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Digestive System

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fei swinton

on 11 February 2013

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Transcript of Digestive System

PROCURING FOOD 1. Filter Feeding a. Sea squirts, amphioxus, and larval living agnathans b. Fishes (spoonbills and tiny clupeids) and basking sharks c. Large mammals (Baleen whales) 2.Bite-tear-swallow Technique (modern sharks) 3. Teleost methods 4.Lamprey method 5.Other methods a.Amphibians, squamate reptiles, some birds and mammals: use long sticky tongues to capture food b. Snakes impale prey with upper jaw teeth c. Birds use a variety of beaks d. Herbivores crop grasses e. Carnivores use “snap-bite-tear” technique 6.Grasping methods a. Primates b.Rodents 7.Food Finding Methods: allow animals to be alerted to the presence and location of food Chemoreceptors (olfactory organs) Mechanical Receptors (inner ear and lateral line organs) Thermal Receptors (loreal pits of snakes) Capsulated touch receptors (snouts of pigs) Visual Receptors Electroreceptors An Overview MOUTH Mouth entrance of the digestive tract opens into the oropharyngeal cavity with teeth and walls perforated by gill slits In tetrapods: the mouth opens into an oral cavity containing teeth,and a tongue of one variety
leads to the pharynx primary palate is pierced anteriorly in fishes and amphibians by internal nares *the roof of the oral cavity is the primary palate in fishes and amphibians, partial or complete secondary palate in amniotes In reptiles
and birds: palatal fissure - incomplete secondary palate, leaving a deep groove called palatal fissure that channelizes respiratory air. In crocodilians and mammals: - entire roof of oral cavity is the secondary palate that
separates nasal passageways from external nares to the pharynx secondary palate In mammals: the oral vestibule separates the gums, or alveolar ridges from the cheeks and lips In rodents: an opening leads from the vestibule into cheek pouches, used for transporting food from fields to burrows some seed-eating birds have a median sublingual seed pouch TONGUE PRIMARY TONGUE No glands and musculature (can’t be independently manipulated ) but may assist the jaw in holding the prey
Forerunner of the tetrapod tongue
Derived from the mesenchyme of III branchial arch GLANDULAR FIELD Secretes a sticky mucus
as Tuberculum Impar in amniotes LATERAL LINGUAL SWELLINGS Derived from mesenchyme of II branchial arch FUNCTIONS OF THE VERTEBRATE TONGUE for gathering and capturing food sense of taste manipulation of food in the oral cavity swallowing thermoregulation grooming human speech streognosis COMPARATIVE ANATOMY OF TONGUE In humans, if the frenulum inhibits movement of the tip of the tongue, making the individual tongue-tied, it can be snipped at its anterior edge. The roof of the tongue of terrestrial urodeles becomes anchored to the basihyal and ceratohyal cartilages in the floor of the pharynx; that of anurans, unlike other tetrapods, becomes anchored to the floor of the oral cavity immediately behind the mandibular symphysis. The “tongue” of agnathans is not homologous with any component of the tongue of gnathostomes. Tongue of terrestrial urodeles and of anurans consists of a primary tongue and an extension that can be flipped out of the mouth Amniote tongue is anchored to skeletal components of hyoid arch When the mouth of a toad is closed, the tongue lies folded back over itself so that the tip is directed toward the esophagus. The mucosa of the tongue contains receptors not only for taste but for other stimuli including stereognosis in amniotes The sticky tongue of the 4-foot-long Great Anteater is 2 feet long. Woodpeckers have a barbed tongue ORAL
GLANDS M u c u s - chief ingredient secreted in oral cavity - lubricates dry food - vary in viscosity and chemical composition - moistens food Other secretions: - serous fluids - toxins - venoms - enzymes Oral glands and their location: Labial Molar Submandibular Intermaxillary Palatal Infraorbital Sublingual open into oral vestibule at the base of lips lie near premaxillary bone lie near molar tooth in the floor of orbit open into the palate open via common papillae under tongue Saliva mixture of oral secretions produced in parotid gland contains ptyalin different composition per organism Goblet cells secrete mucus only source of oral secretions in aquatic craniates perform special functions in male population of catfishes produce a copious nutritious secretion that maintain suitable environment for egg development TEETH They are descendants of the denticles of the dermal armor of early fishes like that of placoid scales they are composed of dentin [a variety of bone, surmounted by a crown of enamel or enameloid] Found in jawed fishes, amphibians, reptiles, and mammals [has achieved their regional specialization]; [there are a few mammals that don’t have bony teeth] and earliest birds, Lacking bony teeth are sturgeons, numerous teleosts including seahorses, whalebone whales, South American and scaly anteaters, and the monotreme Echidna. But even in toothless species they have develop an embryonic set they simply does not erupt or disappears after erupting EARLY FISHES They have bony dental plates of dermal origin which overlay the endoskeletal components of the jaws of early fishes Many had pointed, rounded or jagged surface projections that prevented the escape of live food from the oropharyngeal cavity or that were used for crushing, shellfish, biting flesh or rasping vegetation Jaws of placoderm vary from unossified Meckelian cartilage associated with overlying denticles within the skin to fully ossified dental plates overlying or adjacent to the Meckel’s cartilage They did not possess teeth!!!! But there surface of their dental plates has the same function as the teeth They have flattened edges for crushing, sharp edges for shearing, large spikes for impaling prey, numerous individuals cusps for holding prey, single or paired upper dental plates opposed the lower jaw on each side ACANTHODIANS Each denticle, usually broadly based, some conical, some slender and curved, and some with dozen spiny cusps arranged in whirls, was attached directly to the skeleton of the jaws like that of modern teeth Some acanthodians were toothless, or had only lower teeth The bony denticles of early fishes were the precursors of the teeth of modern gnathostomes Generalization regarding phylogeny: “They are derivatives of bony dermal armor.” The earliest indication of the development of socketed teeth is the ingrowth of a longitudinal ridge of the ectoderm called the dental lamina which extends more or less the length of the jaws

