Comparative Study of Digestive System of Vertebrate
Alimentary Canal Or Digestive System
The term alimentary canal or digestive tract in vertebrates refers to an internal tube, seldom straight and often tortuously coiled, running from an anterior mouth opening in head to a posterior anal or cloacal aperture at the base of tail. It is designed for ingestion, digestion and absorption of food stuffs and egestion of undigested wastes. Major parts of alimentary canal are : oral cavity, pharynx, oesophagus, stomach, and small and large intestines. Chief accessory organs associated with the alimentary canal are : tongue, teeth, oral glands, pancreas, liver, gall bladder, etc. Various modifications of alimentary canal in different vertebrates include :-
- Lengthening of various parts by looping or coiling.
- Formation of diverticula or enlargements (e.g. crop, caecum, stomach compartments).
- Development of internal folds (e.g. spiral valve, villi, typhlosole, papillae, rugae, etc).
Wall of alimentary canal of vertebrates is made of 4 distinct concentric layers or coats :-
- The outermost serosa or serous coat is visceral peritoneum, made by mesothelial cells and a thin layer of connective tissue.
- Beneath serosa, the muscular coat is composed of smooth muscle fibres arranged in outer longitudinal and inner circular muscle fibres, with a network of autonomic ganglionated myenteric plexus of Auerbeach between them.
- Beneath muscular coat lies submucosa, a connective tissue layer containing elastic fibres, nerves, blood and lymphatic vessels, and glands.
- The innermost coat or mucosa is further differentiated into :
- innermost layer of columnar epithelium, often glandular and ciliated, supported by a thin basement membrane.
- Middle thin connective tissue, called lamina propria or corium, having blood capillaries, lacteals and nerves.
- Outer narrow band of inner circular and outer longitudinal muscle fibres, called muscularis mucosa, which separates mucosa from submucosa.
Mouth is the anterior opening leading into oral cavity and is subject to a great deal of variations. In amphioxus, true mouth (enterostome) is located at the end of vestibule, perforating the membranous velum. In cyclostomes (lamprey), it is a circular opening (cyklos = circular + stoma = mouth) at the vortex of the buccal funnel, and permanently open in the absence of jaws or other mechanisms for closing it. In gnathostomes, mouth is usually terminal, although in elasmobranches and sturgeons it is located ventrally. True fleshy and muscular lips occur only in mammals. Muscular lips and cheeks are adaptations for sucking. In fishes, amphibians and most reptiles, mouth is surrounded by unmodified or heavily cornified skin forming immovable lips.
In a broader sense, the term “mouth” is used as a synonym for oral cavity. In fact, oral cavity begins at the mouth and merges with pharynx without a definite line of demarcation. In gnathostome fishes, oral cavity is shallow, loosely organized and roofed with dermal bones usually bearing teeth. In addition to serving as a passageway for food, it is also a passageway for water and serves for aquatic respiration. In amphibians and reptiles, oral cavity is more compact and its muscular floor serves for swallowing food and also used in breathing in the absence of a diaphragm. It reaches culmination in mammals with a space, called vestibule, between lips and teeth, and bounded laterally by muscular cheeks, thus forming an efficient sucking and chewing organ. Nasal sacs in vertebrate embryos originate as ectodermal invaginations of head. In most fishes, nasal cavities are quite independent of oral cavity. In Dipnoi, amphibians and most reptiles, having only primitive or primary palate, nasal cavities open into oral cavity, rather anteriorly, by a pair of internal nares or choanae. In amniotes, with formation of a hard or secondary palate, the respiratory nasal passage becomes effectively separated from oral cavity or food passage, and the internal nares open far posteriorly into pharynx. In birds this palate is cleft so that nasal and oral cavities are in direct communication. In mammals, bony plate is continued posteriorly as a membranous soft palate. Sometimes, a fleshy pendulant process, the uvula, hangs from soft palate into laryngeal pharynx.
Derivatives or accessory organs of oral cavity
These are mainly teeth, tongue, oral glands and anterior and middle lobes of pituitary (adenohypophysis).
