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THE DIGESTIVE SYSTEM OF
VERTEBRATES: CD 
4. General Characteristics of the Digestive System:
Structural Characteristics - Introduction:
Due to species variations, the vertebrate digestive system is best described under the broad headings of headgut (mouthparts and pharynx), foregut (esophagus and stomach), midgut, hindgut, and its ancillary organs (salivary glands, exocrine pancreas, and the biliary system of the liver). The digestive tract tends to be shortest and simplest in carnivores and species that feed on plant concentrates, and longer and more complex in omnivores. However, the low concentrations of soluble nutrients and high levels of structural carbohydrates (cellulose and hemicellulose) in the diet of herbivores requires more efficient mechanisms for the breakdown of forage, and a more complex system for the retention and microbial fermentation of plant material. The principal sites of gut expansion in herbivorous vertebrates are the foregut, midgut, or hindgut.
Structural Characteristics - Headgut:
The headgut serves mainly for the procurement and physical breakdown of food. Articulated jaws are found in all vertebrates other than cyclostomes. Lips, tongue, teeth, or a beak may be used for the prehension of food and most vertebrates use teeth or a beak for the cutting, tearing, crushing, or grinding of food. Deglutition (swallowing) of food is lubricated by mucous-secreting cells in the oral cavity of fish, and multicellular glands in amphibians, reptiles, birds, and mammals. The headgut shows numerous examples of convergence on common functions. Most carnivores, such as sharks, crocodiles, hawks, and tigers use their teeth or beak for grasping, cutting and tearing their prey. However, the headgut of basking and whale sharks, paddlefish, many amphibian larva, the flamingo, and baleen whales (Fig. 4.1a,b) are designed for microphagous filter-feeding on small aquatic invertebrates, and the ant and termite eaters in five of the mammalian orders have long tongues and weak jaws. Most herbivores use their teeth or beak for the reduction of plant material into small particles, but birds and some herbivorous fish use a gizzard or gizzard-like stomach for this purpose.
Figure 4.1a Microphagous filter feeders which consume small aquatic invertebrates (photos: basking shark by Dan Gotshall, Atlantic right whale by Jennifer Campbell, blue paddlefish by John MacGregor).
Figure 4.1b Baleen filter of gray whale (photo by W. A. Sheppe)
Structural Characteristics - Foregut:
The esophagus transports food to the stomach . It is lined with a relatively impermeable multilayer of stratified squamous epithelium in most species, and includes a crop for the storage of food in most birds. The stomach serves as the initial site for the storage and digestion of food in most vertebrates. However, it is absent in cyclostomes, some advanced fish, and the larval amphibians, and its functions are divided between a crop (storage), proventriculus (enzymatic digestion), and gizzard (trituration) in birds. The stomach of all vertebrates, other than the larval amphibians, monotremes, and armadillos, is lined with regions of proper gastric and pyloric glandular mucosa (Fig. 4.2). The proper gastric region secretes mucous, hydrochloric acid (HCl), and pepsinogen (the precursor to the proteolytic enzyme pepsin). HCl and pepsinogen are secreted by the same cell in lower vertebrates but separate cells in mammals. The pyloric region secretes mucous and bicarbonate (HCO3-). The stomachs of some adult amphibians and reptiles, and most mammals contain an additional region of cardiac glandular mucosa, which also secretes mucous and HCO3-, and a fourth region of nonglandular stratified squamous epithelium is found in the stomach of some mammals.
Figure 4.2. Regions of glandular mucosa lining the stomach of vertebrates. Proper gastric and pyloric glandular mucosa are found in the stomach of all vertebrates other than larval amphibians, monotremes, and armadillos. The stomachs of salamanders, reptiles, and mammals contain the additional region of cardiac glandular mucosa near the gastroesophageal junction. (Stevens 2001)
Structural Characteristics - Midgut:
The midgut serves as the principal site of digestion and absorption in all vertebrates. It is lined with a single layer of epithelium comprised of a variety of different cell types that aid in digestion, absorption, secretion of electrolytes, or the production and secretion of hormones or paracrine agents. Its lumen surface is increased by a brush border of microvilli on the absorptive-digestive cells. It is further increased by pockets, folds, or ridges in lower vertebrates, or macroscopic projections of epithelial and subepithelial tissue (villi) in salamanders, birds, and mammals (Fig. 4.3). The microvilli and villi provide an enormous expansion of the surface area for the final stages of digestion and absorption, as shown in Figure 4.4.