Beneath or above the lamina is a linear series of dermal papillae, each designating a site of a future tooth, form at intervals, indenting the lamina, and organizing blood vessels necessary for the development of a tooth primordium

The cells at the periphery of each papilla become organized into a definitive layer of odontoblasts that commence to deposit dentin

As deposition proceeds, odontoblasts slowly withdraw toward the center of the primodium

Withdrawing odontoblasts leave behind evidence of their withdrawal in the form of dentinal tubules

The ectoderm of the dental lamina has organized an enamel organ, consisting of ameloblasts that deposits enamel on the surface of the dentin.

Cementum, a type of acellular bone, anchors the tooth to the bone of the jaw by means of collagenous fibers
vessels and nerves [essential in maintaining the tooth in a healthy state] inside the tooth remains throughout the life of the tooth Armadillos- An enamel organ is present but functionless, hence their teeth has no enamel Mammals- Enamel in mammals are at least deposited by ameloblasts of ectodermal origin Fishes- compact dentin Gnathostomes- Their teeth vary in number, distribution within the oral cavity, position with reference to the summit of the jaw, degree of permanence and shape Blue sucker Early tetrapods -Even today most amphibians and many reptiles have teeth on their vomer, palatine, and pterygoid and sometimes in the parasphenoid as well

-But they are confined in the jaws of crocodilians, fossil toothed birds and mammals and they are least numerous among mammals Acrodont dentition-SEEN ON TELEOSTS, jaw teeth are attached to the outer surface or to the summit of the jawbone Pleurodont dentition
-Seen in anurans, urodeles, and many lizards; attached to the inner side of the jawbone Thecodont dentition
-Occupies bone sockets
-Seen in many fishes, crocodilians, extinct toothed birds and mammals
-The sockets (alveoli) are deepest In mammals Polyphydont dentinion
-Seen in gnathostomes through amniotes
-Have a succession of teeth, and the number of replacements during a lifetime is indefinite but numerous - Nonmammalian gnathostomes replace their teeth in waves
- First wave, even number of teeth are lost; the next wave, odd number of teeth are lost
- The waves ensures a balanced distribution of teeth along the jaws throughout life
- Tooth germs for the next wave of eruptions are already forming
- Tooth germs in sharks are formed in the dermis on the oropharyngeal cavity side of the jaws and while growing migrate onto the cutting edge of the jaw as the teeth that are being replaced move beyond the edge before falling away

- Only in mammals is their a definite number of teeth in a species, with rare exemptions
- Most mammals have two sets of teeth the deciduous or milk teeth [provides constantly changing infant jaw with small temporary teeth], and permanent teeth (diphyodont dentinion) and there is a definite sequence in which teeth erupt
- Eruption of teeth in human beings, 6,1,2,4,5,3,7,8
- Eruption of 8 “wisdom tooth” is delayed in higher primates, and is sometimes imperfect, unerupted or missing - A few mammals only develop a first set of teeth (monophyodont dentition)
- in platypus the deciduous set is replaced by horny epidermal teeth - in toothless whales, the first set is formed within the jaw bone but may not erupt , and if they do they usually shed - freshwater manatee [rate of 1 to 2 mm per month; thin bony sockets separate the roots of successive teeth and the bony septa are resorbed under pressure from the migrating teeth; grasses eaten by manatees contain abrasives that appear to be necessary for the teeth to move forward] from the Amazon River and the Australian Rock Wallaby do not have “sets” teeth being replaced throughout life by the forward migration of new teeth formed at the rear of the jaws  - proboscidians have a slow but constant succession of the molar teeth that move forward from the rear MAMMAL Morphologic Variants in Mammals Heterodont dentition difference in morphology in the front and rear teeth
First arose in late synapsids
