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THE DIGESTIVE SYSTEM

Humans and bacteria have similar metabolic needs in that both require simple molecules (monosaccharides, amino acids, fatty acids, etc.) from which to construct their own unique complex molecules and both need energy, which may be obtained through the degradation of molecules encountered in their environment. Thus, the ability to digest large molecules from the environment to produce energy and to provide molecular building blocks has been a characteristic of living things for billions of years. Bacteria and protists can release digestive enzymes into their environment and, after chemical digestion, absorb the simpler molecules. Controlling the acid levels of digestion is important since many digestive enzymes have an optimum pH at which they work best. The typical eukaryotic cell possesses intracellular organelles called lysosomes which digest molecules in an acidic medium. In many protists and sponges, the intracellular digestion which follows phagocytosis occurs first in an acidic environment, then in an alkaline environment, interestingly similar to the sequence in the stomach and intestine of vertebrates (Barrington, p. 172).

Cnidarians (such as the Hydra in the adjacentimage) do not possess organs or systems, but they possess a number of significant characteristics of the digestive systems of higher animals. They possess a mouth and a gastrovascular cavity. The food which will be digested can be trapped by mucus and moved to the gastrovascular cavity by the action of cilia. Digestive enzymes are secreted into cavity, mostly proteinases from endodermal gland cells. Muscle cells surround the gastrovascular cavity and may be present in multiple layers. Some cnidarians have a pharynx. Some ctenophores possess anal canals which means that indigestible material does not need to exit the body through the mouth, which is the condition of most cnidarians.

In cnidarians, extracellular enzymes don’t break down the food completely—small pieces undergo phagocytosis and are digested further intercellularly (Hickman 185, Fretter). Hydra possess 2 kinds of gland cells in their digestive tracts for the production of mucus and enzymes. The use of extracellular digestion by primitive animals probably introduced a selective advantage for greater differentiation of the gastrointestinal tract. Although extracellular digestion can aid intracellular digestion, eventually it requires a control of the pH in the area of digestion, the ability to mix food and enzymes, and the separation of the digestive area into separate compartments to function well (Beklemishev).

A cross section of a hydra, with its central gastrovascular cavity, is depicted below.

Flatworms of the order Acoela lack a pharynx, an intestine, and an anus—the digestive tract has only one opening, as in cnidarians. The mouth is not located at the anterior end of the animal, but rather more centrally. Although acoels are bilateral animals with a head, the organization of their digestive system with a single, more centrally located opening is more similar to that of cnidarians than to other bilaterans (Hickman).

In acoels, food is digested by phagocytosis by the gut lining instead of by enzymatic breakdown. It is unknown whether this is a reduction from the primitive state of extracellular digestive enzymes found in cnidarians or whether Acoela retain the most primitive type of animal digestion (Fretter, Dougherty).

In more advanced flatworms (turbellarians, macrostomids), there is a muscular pharynx which performs peristalsis (Fretter), is ciliated, and possesses longitudinal and circular muscle layers (Dougherty, p. 197, Beklemishev 2, p. 196; Rieger, from Harrison, 1991). Microvilli increase the surface area of the intestine as in higher animals (Hickman) and many unicellular glands are present (Beklemishev 2, p. 196). The following image is of the pharynx of a planaria which is located in the center of the body rather than in the head.

The following image is a cross section of a planarian pharynx.

As in Acoela, the mouth is not located in the head but is located more posteriorly or even centrally, suggesting a link to more primitive, radially symmetric animals. In flatworm orders Macrostomida and Notandropora digestion is extracellular and a stable epithelial gut lining exists (Beklemishev 2, p. 192). Since they lack a circulatory system, flatworm cells must be located near the gut to obtain food by diffusion, just as in coelenterates. The gut (which is pigmented in the following images) is highly branched as a result and serves a distributive (vascular) function in addition to a digestive function.

While some flatworms have anal canals, most have only one opening of their digestive tracts (Beklemishev 2). Advanced flatworms possess a diverse set of digestive glands although their products aren’t well known (Rieger, from Harrison, 1991).

Nemertine worms show a number of advances over flatworms, to which they are thought to be related. They possess a complete digestive system, with a mouth on one end and an anus on the other. In a few species, the mouth is not at the anterior end of the animal and is located more posteriorly. Food is moved primarily through ciliary action and is digested both extracellulary and intracellulary. In some, the larval forms lack an anus, as is the condition in flatworms (Hickman, p. 227).

Closely related invertebrates often differ in their source of nutrition. For example, although most ascidian urochordates are filter feeders, some species in the family Octacnemidae are carnivores which feed on small invertebrates (Burighel, from Harrison, 1997, p. 221).

THE GASTROINTESTINAL TRACT

In chordates, such as in Amphioxus pictured below, the gastrointestinal tract is ventral to the nervous system and notochord.

The gastrovascular cavity of cnidarians is lined by epithelial cells which secrete enzymes and absorb nutrients. In hemichordates and tunicates, as in higher chordates, microvilli in the GI tract form brush border extending columnar epithelial cells to increase surface area for secretion and absorption (Benito, form Harrison 1997, p. 88). In vertebrates, the absorptive cells may be located on villi which further increase the surface area of the gastrointestinal tract.

lamprey

frog intestine

While the intestinal wall of tunicates is typically smooth, a few species possess grooves (Burighel, from Harrison, 1997, p. 256). Folds can also be seen in the lining of the GI tract in the annelid worm pictured below. Also note the presence of cilia on the epithelial cells to propel the ingested material. The mucus that worms developed to help them burrow could also be used to trap and move particles of food (or vice versa). The cilia which primitive worms used for movement could help move this mucus and food. Among primitive deuterostomes, the use of cilia to propel food is commonly used. Primitive echinoderms and pterobranchs possess ciliated tentacles while enteropneusts possess a ciliated proboscis. Primitive chordates utilize cilia and mucus for food capture in their pharynx (in Amphioxus, the endostyle is a ciliated groove which makes mucus to trap food) (Barrington, 194).

worm

In the primitive condition (as in the adjacent image of the worm), ciliary movement propelled food which had been filtered from sea water and had been lodged in mucus. In tunicates, there is very little muscle along the GI tract and most material is moved through ciliary action (Burighel, from Harrison, 1997, p. 255) Muscle along the GI tract was needed when larger, more solid food items were ingested. The muscularis layer of the GI tract of a frog is pictured below.

