Digestive system of vertebrates

Digestive system of vertebrates

1. Digestive System of Vertebrates Comparative Anatomy By-Sanju Sah St. Xavier’s College, Maitighar , Kathmandu Department of Microbiology

2. Digestive System Digestive system = Digestive tract/ALIMENTARY CANAL + Associated accessory organs Digestive tract = Mouth, buccal cavity, pharynx, oesophagus , stomach, small intestine, large intestine and cloaca /anus. Associated organs = Teeth, tongue, oral glands, liver, pancreas, gall bladder etc. F unction: Ingestion, peristalsis, digestion, absorption, assimilation, egestion

3. Embryonic Development of Digestive Tract One of the earliest system to form during embryonic development. Embryonic archenteron formed of endoderm becomes the lining of adult digestive tract and its derivatives. Splanchnic mesoderm adds connective tissue and smooth (involuntary) muscle layers around the archenteron/ primitive gut.

4. Digestive System (development) The part of archenteron/primitive gut containing yolk or connected by a narrow stalk with yolk sac is called midgut . The part anterior to midgut is foregut and posterior to midgut is hindgut. Fore gut is differentiated into – oral cavity, pharynx, oesophagus , stomach and most of the part of small intestine of the adult body. Hindgut forms large intestine and cloaca.

5. Digestive System (Development) Vertebrates are deuterostomes . A midventral ectodermal caudal invagination called proctodaeum forms the external part of cloaca or anus. Similar midventral ectodermal anterior invagination called stomodaeum forms the buccal cavity. It gives rise to enamel of teeth, epithelial lining of oral cavity and different oral glands like- poison, salivary, mucus glands etc.

6. Digestive System (Histology) Histology of wall of Digestive Tract: Wall of alimentary canal of vertebrates bears 4 concentric layers: serosa , muscularis , sub-mucosa and mucosa. Serosa /visceral peritoneum : the outermost layer made of squamous mesodermal cells. Muscularis : Smooth muscle fibres arranged in outer longitudinal and inner circular muscle layers. Autonomic nerve network ( myenteric plexus) is present between them. Sub-mucosa : It is a connective tissue layer containing elastic fibres, blood vessels, lymph vessels, nerves and glands. Mucosa : It is the innermost layer, further differentiated into 3 layers: muscularis mucosa, lamina propria and epithelium.

7. Outer muscularis mucosa with thin layers of longitudinal and circular muscles. Middle, lamina propria /corium having connective tissue, blood capillaries, lacteals and nerves. Innermost, mucosal epithelium made of columnar cells resting on basement membrane; often ciliated or glandular

8. Digestive System of Vertebrates Mouth- the anterior opening Agnatha /cyclostomes : circular opening at the apex of buccal funnel. It remains permanently open in the absence of jaws or other closing mechanism. Gnathostomes : - Elasmobranchs and sturgeons have ventral mouth, all others have terminal mouth. - In fishes, amphibians and most of reptiles, mouth is surrounded by unmodified or heavily cornified immovable lips. - In birds, mouth is wide and bounded by horny beaks. - In mammals, mouth is surrounded by fleshy and movable lips.

9. Digestive System of Vertebrates 2. Buccal Cavity/ Oral Cavity: In cyclostomes , buccal funnel is cup-like depression acting as a sucker. In fishes , oral cavity is shallow, loosely organized serving as passageway for food and also for water helping in aquatic respiration. It is quite independent of nasal cavities. In amphibians and lower reptiles oral cavity is much compact, muscular floor serves for swallowing food and also used in bucco -pharyngeal respiration and pulmonary breathing in the absence of diaphragm. In Dipnoi , amphibians and most reptiles , nasal cavities open into oral cavity by a pair of internal nares or choanae at anterior region.

10. Digestive System of Vertebrates In birds nasal and oral cavities are in direct communication as the palate (roof of buccal cavity) is cleft. In mammals , oral cavity is highly specialized for efficient sucking and chewing. In higher reptiles and mammals, internal nares open far posterior into pharynx. In mammals , bony palate separating buccal cavity and nasal cavities is continued posteriorly as a membranous soft palate and often a fleshy pendulant process – the uvula, hangs down.

11. Digestive System of Vertebrates Accessory organs of Buccal cavity: Tongue Almost all vertebrates bear tongue. Vertebrate tongue shows much diversity and is not homologous in all. In cyclostomes : it is thick, fleshy extensible structure on the buccal floor bearing horny epidermal teeth for rasping skin and muscles of the prey. In fishes : it is immobile, non-muscular sensory elevation on buccal floor bearing receptors and also teeth in some bony fishes. Most of amphibians - bear sticky tongue attached at the anterior end and free at posterior end. It can be thrust out ( protrusible ) suddenly by rapid injection of lymph into it for capturing prey .