The midgut villi of mammals are surrounded by the crypts of Lieberkuhn (Fig. 4.3), which contain endocrine cells and undifferentiated cells that become absorptive/digestive cells or goblet (mucous secreting) cells as they migrate from the crypts to the tips of the villi. Zones of cell proliferation have been described at the base of folds in advanced species of fish (Hyodo-Taguchi 1970; Stroband and Debets 1978) and amphibians (Martin 1971; McAvoy and Dixon 1977). Crypts have been described in the midgut of a few fish, salamanders, and some reptiles and birds. However the crypt cells appeared to be similar to those of surface epithelium in the fish (Harder 1975a) and reptiles (Luppa 1977), and did not appear to contain endocrine cells in some birds (Ziswiler and Farner 1972).
Figure 4.3. Intestinal villus and crypt of the midgut. Inset shows an enlarged absorptive/digestive cell, with its microvilli or brush border. (From Stevens and Hume 1995)
Figure 4.4. Relationship between lumen surface area of the midgut or small intestine, and body mass. Surface areas are nominal (length x diameter) except where otherwise indicated, and include ceca when present in fish and birds (From Karasov & Hume 1997.)
Structural Characteristics - Hindgut:
The hindgut of most fish and the larval amphibians is short and difficult to distinguish from the midgut in either its structure or function. However, the midgut and hindgut of most mammals, birds, reptiles, and adult amphibians are separated by a valve or sphincter and, due to differences in diameter, are generally referred to as the small and large intestine. The hindgut is also lined with a single layer of epithelium and with crypts and a surface epithelium containing goblet and absorptive cells with a brush border, but an absence of villi. At its junction with the midgut, the hindgut includes a cecum (blind sac) in a few fish and reptiles, and many mammals, and a pair of ceca in many birds. The hindgut of fish, larval amphibians, and most mammals exits the body at the anus. However, the hindgut of adult amphibians, reptiles, birds, and some mammals empties into a cloacal chamber, along with the urinary and reproductive tracts.
The hindgut appears to have evolved in response to the transition of vertebrates from an aquatic to a terrestrial environment (Fig. 4.5). Freshwater fish excrete excess water by glomerular filtration of blood and reabsorption of most of its solutes from the renal tubules. Marine fish adapted to a hypertonic environment high in Na+ and Cl- by reducing or eliminating glomerular filtration and secreting Na+ and Cl- via their gills and by salt glands in some species. However, terrestrial animals must go to considerable lengths to conserve both electrolytes and water. Urinary excretions are released into the cloaca of adult amphibians, reptiles, and birds, and refluxed the length of the hindgut by antiperistaltic muscular contractions in some reptiles and most birds. This increases the retention time of both urine and digesta, providing more time for the reabsorption of electrolytes and water, and the multiplication of indigenous bacteria. In most mammals, the urinary and digestive tract develop separate exits prior to the birth, the kidney is more efficient in its recovery of electrolytes and water, and the antiperistaltic contractions are confined to the proximal segment of a longer hindgut. The hindgut tends to be longer in species that inhabit arid environments and is the principal site of microbial fermentation in most reptiles, birds, and mammals.
Figure 4.5. Adaptations of the nephron and hindgut in relation to habitat The nephrons of fish, amphibians, reptiles, and birds are limited in their ability to concentrate urine. Urine is excreted into the cloaca of amphibians, reptiles, and birds and refluxed into the hindgut, which aids in the recovery of electrolytes and water from the urine and digesta. Microbial digestion of uric acid also aids in the conservation of nitrogen. The majority of mammals excrete their digesta and urine separately. Recovery of urinary electrolytes is aided by the kidney’s loop of Henle. Nitrogen conservation is aided by diffusion of urea into the intestine where it is digested by hindgut microbes into ammonia and absorbed. (Modified from Smith 1943 by Stevens 1977).