- Teeth :- Teeth are hard and pointed structures attached to jaw bones, that aid in food-getting. Two types of teeth occur in vertebrates epidermal and true teeth. Epidermal teeth are horny projections of stratum corneum and best represented in cyclostomes (lampreys). Other examples are conical projections from lips of tadpoles of some species of frogs, serrations on beaks of some turtles and birds, horny plates in duckbill, sirenians and baleen whales, and egg-tooth for cracking egg-shell before hatching in turtles, Sphenodon, crocodiles, birds and monotremes. True teeth occur in all vertebrates except agnathans, sturgeons, some toads, sirens, turtles, modern birds, etc. Teeth are polyphyodont, acrodont and homodont in fish, amphibians and most reptiles, but they are diphyodont, thecodont and heterodont in mammals. Teeth are similar in structure to the placoid scales of sharks, composed of a core of dentine surmounted by a crown of enamel, and are believed to have evolved from bony scales.
- Tongue :- A structure called tongue is found in the mouths of nearly all vertebrates. They show much diversity and not all are homologous with the mammalian organ with which we are most familiar. In cyclostomes (lamprey), it is a thick, fleshy, extensile, rasping organ on buccal floor, armed with horny teeth. The primary tongue of fishes and Necturus is an immobile, non-muscular, sensory elevation on buccal floor, bearing teeth in some teleosts. Tongue of most amphibians (frogs, toads, salamanders) is sticky, attached at the anterior end and free at the posterior end. It can be thrust out of mouth suddenly by rapid injection of lymph, for capturing insect prey called definite tongue. In turtles, crocodilians, some birds and whales, tongue is immobile. In snakes, some lizards, some birds and some mammals (anteaters), it is long, highly protractile and often used for food-capture. In some lizards and snakes, it is forked at the free end, and retractile into a basal sheath. In most mammals (man), tongue is attached to buccal floor by a ligament, the frenulum. Principal role of mammalian tongue is to help in manipulation and swallowing of food. It also bears numerous microscopic taste buds. Human tongue is also of great importance in speech. In amniotes tongue is made by fusion of four parts viz., primary tongue, a muscular and glandular part called tuberculum impar and two lingual folds.
- Oral glands :- Vertebrates exhibit a great variety of glands opening into mouth cavity, and often named according to their location, viz palatine, lingual, sublingual, maxillary, labial, parotid, etc. As a rule, oral glands are absent or few in aquatic forms, but gradually increase in number and complexity in terrestrial forms. Fish and aquatic amphibians have only simple mucous glands. Poisonous snakes have large poison glands. The largest oral glands are enzyme-secreting salivary glands of mammals secretes enzyme called salivary amylase or ptyalin.
- Adenohypophysis :- Pituitary, the most important endocrine gland of vertebrates, consists of three lobes having dual embryonic origin. A ventral evagination of diencephalons, called infundibulum, forms the posterior lobe termed pars nervosa or neurohypophysis. Whereas a dorsal diverticulum of stomodaeum, called Rathke’s pocket, constricts off to form the anterior and middle lobes of pituitary or adenohypophysis.
Region of foregut between oral cavity and oesophagus is termed pharynx which is lined by endoderm. Being concerned with digestion as well as respiration, it shows greater modifications than other parts of digestive tract. In fishes, pharynx is extensive and perforated by gill slits for aquatic respiration. In tetrapods, it is short and a crossroad between respiratory and food passages. From the wall of pharynx in embryo are derived spiracle, gill clefts, lungs, air bladder, tonsils and endocrine glands such as thymus, thyroid and parathyroids.
Oesophagus Oesophagus is a simple, muscular, distensible tube connecting pharynx with stomach. Its length is related to the length of neck. It is very short in neckless vertebrates (fishes and amphibians) but longer in amniotes, reaching extreme in birds, giraffe, etc. It may be lined internally with finger-like fleshy papillae (elasmobranchs), horny papillae (marine turtles) or longitudinal folds. In grain feeding birds (pigeon), oesophagus forms a paired or unpaired membranous sac, or crop, modified for storage of food. In pigeons of both sexes, epithelial lining of crop undergoes fatty degeneration controlled by a pituitary hormone, prolactin, forming ‘pigeon’s milk’ which is fed to nestlings. Oesophagus has no serous coat as it lies outside coelom. In mammals when it passes diaphragm it has serous lining. Food bolus passes down oesophagus into stomach by a muscular wave of contraction and relaxation called peristalsis. Oesophagus exhibit difference from rest part of the alimentary canal. The important differences are — it has no visceral peritoneum lining but outer covering is tunica adventitia. Muscle fibers of the anterior part of the oesophagus are striped, middle part is both striped and non striped and posterior part is only unstriped muscles. But ruminants all along their oesophagus have striped and voluntary muscles. Internal mucosal lining is of stratified squamous epithelial cells.
