which is crucial for supplying the Na/K pump; the more Na+ in the cell, the more K+ may be secreted.

11. Acid/Base Regulation in the Collecting Duct • Intercalated cells secrete hydrogen ions via H-ATPases and H+/K+ exchangers in the apical membrane • Secreted H+ combines with filtered HCO3 to form CO2 and water in the lumen. HCO3 generated intracellularly is transported to the interstitium • Regulating the amount of Bicarbonate reabsorption/secretion is critical in maintaining plasma pH.

12. Regulation of ADH Secretion • Osmorecepters in the macula densa and hypothalamus sense osmolality. Baroreceptors in the cardiac atria and aortic arch sense pressure. Both the osmoreceptors and the baroreceptors give signals to the hypothalamus to either increase or decrease ADH secretion. • ADH travels to the kidney via the bloodstream and causes urine volume to decrease and urine concentration to increase.

13. Osmoreceptors and ADH secretion: As osmolarity increases, osmoreceptors send signals to the hypothalamus and ADH is released from the hypothalamus. As osmolarity decreases, there is a decrease in ADH secretion.

14. Baroreceptors and ADH secretion • Baroreceptors sense pressure. An increase in pressure in the left atrium causes baroreceptors to fire on an inhibitory neuron for ADH release. The result is decreased ADH secretion. • An increase in pressure results in a decrease in ADH secretion.

15. ADH causes increased water reabsorption in the Collecting Duct system via insertion of Aquaporins: • ADH is a peptide hormone. When secreted, ADH acts on the collecting tubule. Through a series of intracellular events, ADH causes the formation of microtubules into aquaporins. As a result, there is a marked increase in water permeability and water is reabsorbed.

16. LM Images of Collecting Ducts • Collecting ducts have a lightly staining cytoplasm and distinct intercellular borders. The epithelium is cuboidal in the cortex and transitions to taller/columnar cells in the medulla. CD Kidney odd 062A

17. More collecting ducts… CD CD

18. The collcting ducts are clearly visible here. They have lightly stained cytoplasm and distinct intercellular borders. • Surrounding the collecting ducts are thin segments of the Loop of Henle and vasa recta capillaries with RBCs inside them. • Picture from: http://www8.georgetown.edu/dml/educ/hist/lab11/14.htm