Aquaporins (AQPs) are a recently recognized family of nodes, endothelial cells lining lymphatic sinuses, and on small, hydrophobic, transmembrane proteins that mediate the vascular endothelium of high endothelial venules as water transport in the kidney and other organs (i.e., liver, well as in the endothelial cells of intestinal lacteals. gallbladder). To date, 13 proteins have been characterized  AQP-2, present in the terminal portion of the distal con-and cloned. The molecular size of AQPs ranges from 26 to voluted tubules and in the epithelium of collecting34 kilodaltons. Each protein consists of six transmembrane tubules and ducts. AQP-2 is under the regulation of an-domains arranged to form a distinct pore. The sites where tidiuretic hormone (ADH) and is thus known as an ADH-AQPs are expressed implicate their role in water transport, regulated water channel. Mutation of the AQP-2 gene such as renal tubules (water reabsorption), brain and spinal has been linked to congenital nephrogenic diabetes cord (cerebrospinal fluid reabsorption), pancreatic acinar insipidus.
cells (secretion of pancreatic fluids), lacrimal apparatus (secretion and resorption of tears), and eye (aqueous  AQP-3 and AQP-4 have also been detected in the bahumor secretion and reabsorption). Most AQPs are selec-solateral cell surface of the light cells of kidney collecttive for the passage of water (AQP-1, AQP-2, AQP-4, ing ducts as well in the gastrointestinal epithelium AQP-5, AQP-6, and AQP-8), whereas others, such as (AQP-3), pancreatic acinar cells (AQP-12), and the
AQP-3, AQP-7, and AQP-9, called aquaglyceroporins, also brain and spinal cord (AQP-4). transport glycerol and other larger molecules in addition to
Current research into the function and structure of the water. Prominent members of the AQP family include AQP proteins may lead to the development of water chan  AQP-1, expressed in kidney (proximal convoluted nel blockers that could be used to treat hypertension, con-tubules) and other cell types such as hepatocytes and gestive heart failure, and brain swelling and to regulate red blood cells. AQP-1 is also expressed in the lymph intracranial or intraocular pressure.
They are shorter, with a less well-developed brush border and with fewer and less complex lateral and basolateral processes. The mitochondria are smaller than those of the cells of the convoluted segment and are randomly distributed in the cytoplasm. There are fewer apical invaginations and endocytotic vesicles, as well as fewer lysosomes.
Thin Segment of Loop of Henle
As noted above, the length of the thin segment varies with the location of the nephron in the cortex. Juxtamedullary nephrons have the longest limbs; cortical nephrons have the shortest. Furthermore, various cell types are present in the thin segment. In the light microscope, it is possible to detect at least two kinds of thin segment tubules, one with a more squamous epithelium than the other. Electron microscopic examination of the thin segments of various nephrons reveals further differences, namely, the existence of four types of  epithelial cells (Fig. 20.20):  Type I epithelium is found in the thin descending and ascending limbs of the loop of Henle of short-looped nephrons. It consists of a thin, simple epithelium. The cells have almost no interdigitations with neighboring cells and few organelles.