12. Fish eating the worm Tongue of fish Cyclostome Tongue

13. Digestive System of Vertebrates Chelonian and crocodilian reptiles , some birds and whales have immobile tongue. Squamatan reptile (snake & lizards), some birds ( eg . Woodpecker) and some mammals ( eg . Anteaters) bear long, highly protrusible tongue often used for food capture. In most mammals , base of tongue is attached with the floor of buccal cavity by fold of mucosal membrane called frenulum . In mammals, upper surface of tongue bears 4 types of papillae providing roughness, some of which bear taste buds. They are circumvalate papillae, fungiform papillae, filliform papillae and foliate papillae.

14. Echidna

15. Tongue of Human

16. Digestive System of Vertebrates b. Teeth 2 types: epidermal teeth and true teeth. Cyclostomes bear epidermal teeth which are horny projections of stratum corneum on the wall of buccal funnel and tongue. True teeth occur in all vertebrates except agnathans , sturgeons, some toads, sirens, turtles, modern birds etc. In fishes and amphibians: dentition is acrodont , homodont and polyphyodont . In reptiles: it is pleurodont , homodont and polyphyodont . In mammals, it is thecodont , heterodont and diphyodont ( monophyodont in moles and Indian squirrel).

17. Pleurodont (Reptiles) Thecodont and heterodont Acrodont (frog)

18. Digestive System of Vertebrates c. Oral Glands: Vertebrates bear variety of glands opening into oral cavity. Oral glands are absent or few in aquatic forms, gradually increase in terrestrial form. Fishes and aquatic amphibians bear only simple mucous glands. In poisonous snakes, upper labial glands produce poison. Birds have sublingual glands and a gland in the angle of the mouth. Mammals have many small mucous glands and true and enlarged enzymatic salivary glands – parotid, sublingual, submaxillary and infraorbital salivary glands secreting mucin and ptyalin.

19. Pharynx Part of foregut between buccal cavity and oesophagus . From embryonic pharynx spiracles, gill clefts, lungs, air bladder, tonsils, thyroid, parathyroid and thymus are derived. It is the common passage for digestive and respiratory tracts.

20. Pharynx In cyclostomes : In Petromyzon only respiratory pharynx is present which is a blind pouch bearing gill slits while in Myxine it is continued into oesophagus . In fishes , it is extensive and perforated by gill slits for aquatic respiration. In tetrapods , it is short and common passage for digestive and respiratory tract. Tetrapods : pharynx is the part of the foregut preceding the esophagus & includes: Glottis – a slit leading into the larynx openings of auditory ( eustachian ) tubes Gullet - opening into esophagus

21. Oesophagus Muscular, distensible tube between pharynx and stomach that passes food by peristalsis. Its length is related to the length of the neck. In cyclostomes , it is a long slender tube opening directly into intestine. In anamniotes or neckless vertebrates (fishes and amphibians)- very short. In amniotes (reptiles, birds and mammals) – long. It internally bears longitudinal folds; in elasmobranchs it is lined by fleshy papillae and in marine turtles by horny papillae.

22. Oesophagus In graminivorous birds it forms paired or unpaired food storage sac called crop . In pigeon , from crop of both the sexes a curdy substance called pigeons milk is produced under the influence of prolactin hormone from pituitary gland. In mammals , oesophagus is long and enters from thoracic cavity into abdominal cavity piercing diaphragm at a point called oesophageal hiatus . Digestive System of Bird

23. Stomach It is sac like part of digestive tract between oesophagus and intestine. - Temporary storage organ of food. - Churning/maceration of food into fine pieces. - chemical digestion of food. True stomach bears gastric glands on epithelial lining. Cyclostomes, chimaeras, lung fishes and some primitive teleosts do not bear true stomach. Elasmobranchs , most of teleosts and tetrapods bear well developed stomach. Stomach is sphinctered at both the ends; anterior- cardiac sphincter and posterior- pyloric sphincter.

24. Stomach Embryonic stomach is straight tube. Lower vertebrates like gar, salamander etc bear straight stomach throughout the life. Most of the amphibians and squamatan reptiles bear elongated and spindle shaped stomach. Chelonian reptiles bear wide curved tubular stomach. In crocodilian and most of the birds it is divisible into anterior glandular thin walled proventriculus and posterior thick walled highly muscular ventriculus or gizzard . Gizzard has inner tough protective lining and also bears pebbles which macerate food.

25. Stomach In mammals , there is greatest diversity in stomach. May be a single sac . May be divided into cardiac, fundic and pyloric region with characteristic glands. In ruminants (cud-chewing mammals) , 4 distinct compartments: rumen, reticulum, omasum and abomasum. Rumen, reticulum and omasum are modifications of oesophagus , serve as reservoir. Abomasum /rennet is true stomach with gastric glands. In camel, omasum is absent. In sanguivorous bats like Vampires, pyloric enlargement serves to store blood.

26. Human Stomach Ruminant Stomach

27. Stomach Ruminant Mammals (Cud-chewing Ungulates) Possess ruminant stomach with 4 chambers. When food is eaten it enters rumen and reticulum which reduce the food to pulp. Microorganisms are present that aid in the breakdown of complex carbohydrates in plant material. The cud is then regurgitated for more chewing. After chewing the cud, the remasticated material passes to omasum and abomasum where physical and chemical processing similar to normal mammalian stomach occurs .

