EXCRETORY STRUCTURE IN FISH .pptx

This document discusses the excretory structures in fish, including the kidney, gill surface, skin, liver, bile, intestine, chloride cells, and rectal gland. It elaborates on the five types of marine teleost kidneys as described by Ogawa in 1961, and the different parts of the kidney, including the endocrine elements in fish kidneys. The document also compares the differences between the nephrons of freshwater and marine teleost fish.

1. EXCRETORY STRUCTURE IN FISH

2. INTRODUCTION Excretion is the removal of metabolic waste into the surrounding water. The excretory process in fish can very tremendously with respect to the volume and chemical composition of the excretory fluid produced. The physiology of excretion and osmoregulation are closely associated and excretion is a mechanism of regulation for both the processes. Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, proper balance of the internal environment of a fish is maintained by excretory system, especially the kidney. Excretion involves the separation and elimination of metabolic waste produced from the body. Various organs are involved in this process.

3. EXCRETORY STRUCTURES IN FISH

4. KIDNEY Organ through which most of the metabolic waste are excreted. Kidneys are paired, elongated structures, placed above the alimentary canal and close to vertebral column. Reddish brown in colour. Kidneys lie on either side of the posterior cardinal veins. Overlain by peritoneum, gut and reproductive organs. The kidney tissue is dark and loose, and is not clearly encapsulated.

5. The kidney shows multiple shapes in different fishes due to fusion at various positions. Made up of many individual units called nephrons. Each nephron contains a renal corpuscle/ malpighian body &renal ducts with various terminal modifications. Teleostean kidney is divided into two parts; head kidney and trunk kidney Head kidney : endocrine in nature It consists of lymphoid, hematopoietic tissue( help in haematopoiesis) and few nephrons/ no nephrons and collecting duct. No excretory function Inter-renal gland, chromaffin tissue, and corpuscles of stannius are present. Trunk kidney : consists of large number of nephrons. Excretory in function

6. Kidney of marine teleost can be divided into five types by Ogawa in 1961. Type 1 : two kidneys are completely jointed along their entire length without any clear distinction between head and trunk kidney. Eg : herrings Type 2 : anterior portion of the kidney are free but the middle & posterior portions are fused. head and trunk kidneys are clearly distinguishable. Eg : marine catfishes, eel Type 1 Type 2 Trunk kidney Head kidney

7. Type 3: anterior and middle portions of the kidneys are free but the posterior parts are fused. Anterior portions has two thin branches. Head and trunk kidneys are clearly distinguishable and separate. eg : mullets, billfish Type 4 : only the extreme posterior parts of the kidneys are fused. Head and trunk regions cannot be distinguishable. Eg : sea horses, pipefishes Type 5: two kidneys are completely separate. Eg : angler fish Head kidney Trunk kidney Type 3 Type 4 Type 5

8. In freshwater teleost type 1, 2, and 3 kidneys are seen, Type 1: salmon & trout Type 2: carps & minows Type 3: killifish & stickleback

9. Based on the presence & absence of glomeruli, two type of kidney; Glomerulus : network of capillaries contained within bowman’s capsule. Glomerular kidney : possess glomeruli. Aglomerular kidney : lack glomeruli. Eg : angler fish , pipe fish, sea house Tubules join to a collecting duct called archnephric duct/ ureter located posterior to cloaca. Distal tubules also absent Basic structure consist of a proximal portion and a collecting duct system.

10. In ancestral vertebrates, kidney possess one nephron for each body segment. The nephron drained into a duct called wolffian/ archinephric duct. This type of kidney is known as Holonephros because kidney extends to the entire length of the body. Eg : larvae of the cyclostomes, but not in any adult Ophisthonephros : the most anterior tubules have been lost, some middle tubules have been associated with testes and multiplication of tubules posteriorly.

11. Pronephros : the tubules of the anterior region become functional in early life The fish kidney has pronephric head region (non functional in terms of excretion) & mesonephric trunk region (functional middle region). Mesonephros : main excretory organ The anterior tubules are reduced and converted into lymphoid organ ,the posterior tubules have excretory function. It consist of mesonephric duct, mesonephric tubules and capillary tufts.

12. Nephron Afferent arteriole Efferent arteriole Glomerulus (fenestrated capillary bed) Basement membrane Proximal segment filtrate neck Consist of two parts bowman’s capsule and renal tubule. Begins as a double walled blind cup called Bowman’s capsule . Bowman’s capsule encapsulate glomerulus (a cluster of small blood vessels). A long, thin tubule is attached to each bowman’s capsule – dialysis unit.

13. Bowman’s capsule is lined by visceral (inner) & parietal (outer) layers of epithelial cells. Inner visceral layer lie just below the thickened glomerular basement membrane. which is made up of podocytes ,which sent foot processes over the length of glomerulus. Foot processes cross each other to form a filtration slit (passage of large molecules & cells restricted by filtration slit). Foot processes have a negatively charged coat called glycocalyx (helps to limit the filtration of negatively charged molecules). The size of glomeruli varies in different groups of fishes.

