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The Layers of the Vertebrate Gut, Their Function In Different Parts of the Gut and in Different Animals

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Introduction

The Layers of the Vertebrate Gut, Their Function In Different Parts of the Gut and in Different Animals For the purposes of the essay the gut is considered to start from the gastroesophageal junction and terminates at the anus. The gut layers are described and their functions in different parts of the gut discussed, beginning at the gastroesophageal junction and working downward through the gastrointestinal tract. Description and discussion of the human and mammalian gut predominates, as it is these animals that have been most studied, but variations in different groups of vertebrates have been included where possible. A cross-section through the gut reveals four separate main layers with each layer performing a functionally distinct role (Fig. 1). Some layers remain relatively unchanged throughout the entire length of the tract whilst other layers are adapted for different functions in different regions of the gut. The inner mucosa is the most variable layer and is sub-divided into three further layers (1) the innermost epithelial lining adjoining the lumen of the tract (2) the lamina propria, a layer of fibrous connective tissue usually containing abundant blood and lymphatic capillaries that enable efficient secretion and absorption at the mucosal surface. It may also contain glands, lymph nodules and diffuse smooth muscle fibres (3) the muscularis mucosae, a number of layers of smooth muscle fibres that improve contact between the epithelium and contents of the lumen for absorption by persistent agitation. ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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