Cretaceous ancestors of cretaceans, sirenians, and some marine carnivores exhibited heterodonty Incisors Located on either side of the mandibular symphysis
Have one horizontal cutting edge and a single root
Best developed in herbivorous animals
May be lacking on the upper jaw or lacking completely Canine teeth Lie next to the incisors
In carnivores, these are spearlike and are used for piercing flesh
absent in lagomorphs Premolars Usually have two prominent cusps (bicuspids)
Also have one or two roots Molars Have three or more cusps (tricuspids)
Usually have three roots
Not replaced by a second set Crown Part above the gum line
Covered with enamel
Crowns of premolars and molars of carnivores and herbivores vary greatly in morphology The crowns of carnivores’ cheek teeth are laterally compressed and usually have two or three cusps.
They are also known as secodont teeth. Carnassial teeth The last upper premolar and the first lower molar
Larger and longer than adjacent cheek teeth. Selenodont teeth The cheek teeth of ungulates.
Specialized for grinding vegetation. Lophodont teeth In elephants, lophodont teeth reach a foot or more in length and may reach a third of a foot in width. The cheek teeth of the remaining mammals exhibit a diversity of molariform styles. Rodents exhibit the largest variety of teeth. Triconodont teeth Cheek teeth of early prototherians Trituberculate teeth Possessed by early therians
forerunners of today’s tricuspids The first eutherians had three incisors, one canine, four premolars, and three molars on each side of the jaw. Teeth, along with the tongue and hyoid, constitute a functional triad that procures, manipulates, and in mammals, masticates foodstuffs at the entrance to the digestive tract, then starts a bolus of food on its way to digestive sites. EP. TEETH PHARYNX GUT WALL Living agnathans – horny teeth in the buccal funnel and tongue Anuran tadpoles – horny teeth on temporary lip Turtles, crocodilians, Sphenodon, birds, monotremes – have a temporary egg tooth *Bony teeth replace a platypus’ first set of bony teeth Turtles and birds – beaks often have serrations that perform some functions of teeth. Adult tetrapod pharynx – part that had pharyngeal pouches in the embryo Three constant features of the tetrapod pharynx Glottis – opening into the larynx Openings of the paired auditory tube – leads to middle ear cavity Opening into the esophagus Additional features in mammal pharynx: Nasal pharynx – above soft palate Oral pharynx – between oral cavity and the glottis Laryngeal pharynx – dorsal to the larynx Isthmus of fauces – narrow passageway from the oral cavity to the oropharynx Pillars of the fauces – muscular folds that arch upward from the side of the tongue to the soft palate (glossopharyngeal), and from the pharyngeal wall to the soft palate (palatopharyngeal) Uvula – fleshy extension at the back of the soft palate Tonsils – small masses of lymphoid tissue Palatine – located in the hollow between the pillars of the fauces Pharyngeal (adenoids) – develop in the mucosa of the nasopharynx Lingual – develop on the tongue near its attachment to the hyoid bone Epiglottis fibrocartilaginous flap that lies in the floor of the pharynx; attached to hyoid bone Important in respiration and food digestion in mammals (regulate air and food traffic) Suprabranchial organs found in Teleosts
pair of elongated muscular tubes that terminate as blind sacs
have elongated gill rakers extending from last two gill arches to the entrances of the tube
contain many goblet cells
traps plankton from incoming respiratory water and concentrate it into mucus-rich masses
absorbs oxygen from atmospheric air and supplies it to bloodstream (in others) Four layers:
Muscularis externa
ESOPHAGUS A distensable muscular tube that extends between the pharynx and the stomach
The glands in its lining secrete only mucus
Striated muscle at the cephalic end of a long esophagus is gradually replaced farther down by smooth muscle, except in ruminants. Esophagus
The lining of the rumen is similar to that of the esophagus, and it produces no digestive enzymes.

