At the gastroesophageal junction in humans and some mammals, the protective stratified squamous epithelium of the oesophagus changes to a protective secretory mucosa comprised of a single layer of columnar cells. Stratified squamous epithelium is well adapted to the high abrasion it is subjected to in the oesophagus. The mucosa of the human oesophagus is deeply folded to allow expansion on passage of food. The submucosa in the lower third of the oesophagus contains small glands that secrete lubricating mucus to reduce friction as food passes down the tract. At the gastroesophageal junction in humans the submucosa and muscularis externa continue relatively unchanged beneath the altered mucosa.
In the human stomach, the mucosa is extensively folded longitudinally into rugae to allow expansion after reception of food (Fig 2). The stomach mucosa is protected from self-digestion by a thick layer of protective mucus and is characterised by long tightly packed tubular glands. Numerous closely set pits are present and their depth varies depending on the region of the stomach in which they are found. Several distinct regions of the stomach can be identified by differences in the mucosa (Figs. 2 and 3). In humans the cardia surrounds the entrance of the oesophagus and is an area comprised mainly of mucus secreting glands. Most other mammals, reptiles and a number of adult amphibians possess a cardiac region at the gastroeosophageal junction. In some mammals the cardia comprises a large area of the stomach and in others forms isolated patches amongst stratified squamous epithelium. A cardiac region is not present in fish. Glands in the fundus and corpus secrete acid and pepsin to start protein digestion and protective mucus to prevent autodigestion. Acid and pepsin secretion is performed by the same glandular cells in birds, adult amphibians, reptiles and fish and by different cells in mammals. Some amphibian larvae and mammals do not secrete acid and pepsin. In humans the pylorus secretes the hormone gastrin and two kinds of mucus. In other mammals, the distribution across the stomach of these regions varies (Fig. 3).
In humans the mucosal pits in the cardia are shallow and the glands secrete protective mucus. In the fundus the mucosa is much thicker and contains more glands that reach approximately a quarter of the distance between the epithelium and the muscularis mucosae. Glandular secretions are squeezed out by the contraction of thin bands of muscularis mucosae running between the gastric glands. Preliminary digestion is enhanced by mechanical mixing of the stomach contents to produce semi-digested chyme. In humans mechanical mixing is achieved by three layers of muscle in the muscularis externa. Elsewhere in the human digestive tract (with the exception of the pyloric sphincter) there are only two layers of muscle in the muscularis externa, an inner circular layer and outer longitudinal layer. The muscularis externa of the stomach wall contains a third incomplete layer of muscle set obliquely to the other two muscle layers. This arrangement allows vigorous mixing of the stomach contents. Some other mammals possess this third oblique layer of muscle near the gastroesophageal junction. In a number of marsupials and herbivorous primates it is restricted to taeniae and is practically absent from the gizzard of birds. The ruminant forestomach has a very intricate arrangement of muscle layers.
In humans the branched glands of the pyloric region consist mainly of mucus secreting cells with a scattering of acid secreting cells. The mucus lubricates the tract to ease the passage of chyme and also protects the entrance to the duodenum from attack by acid and pepsin produced in the stomach. The pits in the pylorus are an irregular shape and deeper than in the cardia and fundus extending at least halfway from the epithelial surface to the muscularis mucosae. A glandular pylorus is present in the majority of mammals, fish, adult amphibians and reptiles.
The whole of the gastrointestinal tract is under the control of the autonomic nervous system, that is to say its control is involuntary and carried out unconsciously. The autonomic nervous system controls every aspect of gut function from glandular secretions to control of the muscles. Autonomic nervous system components called Meissner’s plexus are found in the submucosa of the human stomach. Another collection of autonomic nervous system components called Auerbach’s plexus is found in the muscularis externa between the two layers of muscle. The areolar tissue of the stomach submucosa does not contain glands in any region.
The next section of the gastrointestinal tract is the small intestine which in mammals can be divided into the duodenum, jejenum and ileum. It is in the small intestine that the main absorptive phase of digestion occurs and any increase in the length of the intestine, or of mucosal surface area greatly facilitates the absorptive process. Different groups of vertebrates have developed different ways of achieving this. Sharks, rays, lungfish and lampreys use a spiral fold of tissue running the entire length of the intestine to increase its length. Multiple pyloric ceca provide additional sites for digestive absorption in some bony fish. The intestine of jawless fish has a very thin muscle layer so the transport of luminal contents is supplemented by the action of cilia.
From amphibia upwards intestinal mucosal and submucosal folds serve to increase surface area with mammals showing the highest degree of refinement of this system. Circularly arranged folds of the mucosa and submucosa called the plicae circulares. extend throughout the mammalian small intestine but are particularly extensive in the jejenum where the main absorptive phase occurs. Finger-like villi cover the plicae circulares greatly increasing surface area for more efficient absorption, which is further enhanced by the microvilli extending from the columnar epithelium covering the villi. The villi absorb the various amino acids, sugars etc. where they pass into the circulatory system via the lamina propria running through the centre of each villi.
In mammals the mucosa between adjacent villi is shaped into glandular crypts called Crypts of Lieberkuhn that release substances to complete the process of digestion. Some amphibian adults and reptiles possess Crypts of Lieberkuhn, but they are more developed in birds and mammals. The glandular crypts of the mammalian duodenal mucosa reach the submucosa to divide into branched tubular glands called Brunner’s glands. These glands extend further along the duodenum in herbivores than carnivores. Submucosal intestinal glands are absent in most fishes. The alkaline secretion from the Brunner’s glands prevents the duodenal mucosa from autodigestion. The pancreatic duct (if present) releases alkaline juice and digestive enzymes into the duodenum whilst bile duct secretions emulsify any fats present. In some vertebrates, bony fish for example, the pancreatic duct is absent and secretion is through the bile duct or straight into the pyloric cecum.
Indigestible food passes into the large intestine where water is absorbed. As the remains become increasingly solid they are pushed further along the tract for evacuation. The large intestine of most mammals consists of the colon and rectum, a cecum is also present in some species. The mammalian colonic mucosa consists of both absorptive and mucus-secreting cells, microvilli are also present. The glands have a tightly packed straight tubular arrangement in humans, increasing the surface area of the absorptive cells. The mucus secreting cells protect the colonic mucosa from abrasion and the regular contraction of the thick muscularis mucosae in the colon prevents the glands clogging and facilitates the expulsion of mucus. The colonic submucosa of humans does not contain any glands but large lymph nodes in the mucosa often reach the submucosa.
The colonic muscularis externa of many mammals propels the solids by peristalsis for evacution via the anus. In some mammals, adult amphibians, reptiles and birds the digestive tract terminates in a cloaca. The cloaca is comprised of a coprodeum, urodeum and proctodeum. The proctodeum is the posterior section and terminates at the anus. The reptilian and avian coprodeum and urodeum are lined with absorptive mucosa whilst the proctodeum lining is of a stratified squamous arrangement.
The vertebrate gut is remarkable in its complexity and adaptability. The gut layers have an extraordinary ability to perform a multitude of different functions depending on their position in the gastrointestinal tract and in different animals. This is due to the adaptability of their basic structure, particularly the mucosa. This is further illustrated by their function in animals living in radically different environments. The diets and lifestyles of these animals are as far ranging as the environments in which we find them, yet the gut layers are well adapted to what is required of them.
References
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