28. Small Intestine Principal site for digestion (completed) and absorption (virtually almost all food is absorbed). In cyclostomes, elasmobranchs and primitive bony fishes - it is short, straight and wide tube opening into cloaca. It bears typhlosole (a large internal fold) or spiral valve increasing surface area for absorption. Teleost and tetrapod intestine is devoid of spiral valve, greatly elongated and coiled. Internal surface bears small finger like villi . Small intestine is differentiated into duodenum and ileum.

29. Small intestine Anteriormost widest part – duodenum bears Brunners glands secreting alkaline mucus and crypts of Lieberkuhn secreting intestinal juice containing enzymes. In mammals, behind the duodenum small intestine is differentiated into jejunum and ileum. Increased surface area in Tetrapods is by elongation and coiling of intestine along with folding of internal surfaces Intestine is longer in herbivores than in carnivores because plant matter is more difficult to digest

30. Large Intestine In cyclostomes , intestine is not differentiated into small and large intestine. In most of fishes and amphibians, it is wider than small intestine, straight, short and leads into cloaca (teleosts bear anus). In reptiles, birds and mammals – it is longer and divided into proximal colon and distal rectum opening into cloaca (except in mammals). All mammals except monotremes and almost all teleost fishes lack cloaca, their rectum opens out through anus.

31. Large Intestine In fishes , ileocaecal valve is absent. In tetrapods , ileo-caecal valve is present that prevents entry of bacteria from colon to ileum. In amniotes, a caecum is present; birds bear a pair of them. Primates have small caecum with vestigeal vermiform appendix. Herbivorous mammals bear long caecum and large functional appendix with symbiotic bacteria helping in digestion of cellulosic food. Elasmobranchs bear a caecum and a rectal gland that secrets Sodium chloride.

32. Digestive Tract of Anamniote Vertebrates

33. Digestive Tract of Amniotes

34. Digestive Glands: Liver The largest gland of the body of all vertebrates. Arises as ventral outgrowth/ diverticulum from the floor of embryonic duodenum and divides into anterior and posterior parts. Anterior part proliferates and gives rise to liver proper and bile ducts. Posterior part gives rise to gall bladder, cystic duct and common bile duct or ductus Choledochus . Liver is a network of endodermal epithelial cells but soon mesodermal derivatives incorporate with those to form the adult organ.

35. Embryonic Development of Liver, gall-bladder and pancreas

36. Liver Vertebrate liver differs in shape, color and division into lobes. In cyclostomes , it is small. Single lobed in lampreys and bilobed in hagfishes. In fishes, urodel amphibians and reptiles it is elongated, narrow and cylindrical. In elasmobranch fishes, it is bilobed . In teleost fishes it is 2 or 3 lobed and may be green, yellow, orange, brown or even red. In amphibians, reptiles and birds it is 2 or 3 lobed, while its lobes differ greatly in mammals. Human liver has 2 lobes while that of rabbit has 5 lobes etc.

37. of Human

38. Liver Liver cells secrete bile juice which is stored in gall bladder. Bile juice neutralizes acidic chyme and emulsifies fats. Liver is the vital organ for heat production, metabolic actions like glycogenesis , glycogenolysis , gluconeogenesis , lipogenesis , desaturation of fats, deamination , transamination etc, synthesis of vitamins, phagocytosis of germs, detoxification of drugs, alcoholic products & toxins etc. A gall bladder is present in hagfishes ( cyclostome ) and all higher vertebrates. Gall bladder is absent in lampreys, many birds, aquatic mammals ( Cetacea ) and some artioactyla and all perissodactyla . Gall bladder stores and concentrates bile juice.

39. Pancreas The second largest gland. Heterocrine /mixed gland/compound gland It is formed from one dorsal and one or two ventral diverticulum from embryonic duodenum. Distal portions of diverticula divide and form compound pancreas, one dorsal pancreas and one ventral pancreas.

40. Pancreas In fishes both dorsal and ventral pancreas persist while in tetrapods two unite to form single pancreas. In lampreys, some teleosts , lung fishes and lower tetrapods it is probably exocrine only and distributed diffusely in liver, mesentries and intestinal wall. In elasmobranchs, most of teleosts and higher tetrapods it is well-defined and compact.

41. Pancreas Its exocrine portion consists of pancreatic acini secreting digestive juice and endocrine portion consists of Islets of Langerhans secreting hormones. Pancreatic juice from acini is carried by the ducts which unite to form major pancreatic duct (usually one in higher vertebrates). Major duct joins with common bile duct forming hepato -pancreatic duct that opens into duodenum. Islets of Langerhans bear endocrine cells like alfa cell, beta cell, delta cell and PP cell or F cell) which secrete hormones- glucagon, insulin, somatostatin and pancreatic polypeptide hormone respectively. Glucagon and insulin are involved in glucose metabolism.