lined by a stratified squamous epithelium, which, in terrestrial turtles, birds and few mammals, is cornified, enabling the lining to withstand abrasions caused by roughage in the Esophagus The esophagus is shortest in fishes and neckless tetrapods
In fishes, it serves as a sphincter that closes the passageway to the stomach during the phase of respiration when water is being forces across the gills
Its only function otherwise in fishes is to conduct foodstuffs to the stomach. Comparative Anatomy of the Esophagus
The esophagus of marine turtles is lined by horny papillae that are directed backward, thereby preventing regurgitation while making it easy to swallow slippery seaweed. Comparative Anatomy of the Esophagus
CROP (found in Aves)
– a paired or unpaired membranous diverticulum, or sac, found chiefly in grain eaters, who use it for hoarding seeds and grain until there is no room for them in the stomach Comparative Anatomy of the Esophagus The esophagus of vampire bats has a very narrow lumen through which only fluids can pass. This is correlated with their sanguinivorous diet. Comparative Anatomy of the Esophagus CARDIA and CARDIAC REGION
– no zymogenic cells
– glands are compound tubular with many goblet cells
– a few parietal cells are also present Parts of the Stomach
– characterized by a specific array of simple tubular gastric glands

– in mammals having been the embryonic dorsal border connected to the coelomic roof by the dorsal mesentery (MESOGASTER)
– convex border Parts of the Stomach
– concave border of the stomach


– increases surface area for absorption Parts of the Stomach
– terminal opening of the stomach leading to the duodenum
– no zymogenic cells
– with simple branched tubular glands, many goblet cells and relatively few parietal cells Parts of the Stomach
– ring of smooth muscle
– regulates the entry of chyme from the stomach into the small intestine Parts of the Stomach
– portion of ventral mesentery attached to lesser curvature of stomach and duodenum; hepatoduodenal ligament and gastrohepatic ligament Parts of the Stomach GREATER OMENTUM
– portion of the dorsal mesentery attached to the greater curvature of the stomach
– draped like a curtain between the ventral body wall and the intestines
– covers the abdominal viscera like an apron in humans Parts of the Stomach
– portion of the peritoneal cavity enclosed within the greater omentum
– opening between lesser and greater peritoneal cavities Parts of the Stomach
Absorption of H2O and X-OH

