190The kidney of the teleost is a mixed organ comprising hematopoietic, phagocytic, endocrine, and excretory elements. The kidneys vary greatly between different species of fish, both grossly and histologically. It is usually divided into anterior or head kidney, which is hematopoietic and posterior kidney that contains more renal tubules with a lesser amount of interstitial hematopoietic and lymphoid tissues and thus functions as an osmoregulatory organ. The component structure of the fish nephron varies considerably between marine, euryhaline, and freshwater. Each nephron consists of several segments with specific structure and function. A typical freshwater nephron consists of cytologically distinct regions: renal corpuscles, neck segment, proximal, intermediate, and distal tubules. The fish renal corpuscles consist of a glomerulus and Bowman’s capsule. The neck region is continuous with the parietal and visceral epithelia of Bowman’s capsule and shows a narrow lumen surrounded by ciliated epithelial cells. This segment is usually short, and opens into a wider proximal tubule. The proximal convoluted tubule is the longest and most developed segment of the nephrons. This tubule is lined by eosinophilic-granular columnar cells with a well-developed brush border. Proximal convoluted tubule resorbs 85% of the water and sodium chloride. The intermediate segment has a narrow lumen surrounded by cuboidal cells that often have cilia, which help move the filtrate along the nephron. Within the distal tubules, more water is resorbed and urine concentrated or diluted. Collecting tubules and ducts are located throughout the kidney and are involved in collection of concentrate for excretion and more water resorption. The ureters, which conduct urine from the collecting ducts to the urinary papilla, may fuse at any level and may be dilated, after fusion, to form a bladder. The bladder can be only a simple dilation of the ureters or a true saccular organ emptying outside by a urogenital pore.