Structural Characteristics - Oral Glands, Pancreas and Biliary System:
The headgut of amphibians, reptiles, birds, and mammals contains oral glands, which are referred to as salivary glands in birds and mammals. They secrete mucous that aids in the deglutition of food and serve a variety of other functions in some species. The oral glands of frogs, toads, swifts, woodpeckers, and mammalian anteaters secrete an adhesive material that aids swifts in building their nests and the other species in the capture of prey. The oral glands of amphibians, reptiles, birds, and mammals secrete digestive enzymes in some species, and venom and venom-spreading agents in others. The serous (watery) component of mammalian saliva contains bicarbonate and phosphate buffers, which neutralize end products of microbial fermentation in the stomach of foregut fermenting herbivores.
The pancreas and liver are embryonically derived from the midgut. Pancreatic tissue is distributed along the midgut of cyclostomes and some advanced species of fish, but the pancreas is a compact organ in other vertebrates. It secretes enzymes that aid in the digestion of carbohydrates, lipids, and protein, and fluids that help neutralize the pH of midgut contents. The biliary secretions of the liver emulsify lipids in preparation for their digestion by pancreatic enzymes. Bile is stored in a gall bladder in most vertebrates, for release as needed for lipid digestion, but it is released continuously into the midgut of some fish and mammals.
Motor Activity:
The ingestion and mixing of food with digestive fluids and enzymes, and the passage of digesta through the alimentary tract are accomplished by its motor or muscular activities. Food is passed through the esophagus of some fish, amphibians, and reptiles with the aid of ciliated epithelium. However, with the exception of embryonic fish and larval amphibians, ingesta and digesta are transported through the digestive tract principally or entirely by muscular activity. The esophagus and gastrointestinal tract are enveloped with an inner layer of circular muscle and outer layer of longitudinal muscle (Fig. 4.6). The circular muscle is thickened to form valves or sphincters in some regions, and a gizzard or gizzard-like stomach in some fish and most birds. The longitudinal layer is thin or incomplete in the esophagus, gizzard or hindgut of some species, and concentrated in bands of muscle in the stomach or hindgut of some mammals.
Figure 4.6. Cross-section of the intestine. (Stevens 2001.)
Esophageal muscle is striated in fish and over varying lengths in mammals. However, the esophagus of amphibians, reptiles, and birds, and the entire gastrointestinal tract of all vertebrates is comprised of smooth muscle, which reacts more slowly and demonstrates a greater degree of passive distention. Food and digesta are transported along the digestive tract by sequential stationary or moving (peristaltic) waves of contraction and retained at some sites by sphincters, valves, or antiperistaltic waves of muscular contractions. The motor activities associated with the ingestion and physical breakdown of food in the headgut, and the final defecation of waste products are under voluntary control. However, the motor activities of the remainder of the digestive tract are under the involuntary control of the nervous and endocrine systems.
Digestion and Absorption:
The principal sources of nutrients in the diet of vertebrates are carbohydrates, lipids, protein, vitamins, minerals, and water. Food is taken up by phagocytosis into the midgut intestinal cells of larval amphibians and neonate mammals, and digested by intracellular enzymes. However, in all other vertebrates food is digested by enzymes secreted into the gastrointestinal tract and located in the lumen-facing membranes or cell contents of midgut epithelial cells. The lumen of the gastrointestinal tract is also populated with indigenous bacteria, which are found in highest concentration in the hindgut of terrestrial vertebrates and foregut of some species. These bacteria produce short-chain fatty acids (SCFA) by the fermentation of carbohydrates, including the structural carbohydrates of plants (cellulose, hemicellulose, pectins) if given a sufficient retention time. They can also utilize nitrogenous compounds for the production of ammonia and bacterial protein, and synthesize B-vitamins required by their host.
In carnivores and omnivores, the SCFA are derived largely from starches that escape digestion in the midgut and the endogenous carbohydrates in mucous and sloughed epithelial cells. The ammonia and microbial protein nitrogen is derived mainly from the digestive enzymes and urea (or uric acid) released into the digestive tract. However, herbivores can subsist on a low -starch, low-protein diet of the leaves, petioles and stems of plants by ingesting large quantities of plant material and retaining it for periods long enough for the microbial fermentation of the structural carbohydrates in cell walls and the release of cell contents.
Monosaccharides, amino acids, and water-soluble vitamins are absorbed by a variety of mechanisms that provide carrier-mediated transport across the membranes of midgut intestinal cells. Lipid-soluble nutrients such as the long-chain fatty acids and fat-soluble vitamins, are absorbed by passive diffusion. The SCFA and ammonia produced by gut microbes appear to be absorbed by both carrier-mediated transport and passive diffusion.
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