Converts bolus into chyme

Serves as storage and macerating site for ingested solids via vertical peristalsis (churning)

Secretes digestive enzymes Functions of the Stomach Agnathans, chimaeras and lungfishes
- have no definitive stomach
Jawed fishes
- Epithelium is sometimes ciliated; gar stomach is almost straight; sharks exhibit the more common J shape; entire stomach of some teleosts is one large cecum Comparative Anatomy of the Stomach Amphibians
- Not distinguishable grossly from the esophagus
Crocodilians and Birds
- divided into two parts:
1. PROVENTRICULUS- secretes digestive enzymes
2. GIZZARD- lined w/ a horny membrane; simply a grinding mill that makes a mash of food mixed with gastric secretions Comparative Anatomy of Stomach Ruminants
1. RUMEN – fermentation chamber; site of anaerobic bacteria adding cellulose
2. RETICULUM – digests food material via peristalsis; “chewing the cud”
3. OMASUM – temporary holding chamber
4. ABOMASUM – true glandular stomach Comparative Anatomy of the Stomach INTESTINE L I V E R Begins at pyloric center and ends at cloaca/anus Tetrapods:
- Made of small & large segments, each w/ a distinctive epithelium
- Does not exhibit differentiation Intestine of different organisms Living agnathans, chondricthyes, basal bony fishes
Intestine is quite straight
Few Teleosts
- Exhibit intestinal coiling **Spiral Intestine – a spiral valve (typhlosole) is suspended w/in its lumen; has same function w/ intestinal coils  increases epithelial area available for absorption **Postvalvular intestine – beyond the spiral intestine; leads to cloaca

*Intestinal ceca – major adaptations for increasing the absorptive area of teleost intestine  Pyloric ceca- most common; diverticula near pylorus *rectal gland – finger-shaped; a cecumlike duct w/c empties into the short postvalvular intestine of sharks; no digestive function; it only extracts & excretes excess sodium chloride from blood They have small & large intestine
Duodenum – short curve; 1st segment of small intestine
(the remaining part is coiled to one degree or another except in urodeles in Lizards, birds, & mammals, it’s line w/ villi – fingerlike projections that give a velvety appearance of the lining)
**Villi: short/low (bird), tall (mammals) TETRAPODS: Small Intestine *villi – increase absorptive surface of intestine
*lacteals – located w/in the villi; dead-end lymph vessels where digested (hydrolyzed) lipids go after being absrobed in epithelium *chyle – milky fluid that lacteals pump by its sudden shortening  into a larger lymphatics that epty into a major vein near the heart

- Lipids are then carried by bloodstream to liver (chief site of synthesis of cholesterol & other sterols & lipids, and to other tissues for storage) The small intestine beyond duodenum is divided into:
(basis: shape of villi, nature of epithelial lining, size of lymph nodules in mucosa) Mammals Small nodules are present throughout the intestine, but in ileum they aggregate into large masses – Peyer’s patches

Amniotes: their small intestine terminates at ileocolic sphincter – regulates the ejection of contents of ileum into large intestine Chief site of digestion & absorption of nutrients (although ptyalin, pepsin & cellulase split starch, proteins & cellulose in the stomach)
However, the final stage of digestion that results in absorbable nutrients takes place in small intestine in the presence of intestinal juice & pancreatic enzymes SMALL INTESTINE *intestinal juice – its enzymes secreted by glands in epithelial lining of crypts & by compound alveolar glands near pylorus
- Its enzymes split polypeptides into amino acids, disaccharides into monosaccharides (w/c are absorbable molecules already) *Pancreatic juice – gives an (1) amylase – acts on carbohydrates, (2) lipase – acts on lipids to yield fatty acids & glycerol, and (3) proteolytic enzymes – continue that digestion initiated by pepsin Rarely coiled; ceca are common
Mammals, some reptiles, birds: it is divisible into (1) colon – commences at ileocolic sphincter and (2) rectum – straight terminal portion in pelvic cavity LARGE INTESTINE Colon of some mammals has ascending, transverse & descending portions w/ pronounced flexures between them