Many invertebrates possess circular and longitudinal layers of muscle along the GI tract, although in many (such as squid, sea cucumbers, and some insects), the circular layer is outside the longitudinal layer unlike the organization in vertebrates (Hoar, 1983, p. 425). Peristaltic waves of muscular contraction occur in worms and echinoderms (Hoar, 1983, p. 425). Jawless fish only have thin muscle layers around the GI tract and rely on cilia to move food. In gnathostomes, there are inner circular and outer longitudinal muscle layers. Cilia exist in the gastrointestinal tracts of some fish, amphibians, and reptiles (Stevens, p. 20). A few fishes such as the tench (Tinca) possess striated muscle along their GI tract. (Hoar, 1983, p. 425).

The layers of the human GI tract are essentially the same as those of the frog depicted in the following images. The lumen of the GI tract is lined by a mucosa layer which contains the epithelia which secretes digestive enzymes and absorbs food. The human duodenum possesses simple columnar epithelia located on villi, just as in the frog duodenum below. Beneath the mucosa is the submucosa, the bands of longitudinal and circular muscle of the muscularis layer, and the serosa lining of the GI tract (Stevens,p. 273).

mammal

HUMAN MODEL

The nervous plexuses of the submucosa and muscularis layers of the gastrointestinal tract (referred to as the enteric nervous system) consist of a diffuse nerve net which is largely independent of the central nervous system. This nerve net is similar to the nerve net which surrounds the gastrovascular cavity of cnidarians and evolved prior to the brain and nerve cords of bilateran animals. The connections between the enteric nervous system and the central nervous system evolved gradually in vertebrates. In fish, there is no vagal stimulation of the GI tract beyond the stomach. (Stevens,p. 273). In amphibians, there is some sacral innervation of this system. In amniotes the only cholinergic excitation of the gut comes from the parasympathetic division of the ANS and there is sacral parasympathettic stimulation of the hindgut (Stevens, p. 274)

Like mammals, teleosts regulate digestion with peptide hormones like CCK, gastrin, galanin, glucagons-like proteins, and various somatostatins (Nelson, 2006).

ENZYMES

Digestion relies on the function of enzymes which are able to break down specific molecules in the diet. The enzymes which humans use to break down their food are not unique to humans and many belong to gene families which originated before the evolution of animals. Serine proteases are a large family of enzymes in the human genome which function in diverse physiological processes ranging from digestion to coagulation (OMIM; Yosef, 2003). This is an ancient gene family which includes eubacterial digestive enzymes and the vertebrate digestive enzymes trypsin and chymotrypsin. Cnidarian digestive enzymes include a trypsin-like digestive enzyme that functions in an alkaline environment (Hyman, 393). Trypsin is produced in the pancreas of all gnathostomes and in the intestinal mucosa of hagfish (Stevens).

Vertebrates use both endopeptidases (which cut proteins in the middle of the chain but only at specific amino acids) and exopeptidases (which remove amino acids from the ends of chains, regardless of the amino acid). No pepsin is known in jawless fish but this enzyme is known in gnathostomes. Chymotrypsin is known in bony fish and higher vertebrates; elastase is known from gnathostomes. Aminopeptidase and carboxypeptidase are known from jawless fish (Stevens).

Cnidarians can digest most types of biomolecules but most cannot digest starches (Hickman, 137; Hyman). Amylase is secreted from pancreas in all vertebrates and from the salivary glands in many mammals, including the echidna (Stevens, p. 160). Maltase, isomaltase, and trehalase are known in all tetrapods. Amphibians lack sucrase but it is present in some fish (Stevens). The echidna and some marsupials also lack sucrase (Stevens). Low levels of lactase are present in birds which is interesting since mammals use lactase to digest the milk sugar lactose. Chitinase can break down the exoskeleton of arthropods and is present in jawless fish, cartilaginous fish, bony fish, and all groups of tetrapods including some mammals (but not humans) (Stevens).

In vertebrates, pancreatic lipase is the most important enzyme in the digestion of lipids. (Stevens, p. 168). In amniotes, the primary bile salt is cholesterol (as opposed to sulfated alcohols in fish and amphibians) (Stevens).

The myelin P2 superfamily of proteins includes proteins known as fatty acid-binding proteins (FABPs) known from nematodes through mammals. Vertebrate proteins seem to belong to distinct groups functional in the heart, liver, and intestine. The FABPs expressed in the livers of lampreys and sharks belong to the heart subgroup, suggesting that a change in the FABPs expressed in livers occurred early in the vertebrate line after the shark lineage diverged from that leading to bony fish (Baba, 1999).

Goblet cells exist in acorn worms and secrete some enzymes. (Benito, form Harrison 1997, p. 26). In tunicates, there are 6 types of cells lining the stomach region. Some possess cilia, others are absorptive, others seem to be similar to acinar cells of the vertebrate pancreas, and others are endocrine cells, whose peptides include secretin, gastrin, and somatostatin (Burighel, from Harrison, 1997, p. 255).