The sac-like muscular enlargement of digestive tract between oesophagus and intestine is called stomach. It is held in place by mesogaster, a part of dorsal mesentery. It serves for temporary storage and maceration of solid food and for preliminary stages of digestion. A true stomach is not present in protoehordates, cyclostomes, chimaeras, lung fishes and some primitive teleosts. Only when its epithelial lining contains gastric glands, is it properly called a true stomach. Absence of stomach is considered to be an ancestral trait. A well-developed stomach occurs in elasmobranchs and tetrapods. The anterior end of stomach connecting to oesophagus is nearer heart and therefore called cardiac end. Main sac-like portion is termed body. The caudal end connected to intestine is called pyloric end. It terminates at a pyloric valve or sphincter called pylorus. In many vertebrates, especially fishes, one to several hundred finger-like pyloric caeca may be present at the junction of pylorus with duodenum. Stomach is straight in vertebrate embryos and may remain so throughout life in lower vertebrates (cyclostomes, gar, Belone, salamander, etc.). it is long and spindle-shaped in Proteus, Necturus, snakes and many lizards with elongated and narrow bodies. It forms a wide curved tube in turtles and tortoises. More often, flexures develop producing a J-shaped or U-shaped stomach (elasmobranchs, Gobius, seal, man etc.). As a result, stomach exhibits a short concave border, called lesser curvature, and a large convex border, called greater curvature. Expansion of greater curvature forms the so-called fundus region. In Polypterus, stomach appears like a blind pouch due to fusion of cardiac and pyloric limbs along lesser curvature. In crocodiles and seed-eating birds, stomach is divisible into an anterior, thin-walled proventriculus with gastric glands, followed by a thick-walled highly muscular gizzard or ventricuius. The latter has a tough, horny lining and contains small stone pieces or pebbles called gastroliih which help in maceration of food. Stomach of mammals shows greatest modifications. It may be a simple sac or divided into cardiac, fundic and pyloric regions, each region with its characteristic gastric glands. In cud-chewing mammals or ruminants (cow), stomach has 4 well-defined chambers or compartments . Of these, the first three chambers (rumen, reticulum, omasum) are claimed to be the modifications of oesophagus and serve as reservoirs of food. Only the last chamber (abomasum or rennet) represents true stomach containing gastric glands, comprising of the usual parts— cardiac, funduc and pyloric. In camel, omasum is absent and the pouch-like water cells projecting from rumen and reticulum probably help in digestion but do not serve for storage of water as generally believed stomach is wanting in monotremes, it is represented by a wide sac devoid of glands internally and is lined throughout by stratified epithelium. Histological stomach shows two peculiarities. The muscularis mucosa consists of an outer longitudinal layer and inner circular muscle layer. Epithelial lining of stomach is thick and provided with several types of glandular cells forming gastric glands. Gastric glands are of three types Cardiac, fundic and pyloric glands. Cardiac and fundic glands secrete mucus only but fundic glands have three types of cells— mucus neck cells producing mucous, oxyntic cells producing HC1 and Zymogen cells producing pepsin.
The part of digestive tract following stomach is intestine, in which digestion and absorption of food take place. Hence it is the most important part of digestive tract and undergoes several modifications in vertebrates. In cyclostomes (lamprey), elasmobranchs (dogfish), some primitive bony fishes (sturgeon, Polyodon) and an occasional teleost, intestine is a short, straight and wide tube. Its lumen cantains a typhlosole or spiral valve which compensates for the short absorptive area. This valvular or spiral intestine is equivalent to the small intestine of higher vertebrates. Teleost and tetrapod intestine is without a spiral valve, greatly elongated, coiled and further differentiated into an anterior small intestine and a posterior large intestine. Small intestine is the chief site of digestion and absorption. Its internal surface bears numerous, small finger-like projection or villi which increase the absorptive area. Many bony fishes have one to several pyloric caeca, arising from small intestine. A large number of digestive glands are also present in small intestine, they are tubular called crypts of Lieberkuhn found all along the length of intestine and secrete mucus and group of enzymes called saccus entericus. First part of small intestine is known as duodenum. It is short, starts at the pyloric valve and terminates beyond the entrance of ducts from pancreas and liver into it. It has characteristic Brunner’s glands in submucosa and also secretes hormones (secretin, cholecystokinin) for stimulating pancreas and gall bladder to release their juices. Duodenum is followed by remaining small intestine, called ileum, which is narrow, greatly elongated, and much coiled. Only in mammals, small intestine beyond duodenum is divided into an anterior two-fifth jejunum and a posterior three-fifth ileum. Nodules of lymphoid tissues called Peyer’s patches are found in ileum. However, the division is somewhat arbitrary and based on differences in shape of their villi and in nature of their glands and walls.