14. The renal artery supplies blood to the glomerulus. Renal artery arising from distal aorta. Renal portal system supplies blood to the tubules. To reach each nephron renal artery branches repeatedly forming arterioles Glomerulus is a blood vessel tightly coiled with efferent and afferent arterioles & is covered by thin kidney cells to form bowman’s capsule. Afferent arterioles deliver blood to glomerulus for filtration. Filtered blood drains away by efferent arterioles A network of peritubular capillaries are connected to the efferent arteriole, which is closely associated with the nephron tubule & helps in reabsorption of water, ion, and nutrients from the filtrate in the nephron tubule into it. The wall of afferent arteriole contains renin secreting juxtaglomerular cells (site of synthesis and secretion of enzyme renin, it plays an important function in renin-angiotensin system).

15. Renal tubule has three distinct regions: neck region, proximal convoluted tubule & distal convoluted tubule. Neck region is variable in length Proximal convoluted tubule (PCT) refers to the convoluted portion of the nephron between bowman’s capsule and loop of henle . PCT lined by a single layer of cuboidal cells & have brush borders, mitochondria and lysosomes. PCT help to concentrate salt in the interstitium (tissue surrounding the loop). U- shaped loop of henle help to link proximal tubule to distal tubule. Blood enter through afferent arteriole Blood goes out through efferent arteriole Capsule wall Capsule lumen T.S of DCT T.S of PCT

16. This loop have descending and ascending limbs. Distal convoluted tubules (DCT) refers to the convoluted portion of the nephron between the loop of henle and the collecting duct. Lumen is made up of simple cuboidal epithelium. The cells lining DCT have numerous mitochondria, which help in active transport by using ATP. Distal tubules from many nephrons connect to a common collecting duct.

17. Endocrine elements in fish kidney Thyroid follicle : scattered in the haemopoietic tissue of the head, mesonephric lobe (air breathing fishes) and in trunk kidneys. Corpuscles of stannius : located on the dorsal side of the middle to posterior part of the kidney, and it arises from the embryonic pronephric duct. work in association with pituitary gland, which exerts hypercalcemic effect, to balance relatively constant level of serum calcium .

18. Inter renal tissue : homologous to adrenal cortex in mammals. Secretes mineral corticoids (concerned with fish osmoregulation) & glucocorticoids (regulate carbohydrate metabolism) Chromaffin cells : homologous to adrenal medulla in mammals. Secrete adrenalin and noradrenalin. Inter renal tissue & chromaffin cells are found diffused in head kidney near the post cardinal vein.

19. The difference between the nephrons of freshwater and marine teleost are; Freshwater fish Marine fish 1 Renal corpuscles are more in number Renal corpuscles are less in number 2 Glomerulus are highly vascularised Poorly vascularised, sometimes non functional or completely absent 3 Neck segment along with ciliated epithelium Neck segment short 4 Distal segment present absent (to prevent reabsorption of salts into the body)

20. THE GILL SURFACE Gill can excrete carbon dioxide and ammonia (through passive diffusion). Chloride cells : important in ion excretion They are special type of columnar cells. Which secrete Cl‾ from blood to seawater. The basal portion towards the blood & apical upper part towards the seawater. Basal and later sides infolded to make the smooth tubular system. The cell is covered by overlapping epithelial cells except in apical region. This cells have a large nucleus and many closely packed mitochondria. Pavement cells are connected by deep tight junctions Apical crypt Golgi Basel lamina mitochondria nucleus Glycogen granules mucus Mucous cell

21. THE SKIN Minor role in excretion. Copious mucus secretion from gills/ skin may act as a vehicle for excreta. The mucus help in loss of unspecialized heavy metals as well as aluminium. The amphibious killifish Kryptolebias marmorotus may shift its route of nitrogenous excretion to the posterior end of skin / kidney when out of the water. VIA LIVER, BILE, INTESTINE minor role in excretion Its products are eliminated via bile and intestine / via blood &urine. In the winter flounder, Psedopleuronectes americanus the intestine cells have apical membrane which help in the excretion of potassium ions. In some fishes, posterior intestine have rectal gland. Liver is the major site of ornithine cycle activity in ureogenic & ureotelic fishes.

22. RECTAL GLAND It is a small finger like blind ending structure attached to the rectum in many elasmobranchs and the coelacanth fishes. Help to secrete sodium chloride (NaCl) from the body. The rectal glands are compound tubular glands with a central canal which continues as a duct (start from the posterior ventral part of the gland).

23. The rectal gland has three layers; An outer capsule containing small arteries and a peripheral connective and muscle tissue layer. b) A middle glandular layer consisting of a zone of secreting tubules. c) An inner layer having veins and ducts arranged around a central canal that terminate in a central duct. Mitochondria, Golgi apparatus, smooth and rough endoplasmic reticulum also is present.

24. CONCLUSION Organisms in different environment uses different body parts for excretion. The organs involved are kidney, gills, liver, rectal glands, chloride cells and skin. Kidneys and gills are the chief organs of excretion in fishes (removing waste materials produced during metabolism). Functional unit of kidney is nephron. In marine teleost, kidney remove magnesium & sulphide ions along with sodium, chloride, potassium & calcium ions. Trimethylamine oxide (TMO) is also removed along with small quantity of ammonia and urea. The gills remove ammonia & urea along with carbon dioxide. Chloride cells in gills important in ion excretion. Elasmobranchs have a small rectal gland which eliminate excess ions.