**In Humans: descending colon ends in a sigmoid flexure Ceca are rarebeyond duodenum in fishes & amphibians, but ceca beyond ileocoloic sphincter (ileocolic ceca) are common in maniotes
Birds have two of them
Mammals (that feed on fibrous vegetation, fruits, grains, seeds) – ileocolic cecum may exceed the capacity of large intestine *Appendix – w/ lumen & histology the same as that of cecum terminates the cecum in anthropoids, rodents & mammals The diet has low nutritional value that mandates a large gut capacity
The colon on all tetrapods recovers water from residual contents (feces) of intestine – water reclamation is necessary to replace those w/c were ‘borrowed’ during the process of digestion Liver arises from the midventral aspect of the midgut as a hollow cecumlike diverticulum, the liver bud. The bud grows cephalad in the ventral mesentery of the stomach.
The growing tip of the bud gives rise to numerous sprouts that become the liver and the gallbladder.
The cephalic pole of the liver becomes anchored to the embryonic transverse septum by a coronary ligament. Liver and Gallbladder Hepatic ducts – drained the lobes of the liver
Cystic duct – drained the gallbladder
Common bile duct – site where the hepatic and cystic ducts converge which empties in the duodenum in the latter.

A short terminal segment of the common bile duct is embedded in the wall of the duodenum, where it constitutes an ampulla of Vater. Hepatoduodenal ligament – connects the duodenum and liver
Gastrohepatic ligament – connects the pyloric stomach and the liver.
Hepatoduodenal ligament and gastrohepatic ligament constitute the lesser omentum.
Lesser omentum serves as a bridge that conducts the common bile duct to the duodenum and the hepatic artery and hepatic portal vein to the liver.
Falciform ligament – embryonic mesentery ventral to the liver remains in adults It produces bile (an alkaline fluid containing bile salts that emulsify lipids in the small intestine which aids digestion)
The remains in the splitting of hemoglobin molecule is converted into red and green pigments (bilirubin and biliverdin) that are excreted as part of the bile.
The liver removes from the circulation glucose in excess of immediate tissue needs.
The liver removes dietary amino acids from the hepatic portal system and deaminates them. The byproducts of deamination include ammonia, uric acid and urea. Roles of the liver Later stage Early stage Pancreas consist of two components:
Exocrine portion – produces digestive enzymes in alveoli (acini)
- the enzymes are transported via pancreatic ducts to the duodenum.
Endocrine portion – bears pancreatic islets (islands of Langerhans), lacks ducts and secretes hormonal products, insulin and glucagon into the bloodstream. Exocrine Pancreas Pancreas varies from diffuse to compact.
Diffuse – pancreatic tissue is distributed along the blood vessels in the ventral mesentery of the stomach and duodenum.
Compact – may consists several definitive lobes.
Pancreas arises as one or two ventral pancreatic buds off the liver bud and as a single dorsal pancreatic bud.
Ventral buds – invade ventral mesentery of the duodenum and stomach and forms ventral lobe (body) of the pancreas.
Dorsal buds – becomes dorsal lobe (tail) of the pancreas. Accessory pancreatic duct – when one duct is larger, as in cats and humans, the other is relegated to this status.

- (Latin, “sewer”)
Where urinary and genital tracts also empty
In lampreys, chimaeras, living female coelacanths, and ray-finned fishes , the embryonic cloaca becomes increasingly shallow or disappears as development proceeds and the digestive tract opens independently to the exterior.
In therian mammals, the cloaca becomes partitioned during embryonic development into two or three separate passageways, one becomes rectum leading to an anus. FISH FIN :)
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