Large intestine of most fishes and amphibians 0colon or rectum) is wider than small intestine. It is straight, short and leads into a posterior terminal chamber, the cloaca. Cloaca also receives the urinary and genital ducts and opens to outside through a cloacal aperture. In reptiles, birds and mammals, large intestine is longer and divided into a proximal colon and a distal rectum, the latter ending into cloaca. All mammals except the monotremes and many bony fishes, lack a cloaca. Their rectum opens directly to outside through anus, while the urinary and genital ducts also open independently. Rectum of mammals is derived by partitioning of embryonic cloaca and, therefore, it is not homologous with the rectum of other vertebrates. In tetrapods, and ileocaecal valve or ileocolic sphincter is present at the junction of small and large intestines, but absent in fishes. It prevents bacteria in colon from entering ileum. In amniotes, at the ileocolic junction is found an ileocolic caecum, usually two in birds. This contains cellulose-digesting bacteria and is very long, even coiled, in such herbivorous mammals as rabbit or horse feeding heavily on cellulose. Man, monkeys and apes have a small caecum, bearing a vestigial vermiform appendix. The rectal gland of elasmobranchs is a caecum that secretes sodium chloride. Bursa fabricii is a blind pouch of lymphatic tissue arising from dorsal wall of proctodaeum in young birds, but atrophies in the adults.
Liver occurs in all vertebrates and is the largest gland of the body. It arises as a single or double ventral diverticulum from the floor of embryonic duodenum. This liver bud or rudiment, soon divides into anterior and posterior parts. Anterior part branches repeatedly to become the liver proper of the adult animal. Posterior part forms the gall bladder, cystic duct and common bile duct receiving numerous hepatic ducts from the liver proper and emptying into duodenum. Shape and division into lobes of adult liver varies in different vertebrates. Colour is also variable, especially in teleosts, where it may be green, yellow, orange, brown or red. A true liver is absent in protochordates. Hepatic caecum of amphioxus and digestive glands of ascidians have somewhat different characters. In cyclostomes, it is small and single-lobed in lampreys but two-lobed in hagfishes. Liver is elongated, narrow and cylindrical in fishes, urodeles and snakes, but short, broad and flattened in birds and mammals. It is bilobed in elasmobranchs, two or three-lobed in teleosts, amphibians, reptiles and birds, and many-lobed in mammals. Liver is relatively large in carnivores than herbivores. Liver cells manufacture alkaline bile which is stored in gall bladder before getting released into duodenum. Bile contains no digestive enzymes, nevertheless it neutralizes acidity of liquid food or chyme entering intestine and aids in fat digestion and absorption. No other gland of vertebrate body has more varied functions than that of liver. But, curiously enough, it has never acquired any endocrine function. A gall bladder is present in hagfishes and all higher vertebrates. However, it is absent in lampreys, many birds (pigeon) and many mammals (rats, hyrax, whales some Artiodactyla and all Perissodactyla). Gall bladder is not essential and can be removed surgically without any harm.
Pancreas is also a constant structure of all vertebrates and second largest digestive gland after liver. Typically, pancreas arises as one or two ventral diverticula from liver bud, and one dorsal diverticulum from embryonic duodenum. It is endodermal in origin from embryonic archenteron. Distal portions of diverticula divide to form acinous type glands, one dorsal pancreas and one ventral pancreas. Both may persist, as in fishes, but more generally the two unite to form a single gland as in tetrapods. Proximal portions of diverticula form pancreatic ducts all of which may persist. But usually the ducts undergo reduction or fusion, so that only one or two pancreatic ducts remain as in mammals. The ducts open into duodenum separately or jointly or one of them may unite with the common bile duct Pancreas plays a dual role. It is partly exocrine secreting digestive enzymes through pancreatic ducts into duodenum, and partly endocrine secreting hormones such as insulin. A somewhat detailed treatment of pancreas can be seen in Chapter 50. No pancreas is present in lancelet. In lampreys, some teleosts, lungfishes and lower tetrapods, it is distributed diffusely in liver, mesenteries and intestinal wall and probably only exocrine in function. In elasmobranchs and higher tetrapods, pancreas is well-defined and compact.